Appunti manufacturing and assembly technhonologies.pdf

 ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 ASSEMBLY 1- THE PRODUCTION PROCESS IN THE AUTOMOTIVE INDUSTRY A vehicle is a very complex system made of thousands of parts,but only few o...

 ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 ASSEMBLY 1- THE PRODUCTION PROCESS IN THE AUTOMOTIVE INDUSTRY A vehicle is a very complex system made of thousands of parts,but only few of them are visible. Under it skin there are a lot of assemblies that can be disassembled themselves. The auto components industry is predominantly divided into five segments: body and chassis Engine parts Drive transmission and steering parts Suspension and brake parts Electrical parts The main assemblies that are present in a vehicle are: body (divided into upper and under body) Closures (doors, hood ecc…) Front mechanical unit (front suspension, powertrain…) Rear suspension Fuel supply Exhaust system Electric system Interior trims Ecc… 1.2-Production flow The production flow of a vehicle is made of several steps: The process starts with the stamping of the parts that will form the body and the closures.  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 The body is obtained by welding together the stamped parts; so the body is made only by parts welded together and so cannot be disassembled. The body in white BIW is then obtained by mounting the closures and the two front fenders on the body previously welded. Closures and fenders are not welded to the body but joined to it by means of hinges. BIW is the complete assembly of all the parts made of steel or aluminium. The completed BIW is then subjected to the painting process. The painting is not done only for esthetical reasons, but it is also an important phase that allows to protect the BIW from the external agents. Painting is a very complex process, executed by applying many different layers of paint, each one with a different purpose. Galvanizing : applied on the iron sheet directly by the supplier in order to prevent the metal to oxidise Phosphating (3-5 μm) Anticorrosive primer or cataphoresis ( 20-25 μm) Filling primer Color ed base coat Top coat During the printing process the BIW will go three times in the oven : one for the cataphoresis baking, one for primer baking and one for base/top coating baking. Due to the greater thickness of the base/top coating, this operation is slower than the others, so the line is split into two ovens in order not to slow down the process. Overhauling is made after primer baking.  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 1- WELDING TECHNOLOGIES The term Joining refers to the processes that are used to join two or more parts into an assembled entity. Joining processes are classified as: - Welding (e.g. laser, arc, spot, friction welding): two (or more) parts are coalesced at their contacting surfaces by application of heat (when we melt the parts)and/or pressure(typical of friction welding) - Mechanical joining (e.g. riveting, clinching): two (or more) parts are coalesced at their contacting surfaces by mechanical methods that fasten parts together. Also called “joining by forming” 2.1-WELDING (general though) Many welding process are made by heat alone with no pressure applied, in this case we are talking for example about laser and arc welding; others by a combination of heat and pressure as spot and friction welding; others made only by pressure with no heat as in explosive welding. Important is to know that in some welding processes is added a filler material We can have fusion welding processes in which we can have chemical or electrical sources. For example the electromagnetic laser beam or the resistance related to joule effect  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 Then we also have solid state welding in which the joint is obtained with no melting part, but we only heated up in temperature the material so in this way we transform rigid material into soft material. Let’s see more in detail: Welding processes can be divided into two major categories: Fusion welding - coalescence is accomplished by melting the two parts to be joined, in some cases adding filler metal to the joint? - Examples: arc welding, resistance spot welding, oxyfuel gas welding Solid state welding - heat and/or pressure are used to achieve coalescence, but no melting of base metals occurs and no filler metal is added - Examples: forge welding, diffusion welding, friction welding But we can also made another classification of welding processes: Homogeneous welding : the filler metal ( third body to facilitate the welding operation) is the same as the base material. Heterogeneous welding: the filler metal is different from the base material; the base material does not melt and does not affect the chemical composition of the weld. Autogenous welding: the base material melts and affects the chemical composition of the weld. Another classification: Manual welding: this type has some issues as not uniform weld joint quality because this flessibile welding operation is performed by a human operator so the quality depends on its skills; Automatic welding: a robot perform the operation but it’s very expensive. We can made a classification of manual and automatic welding using the arc time that is defined as the time arc divided by hours worked: manual welding arc time =20% Machine welding arc time = 50% At the end welding is so important because provides a permanent joint, because welded components become a single entity. Usually is the most economical way to join parts and in some cases is not restricted to a factory environment.But at the same time we have such a huge number of limitations when we talk about welding: some welding operations are performed manually and are expensive in terms of labour costs  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 Most welding processes use a high energy (we spent fuel,gas, current ecc.) and are dangerous for human health. Welded joints are not easy to disassembly, can have internal defects that are difficult to find. 2.2 - TYPES OF JOINTS we have five basic types of joints for bringing two parts together for joining, these five types are not limited to welding: Butt joint: the parts lie in the same plane and are joined at their edges Corner joint: the parts in a corner joint form a right angle and are joined at the corner of the angle Lap joint: this joint consists of two overlapping parts Tee joint : one part is perpendicular to the other Edge joint: the parts in an edge joint are parallel with at least one of their edges in common, and the joint is made at the common edge(s). 2.3-TYPES OF WELDS the differences among weld types are in geometry, so joint type, and welding process: FILLET WELDS (SALDATURA AD ANGOLO) Is used to fill in the edges of plates created by corner, lap, and tee joints. This type of welding improves the quality of the joint, for example, we add filler metal as third body that is melted in the joining region. Filler met is used to provide a cross section approximately the shape of a right triangle. This type of weld is common in arc and oxyfuel welding, and requires minimum or no edge preparation. Various forms of fillet welds: (a) inside single fillet corner joint; (b) outside single fillet corner joint; (c) double fillet lap joint; and (d) double fillet tee joint. Dashed lines show the original part edges.  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 GROOVE WELD (SALDATURA A SCANALATURA) This type of welding requires part edges to be shaped into a groove to facilitate weld penetration, but at the same time edge preparation increases costs. This type of welding is closely associated with butt joint. Why the preliminar operation? Because we need to prepare the surface making some grooves. This is common when we are working with thicker parts (1cm) If we don’t make the groove we need high energy to melt all the part and at the same time we are heating up all the surface. Is not a process that we can do just in one time, we have to make several layers of weld beads. Some groove welds: (a) square groove weld, one side; (b) single bevel groove weld; (c) single V- groove weld; (d) single U-groove weld; (e) single J-groove weld; (f) double V-groove weld for thicker sections. Dashed lines show original part edges. PLUG (FORO) AND SLOT (FESSURA) WELDS Plug welds and slot welds are used for attaching flat plates using one or more holes or slots in the top part and then filling with filler metal to fuse the two parts together. Plug weld= 2 overlapped parts with a hole that can be filled with filler metal Slot same process but we need more material. SPOT WELD (PUNTO DI SALDATURA) 2 sheets overlapped together and joint occurs at the interface of two sheets (only when are overlapped). Used for lap joints.  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 We don’t make a linear weld, because in that case we need to provide high energy heating up the surface, but in this case the material is nor rigid so it deform. So with this process we are reducing the heating up effect SEAM WELD (CORDONE DI SALDATURA) A seam weld is similar to a spot weld except it consists of a more or less continuously fused section between the two sheets or plates. FLANGE WELD (SALDATURA A FLANGIA) A flange weld is made on the edges of two (or more) parts, usually sheet metal or thin plate, at least one of the parts being flanged. SURFACING WELD (SALDATURA DI AFFIORAMENTO) A surfacing weld is not used to join parts, but rather to deposit filler metal onto the surface of a base part in one or more weld beads. The weld beads can be made in a series of overlapping parallel passes. Purposes: - increase the thickness of the plate - provide a protective coating on the surface.  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 2.4- THERMAL CYCLE IN WELDED JOINT It’s import for fusion operations. The thermal cycle in welded joints depends mainly on - welding technique (e.g. power density) - material - size of the parts to be welded - location from the welding source  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 Fusion zone (zona fusa): a mixture of filler metal and base metal melted together homogeneously due to convection as in casting. Epitaxial grain growth (like casting). Weld interface ( it’s a transition zone between fusion and heat affected)– a narrow boundary immediately solidified after melting. Heat Affected Zone HAZ (zona termicamente alterata) – below melting but substantial microstructural transformation, even though the same chemical composition as base metal (like a heat treatment) – usually degradation in mechanical properties. Unaffected base metal zone – residual stresses Let’s focus on HAZ: this is the zone in which metal has experienced temperatures below melting point, but high enough to cause micro structural changes in the solid metal. In HAZ the chemical composition is the same as the base metal, but its properties and structure have been altered and the effect of that on mechanical properties is negative. We can made a difference between Twip and Trip steel. Effect of HAZ for twip steel = grain growth Effect of HAZ for trip steel= microstructural modification (austenite- martensite-bainite ecc…) A way to understand if the microstructure is affected to changes is the quantity of carbon equivalent : In welding, equivalent carbon content (C.E) is used to understand how the different alloying elements affect hardness of the steel being welded.  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 2.5-PHYSICS OF WELDING Fusion is most common means of achieving coalescence in welding To accomplish fusion, a source of high density heat energy must be supplied to the faying surfaces, so the resulting temperatures cause localized melting of base metals (and filler metal, if used) For metallurgical reasons, it is desirable to melt the metal with minimum energy but high heat densities Main welding processes parameters: 1) Power density Power transferred to work per unit surface area, W/mm2.If power density is too low, heat is conducted into work, so melting never occurs. If power density too high ( the material is not melted but tends to vaporize), localized temperatures vaporize metal in affected region, strong thermal gradient, distortions. The minimum power density required to melt most metals in welding is about 10 W/mm2 Above around 105W/mm2 the localized temperatures vaporize the metal. PD in reality is not too easy to evaluate - power source (e.g., the arc) is moving in many welding processes;- power density is not uniform throughout the affected surface; it is distributed as a function of area.  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 Heat transfer in fusion welding The quantity of heat required to melt a given volume of metal depends on: - the heat to raise the temperature of the solid metal to its melting point - the melting point of the metal - the heat to transform the metal from solid to liquid phase at the melting point. To a reasonable approximation, this quantity of heat can be estimated by the following equation Not all of the energy generated at the heat source is used to melt the weld metal. There are two heat transfer mechanisms at work, both of which reduce the amount of generated heat that is used by the welding process. heat transfer factor f1 , (from 0 to 1) ratio of the actual heat received by the workpiece divided by the total heat generated at the source melting factor f2, (from 0 to 1) proportion of heat received at the work surface that can be used for melting. -heat energy available for welding, Hw -total energy of the welding process, H  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 The larger is the conductivity of the metal, the higher the dissipation. heat transfer factor f1 determined largely by the welding process and the capacity to convert the power source (e.g., electrical energy) into usable heat at the work surface. melting factor f2 depends on the welding process, but it is also influenced by the thermal properties of the metal, joint configuration, and work thickness. Note: Metals with high thermal conductivity (Al, Cu) have a rapid dissipation of heat away from the heat contact area. The problem is exacerbated by welding heat sources with low energy densities (e.g., oxyfuel welding) High power density combined with a low conductivity work material results in a high melting factor. -Balance equation between the energy input and the energy needed for welding: -Hw net heat energy (J) -Um unit energy required to melt metal,J/mm3 V volume of metal melted, mm3. Most welding operations are rate processes; that is, the net heat energy Hw is delivered at a given rate, and the weld bead is made at a certain travel velocity. It is therefore appropriate to express the rate balance equation: -RHw rate of heat energy delivered to the operation for welding, J/s -RWV volume rate of metal welded mm3/s In the welding of a continuous bead, the volume rate of metal welded is the product of weld area Aw (mm2) and travel velocity v (mm/s)  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 1) WELDING SPEED Welding speed (m/min) is the speed of the welding torch and, hence, is directly related to welding productivity If we move slow, we melt all the material If we move fast we melt only the surface 2) DEPOSITION RATE is the metal deposed in the weld per unit of time. Deposition rate commonly ranges from 1 kg/h (Oxyacetilene, precision TIG) to 100 kg/h (SAW with multiple electrodes). Depends on how much filler metal we are using. If we increase the number of sources (as the multi arc) we increase the deposition rate. From power density depends on - Penetration depth: a high power density maintain a local heating of the base material and limit the thermal dispersion from the weld, thus favoring the heating through the metal thickness. - Shape factor of weld (i.e. ratio between weld width and depth): a high power density allows of obtaining narrow and depth welds. - Distortions and residual stresses: high power density reduces the fusion and the heat affected zone, and favors uniform weld, thus reducing residual stresses and distorsions.  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 1- Fusion and solid-state Welding Processes Welding. Welding is a process by which the base metal as well as the filler metal is melted, and each forms a molten material or a weld pool. This weld pool solidifies to make a strong joint. Brazing and soldering. The filler metal is melted in-between the parts that must be joined. The wetting that is formed in between the joints gets solidified and gives the joint more strength. The metals that have to be joined together are not heated to their melting points, but only the filler metal is heated just above the melting point. Temperature is a lower than that of the temperature used in welding. 3.1-FUSION WELDING PROCESSES ARC WELDING A fusion welding process in which coalescence of the metals is achieved by the heat from an electric arc between an electrode and the work. The electric energy from the arc produces temperatures near 500 °C, hot enough to melt any metal. Most AW processes add filler metal to increase volume and strength of weld joint. A pool of molten metal is formed near electrode tip, and as electrode is moved along joint, molten weld pool solidifies in its wake  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 An electric arc is a discharge of electric current across a gap in a circuit?it is sustained by an ionized column of gas (plasma) through which the current flows. To initiate the arc in AW, electrode is brought into contact with work and then quickly separated from it by a short distance. The problems related to AW are: ultraviolet radiation that can ruin human vision (for this reason welder must wear a helmet with a dark viewing window) sparks Spatters Dangerous fumes The two basic types of AW ELECTRODES are: consumable : consumed during welding process; source of filler metal. They can be: -welding rods or stick (23-46 cm) but they must be changed ggggg frequently - weld wire that can be continuously fed from spools. Non-consumable : filler metal must be added separately. Made of tungsten which resists melting. Gradually depleted during Iiiiiuwelding (vaporization is principal mechanism). Any filler metal must be grvfsupplied by a separate wire fed into weld pool 3.2-SHIELDING At high temperatures, metals are chemically reactive to oxygen, nitrogen, and hydrogen in air. Mechanical properties of joint can be seriously degraded by these reactions. During welding, the arc and weld region must be shielded from surrounding air. Arc shielding is accomplished by: Flux (fondente) (lower density with respect to the metal. This flux produces some gas in order to protect the weld pool, these gases are produced by organic material, otherwise flux can realise some elements like manganese that can be stabilised with sulphurous. It’s a substance that prevents formation of oxides and other contaminants in welding, or dissolves them and facilitates removal. Its effects can be:  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 -Provide protective atmosphere for welding, which displace air -Provide deoxidizers and other scavengers that purify the weld -Produce slag, which provides a physical protection or “lid” over the weld pool -Stabilize arc -Reduce spattering Types of flux: SHIELDING GAS. Another way to protect the welding pool; we provide gases that create a cloud around the welding pool. Shielding gases are inert (e.g., Ar, He, Ar+He) or semi-inert gases (Ar + CO2 (here we have CO2 because it operates like a gas that removes C from the pool)) that are commonly used in several welding processes. Argon is the most used because less expensive and because it’s heavier than helium and can stay longer on the surface. Most notably in arc welding. 3.3-ARC LENGTH During homogeneous welding it’s important to maintain a constant gap. Due to this reason we need to provide a constant heat input to the surface, so the electrical generator know that we change the gap and it modify the electricity (only for small variation-gap)  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 If the arc length is too long (e.g. length > 7-8 mm): - electric arc difficult to be controlled - considerable heat loss - poor weld penetration - more likely the absorption of oxygen and nitrogen - poor mechanical and metallurgical properties If the arc length is too short (length < 3 mm) - excessive overheating of workpiece - electrode bonding and arc shutdown - poor mechanical and metallurgical properties  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 Fusion and solid-state welding processes SHIELDED METAL ARC WELDING (SMAW) (saldatura ad arco con elettrodo rivestito) This type of arc welding uses a consumable electrode consisting of a filler metal rod coated with chemicals that provide flux and shielding; sometimes is called stick welding. Basically we melt the metal of the electrode, then we release the flux which with his organic gases protects the melted pool. obs= the slag can be easily removed with a brush after the process. obs= cellusose generates gases and the oxides generates the covering layer. If we talk about electricity we must have to know that each welding process requires specific polarity depending on the welded metal and electrode selection. In electric welding we can both have AC or DC. DCEP= direct current electrode positive ( also defined as REVERSE POLARITY) in this case we have lower heat to the surface. DCEN= direct current electrode negative (also defined as STRAIGHT POLARITY) here we have larger heat input to the surface. DCEN is not always the preferable polarity for welding because in DCEP we remove easily the oxide layer.  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 AC is not used in SMAW because the change in polarity makes the electrical arc not stable so we use DC. DCEP is the most commonly used because we provide a better penetration due to the fact that the heat is concentrated on the electrode. DCEN can be used when we weld thin material or performing surfacing welds because we don’t need significative amount of heat from the electrode. summing up, Shielded arc welding is usually accomplished by means of an electric arc formed between the work and the coated metallic electrode. Let’s see what happens! When the arc is struck it almost instantly creates a temperature of about 650 degrees Fahrenheit, this melts both the base metal and the metal in the electrode. The metal inside the electrode is melted off in tiny droplets and mixed with the base metal. The force of the arc and the forward travel of the electrode causes the mixed molten metal to be pushed to the rear of the crater where it cools to form a bead. The movement of this metal toward the rear of the crater and the depth of the crater are an excellent check of the quality of the work. At the same time, as the metal is melting, the coating on the electrode is being consumed, this takes place slower than the melting of the electrode which shields the arc and helps direct the flow of metal; it also permits the use of higher currents with the resultant faster deposit of weld metal. A gas is formed covering the arc with a protective shield that prevents the exposure of the molten metal to the air and prevents the formation of harmful oxides and nitrides in the deposit also from the coating chemicals acting as cleansing agents enter into the metal to help to remove the impurities. These chemicals with the impurities float to the top and cooling form a coating or slag over the bead. This slag causes the molten metal to cool more slowly and has an annealing effect. In stick welding an electric current flows through a metal electrode or stick. An arc forms at the end of this electrode and the work piece. Arc melts both the metal in the rod and the metal and the pieces to be joined. The metal from the electrode is ejected into a molten weld pool and mixes or coalesces with the workpiece. Metal stick welding always adds filler metal to the joint. Because the electrode is constantly melting away and becoming part of the welded structure, stick welding is known as a consumable electrode process. Covering of the electrode real eases protective gases the shield the weld and help stabilise the arc. The remaining covering melts and covers the molten weld pool with a protective slag layer, this slag layer protects and helps shape the weld as it solidifies but it must be removed when the weld is cooled since the gases generated by the flux covering are able to completely protect the molten weld there is no need for other shielding equipment such as high pressure gas cylinders. Compared to other welding processes, stick welding equipment is often very simple, sometimes the only controls on the machine are current and polarity. Many important welding variables come from how the person doing the welding position and moves the electrode , for this reason the quality of welds produced by this process depends greatly on operator skill and stick welding to insulated wires are connected to the welding machine one lieb goes to a clamp which is connected to the work, this is called the work lead, the other wire goes to an electrode holder , the uncovered or end of the electrode is placed into the electronic holder. The polarity of the welding setup refers to how the electricity flows from the machine to the workpiece and back, the current will either be AC or DC. In AC the direction of the flow of the electricity changes direction many times every second. DC is like the flow of electricity from a car battery, one lead is always positive and the always is negative. Dc can be set up two ways in stick. One way to connect is DCEN which is ofte called straight polarity, the electrode is connected to the negative terminal and the work to the positive terminal. Another way is DCEP or reverse polarity. So in the ending, arc welding is a process that uses a consumable electrode rod that is covered with a flux material, when heated the flux releases shielding gases the protect the weld; a layer of slag covers the weld while it solidifies and is removed when the metal cools. Stick electrode are not all the same and must be chosen for the specific job they need to do. At the end, for SMAW we can say that this type of welding process is not used in automotive industry.  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 Gas metal arc welding (GMAW) (saldatura a filo metallico continuo in gas ) - This type of welding process is widely used for joining a variety of metal combinations: ferrous alloys as well as alloys of alluminium and copper, also here we have the presence of a filler metal and the main parameters influencing the quality are current and speed of arc travel.In this case we don’t have a covering flux but we need to provide an external gas. These gases can be: - Active gases (MAG) - Inert gases (MIG) → shielding gas is supplied through the weld gun to protect the welding pool. We use this process for joining dissimilar metals. One of the main advantages of this process is the high quality welds and minor weld splatter, but at the same time this process has a lot of limitations: is unsuitable for outside welding because the equipment setup is complex and is not portable; are required addition components and is limited only for horizontal welding technique and needs a surface preparation. The MIG welding process requires DCEP polarity, but we also have in industrial process in which we have different metal transfer configurations the usage of AC. ( One example of these different metal transfer configurations can be: spray, globular (generating a larger droplet) or short circuit (the melting is provided by a short circuit when the electrode touches the work piece)). We have several types of metal transfer : - Spray metal transfer mode - Globular metal transfer mode (globule is the liquid from the electrode like a large droplet) - Short circuiting metal transfer mode - Pulsed arc metal transfer mode ( we modify the amperage in order to control the droplet released of the work material. When we reach the peak of amperage we release the material. Increasing the amperage we make the arc stronger) - High deposition metal transfer mode - Buried arc metal transfer mode GMAW- cold metal transfer (CMT) This type of GMAW is the most used in automotive industries ( for framework and body panel). CMT is a pulsed arc metal transfer mode with a mechanical control of the electrical arc. This means that the arc length is independent from the workpiece surface and welding speed. Also in this case we are talking about a welding process with inert gas. The main features of this process are: - Lower heat input → good for sheet material - Precise control of arc length - Nearly without metal splashes - High arc stability  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 This process starts with the electrode retracted when the liquid drops on the surface and then we push the electrode on the surface but we don’t have a short circuit because the current is not passing. We say that this process has a very lower heat input and as a consequence a low heat affected zone, this due to the controlled movement of the electrode and the current value. This process is not for thicker parts but only for sheets; with respect to the standard GMAW here we have a process with lower amount of energy. Summing up, What is MIG welding? GMAW A thin wire acts as the electrode, this wire is fed from a spool mounted on a gun or inside the welding machine through a flexible tube and out of the nozzle on the welding gun or torch the wire is fed continuously when the trigger in the welding gun is pulled when this trigger is pulled it also switches on the welding current and a shielding gas, an electric arc forms between this wire electrode in the workpiece and heats both metals above their melting point , these metals mix together or coalesce and solidify to join the work pieces into a single piece, the metal in these parts to be joined is called the base metal and the metal that comes from the melting wire electrode is called filler metal, MIG welding adds filler metal to the joint because the wire electrode melts as it’s being used , MIG is called a consumable electrode process. Shielding gas is also fed through the welding lead it goes through a gas diffuser and flows out a nozzle, this shielding gas which is often a mix of argon and CO2 protects the molten metal from reacting with oxygen water Vapor and other things in the atmosphere. We have DCEP Flux-cored arc welding (FCAW) ( saldatura con filo animato) Is an adaptation of shielded metal arc welding, to overcome limitations of stick electrodes - Electrode is a continuous consumable tubing (in coils) containing flux and other ingredients (e.g., alloying elements) in its core - Presence (Gas-shielded FCAW) or absence (Self-shielded FCAW) of externally supplied shielding gas distinguishes the two types: Self shielded FCAW → core includes compounds that produce shielding gases. So we have self generated shielding gas by organic material. DCEN polarity in order to have a more stable arc. Gas shielded FCAW→ uses externally applied shielding gases. Is more expensive but we have better quality. Here the shielding gas cover better the weld pool. DCEP polarity to ensure better melting of the consumable electrode FCAW in general has a lot of advantages: - Excellent weld penetration - Suitable for thicker joints - Flexibility in terms of torch movement and orientation - The highest metal deposition rate But at the same time we have an high level of noxious fumes, higher cost ecc.  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 Submerged arc welding (SAW) (saldatura ad arco sommerso) Uses a continuous, consumable bare wire electrode, with arc shielding provided by a cover of granular flux. Electrode wire is fed automatically from a coil. Flux introduced into joint slightly ahead of arc by gravity from a hopper. Completely submerges operation, preventing sparks, spatter, and radiation. Properties: - High penetration - High welding speed - Sound joint, smooth and uniform - Metal thickness > 1.5 mm - Mainly for planar and curvilinear welds this process is used for most steels (except hi C steel) but it’s not good for nonferrous metals (like Al) due to the not complete protection from oxidation and because increasing the T we have volumetric changes that generates internal stresses (volume increasing due to the transformation into martensite) Gas tungsten arc welding (TIG for European conv or GTAW for American conv) This process uses a nonconsumable tungsten electrode ( Tm has a very high melting point) and an inert gas for arc shielding. We can have the presence of filler metal or not (when we have the presence of filler metal it is added to weld pool from separate rod or wire). It’s used for thin materials. Obs= the shielding gas enter coaxially to the torch. Depending on the welded material, the TIG welding process uses DC or AC output. DC: Mild steel, stainless steel, and carbon steel. DCEN is the preferred polarity because the current and heat are focused on the welded metal (DCEP focuses too much heat on the tungsten electrode; DCEP breaks aluminum oxide residue efficiently, but poor penetration and burning of tungsten electrode) AC : alluminium and magnesium alloys ( 50 Hz or 20-500 Hz). This process allows us to have high quality welds because a welder has more control over welding than the other arc welding technologies; we have little or no post-weld cleaning because we have no flux. At the  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 same time this process requires high operator skills, and compared to consumable electrode AW processes here we have low productivity and higher costs. Summing up, Power source in ARC WELDING In all arc-welding processes, power to drive the operation is the product of the current I passing through the arc and the voltage E across it. This power is converted into heat, but not all of the heat is transferred to the surface of the work. Convection, conduction, radiation, and spatter account for losses that reduce the amount of usable heat. The effect of the losses is expressed by the heat transfer factor f1.Heat transfer factors are greater for AW processes that use consumable electrodes because most of the heat consumed in melting the electrode is subsequently transferred to the work as molten metal. Melting factor f2 further reduces the available heat for welding. The resulting power balance in arc welding is defined by: E is the voltage (V), I the current (A) Plasma arc welding (PAW) (saldatura al plasma) Special form of GTAW in which a constricted plasma arc is directed at weld area Tungsten electrode is contained in a nozzle that focuses a high velocity stream of inert gas (argon) into arc region to form a high velocity, intensely hot plasma arc stream Temperatures in PAW reach 28,000 C, due to constriction of arc, producing a plasma jet of small diameter and very high energy density  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 This process can be used to weld almost any metals but is very expensive and has a larger torch than other AW so tends to restrict access in some joints. Resistance welding (RW) (saldatura a resistenza) A group of fusion welding processes that use a combination of heat and pressure to accomplish coalescence Heat generated by electrical resistance to current flow at junction to be welded Principal RW process is resistance spot welding (RSW) Resistance spot welding (RSW) (saldatura per resistenza a punti) Resistance welding process in which fusion of faying surfaces of a lap joint is achieved at one location by opposing electrodes - Used to join sheet metal parts using a series of spot welds - Widely used in mass production of automobiles, appliances, metal furniture, and other products made of sheet metal: Typical car body has ~ 5-10,000 spot welds (80-90 Million annual production of automobiles in the world is measured in tens of millions of units Components in Resistance Spot Welding -Parts to be welded (usually sheet metal) -Two opposing electrodes -Means of applying pressure to squeeze parts between electrodes -Power supply from which a controlled current can be applied for a specified time duration  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 Spot welding cycle The heat required to bring the material to melting is generated by Joule effect due to the passage of an electric current flowing between two aligned and opposite electrodes in contact with the base material. The amount of heat transferred during the process can be expressed as η, efficiency 0.2-0.3 R, circuit resistance (i.e. resistance of electrodes and sheet stack) t, welding time I, welding current R, t, and I must be kept under control during the process, so as 1) to obtain the melting only at th interface between sheets; 2) avoid improper metal expulsion! -electrode current: 3-15 kA for steel 25-40kA for aluminum alloys (electrode voltage: 1-3 V) -electrode force: 1-7 kN -welding time: 100-1000 ms Medium frequency direct current (MFDC) This type is used a lot in automotive. we have the presence of a three-phase 50 Hz AC supplied to an inverter. This inverter converts the 50 Hz current to a 1000-4000 Hz frequency and is fed to a transformer, integrated into the welding gun. Then the current available is always direct current DC.  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 Welding lobe (iso standard) The weldability lobe is constructed from a series of a weld growth curves at constant force and shal be produced at a predetermined value of electrode force, and the specified limits are determined by varying weld current and weld time. Constant force Constant weld time At the minimum weld time (e.g. 5 cycles when welding uncoated/coated steels), the welding current is increased progressively in order to determine the stuck weld condition or √3.5t limit, a nominal weld diameter (e.g. weld diameter equal to √5t) and the splash limit. Electrodes in RSW We have several shapes and are made of copper alloys because alloy elements provides mechanical strength and wear resistance. Spot welding vs TIG welding Resistance spot welding is a cost effective way to join two or more overlapping pieces of metal together. In addition, spot welding is a huge time savor compared to TIG or MIG welding. Many engine come to VIP for advice to save time and money on sheet metal assembliesthat require welding. One great method that we regularly recommend (depending on the assembly and function of the parts) spot welding as an alternative. Below shows a comparison between spot welding and welding (TIG or MIG). Resistance seam welding (RSEW) (saldatura continua/ a rulli) This process provides high welding speeds, but its applicabilioty is limited by the component shape and wheel access. The joint here is produced progressively along the length of the weld; the weld may be made with overlapping or continuos work pieces. In automotive industries this process  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 is used to produce fuel tanks. In this process the wheels acts like an electrode to produce a series of overlapping spot welds along lap joint Resistance projection welding ( RPW) (saldatura a proiezione per resistenza) We are talking of a resistance welding process in which coalescence occurs at one or more small contact points on parts; this contact point is determined by design of parts to be joined. We don’t have the presence of filler metal and we don’t need high operator skill but is limited to lap joints for RW processes and has high costs. Resistance projection welding (RPW): (1) start of operation, contact between parts is at projections; (2) when current is applied, weld nuggets similar to spot welding are formed at the projections. (a) and (d) Projection welding of nuts or threaded bosses and studs. (e) Resistance- projection-welded grills. Power source in resistance welding The heat energy supplied to the welding operation depends on current flow, resistance of the circuit, and length of time the current is applied. This can be expressed by the equation H heat generated, J (to convert to Btu divide by 1055); I current, A; R electrical resistance, Ω; t time, s. The current used in resistance welding operations is very high (typically 5000 to 20,000 A), although voltage is relatively low (usually below 10 V).  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 Oxyfuel gas welding (OFW) (saldatura ossi-combustibile) Group of fusion welding operations that burn various fuels mixed with oxygen ▪ OFW employs several types of gases (es. acetilene, propylene, propane, natural gas, hydrogen, MAPP…) which is the primary distinction among the members of this group ▪ Oxyfuel gas is also used in flame cutting torches to cut and separate metal plates and other parts ▪ Most important OFW process is oxyacetylene welding because it is capable of higher temperatures than any other (up to 3500C) and most concentrated flames. Oxyacetylene welding (OAW) Fusion welding performed by a high temperature flame from combustion of acetylene and oxygen ▪ Flame is directed by a welding torch ▪ Filler metal is sometimes added ▪ Composition must be similar to base metal ▪ Filler rod often coated with flux to clean surfaces and prevent oxidation Maximum temperature reached at tip of inner cone, while outer envelope spreads out and shields work surfaces from atmosphere fiocco zona di saldatura dardo  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 ▪ First stage reaction (inner cone of flame), primary exothermic reaction: C2H2 + O2 → 2CO + H2 + heat (444 kJ) ▪ Second stage reaction (outer envelope), the combustion products combine with the oxygen from atmosphere in the welding (reducing) region, secondary exothermic reactions: 2CO + O2 → 2CO2 + heat (573 kJ) H2 + 0.5O2 → H2O + heat (234 kJ) ▪ Neutral: stechiometrique amount of C2H2 + O2 (low carbon steel, mild steel), no acetylene feather, no addition or subtraction of any element from the weld pool white cone no feather bluish to orange ▪ Oxidizing: excess of O2 (1.5:1) (brass, bronze, copper alloys). Higher temperature, blue flame, some O2 enter the weld pool. In Cu alloys, oxygen ensures that zinc fumes are not released. white cone Welding Handbook Vol. 1, American Welding Society (AWS) nearly colorless ▪ Reducing or Carburizing: excess of C2H2 (0.9:1) (alloy steels, aluminum alloys). Inner core longer, prevention of oxidation and some carbon from C2H2 may enter the weld pool (i.e. carbides in the welded joint) white cone  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 ▪ Together, acetylene and oxygen are highly flammable ▪ C2H2 is colorless and odorless ▪ It is therefore processed to have characteristic garlic odor ▪ C2H2 is physically unstable at pressures much above 15 lb/in2 (about 1 atm) ▪ Storage cylinders are packed with porous filler material (such as asbestos) saturated with acetone (CH3COCH3) ▪ Acetone dissolves about 25 times its own volume of acetylene ▪ Different screw threads are standard on the C2H2 and O2 cylinders and hoses to avoid accidental connection of wrong gases Power source in oxyacetylene welding The flame in OAW is produced by the chemical reaction of acetylene and oxygen in two stages. Total heat liberated during the two stages of combustion is 55 x 106 J/m3 (1470 Btu/ft3) of acetylene. However, because of the temperature distribution in the flame, the way in which the flame spreads over the work surface, and losses to the air, power densities and heat transfer factors in oxyacetylene welding are relatively low: f1 = 0.10 to 0.30. Laser beam welding (LBW) (saldatura laser) The word laser stands for:  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 Light Amplification byStimulated Emission of Radiation Before seeing the welding process we need to talk about the principles behind this type of welding operation. Laser beam absorption The first basic rule for selecting a laser source for material processing is to know the wavelength of the laser beam, because different materials have different absorption rates at different wavelengths. Nd:YAG (Neodymium-doped Yttrium Aluminum Garnet) laser is absorbed well by Aluminum and Steel, and the 10600nm wavelength laser beam of a CO2 laser is absorbed well by Fe-based alloys and organic materials like paper, wood, leather, plastic, and cloth The beam parameter product is called BPP and is often used to specify the beam quality of a laser beam: higher BPP →lower beam quality. Physics background: - When one atom in E2 decays spontaneously, arandom photon results which will induce stimulated photon from the neighboring atoms - The photons from the neighboring atoms - In actual case, excite atoms from E1 to E3. will stimulate their neighbors and form an - Exciting atoms from E1 to E3 ► optical pumping -Atoms from E3 decays rapidly to E2 emitting hν3 avalancheof photons. - If E2 is a long lived state, atoms from E2 will not decay to E1 rapidly - Condition where there are a lot of atoms in E2 ► populationinversion achieved! i.e. between E2 and E1. - Large collection of coherent photonsresulted. Let’s focus now on the welding process that uses this principle: we have the presence of shielding gases to prevent oxidation and the fusion is achieved by energy of a highly concentrated light beam focused on joint. We don’t need filler metal and we use this process for small parts. The nice thing of this process is the high weld precision and velocity but at the same time we must need an accurate beam joint alignment. It’s very expensive!  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 There are two types of heating “modes” used to describe the resulting melting ofthe metal during laser welding: - “conduction mode” heating - “keyhole mode” heating These modes of heating are created by different power densities and produce different results. Conduction mode: the power density is great enough to cause the metal to melt. Weld penetration is achieved by the heat of the laser conducting down into the metal from the surface. Welds are typically wider than they are deep. Keyhole mode: the power density is great enough that the metal goes beyond just melting. It vaporizes. The vaporizing metal creates expanding gas that pushes outward. Thiscreates a keyhole or tunnel from the surface down to the depths of the weld Solid state welding (SSW) For these types of welding processes the coalescence of part surfaces is achieved by pressure, heat or a combination of both (but if we use both that mean that the heat input is not sufficient to melts the parts); we don’t need filler metal. The most important processes are: - friction welding - friction stir welding - explosion welding explosion welding (EXW) is a SSW process in which rapid coalescence of two metallic surfaces is caused by the energy of a detonated explosive; no filler metal; no external heat; no diffusion occurs because time is too short. Friction welding (FRW) Friction welding is a form of solid-state welding where the heat is obtained from the mechanically induced sliding motion between the parts to be welded. When certain temperature is reached, the rotational motion is seized and the pressure applied welds the parts together. This welding process can be controlled by regulating the time, rotational speed and pressure.  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 Is used for shafts and tubular parts; at least one of the parts must be rotational and flash must be removed. Friction stir welding (FSW) Friction stir welding (FSW) is a solid-state joining process that uses a non- consumable tool to join two facing workpieces without melting the workpiece material. Heat is generated by friction between the rotating tool and the workpiece material, which leads to a softened region near the FSW tool. While the tool is traversed along the joint line, it mechanically intermixes the two pieces of metal, and forges the hot and softened metal by the mechanical pressure, which is applied by the tool, much like joining clay, or dough. FSW invented by TWI (The Welding Institute, UK) in 1991. for 1-7xxx we use tool steel (100-500 euro) for 7xxx, steels, Ti alloys, Cn alloys we use W-Co alloys or W- Re alloys (very resistant alloys) (>1000 euro)  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 FSW can operate in two conditions: - Tool position control The vertical position of the tool is kept constant during welding. As a result, the vertical force changes during welding. - Vertical force control The vertical force that the tool is applying on the metalis kept constant during welding. As a result, the vertical position of the tool changes during welding. It generally ensures a better joint quality. We can have two different machines for this process: - Gantry machine (higher vertical force, for strong metals) - Anthropomorphic robot (less expensive and only for soft materials) Standard FSW leaves a hole when the tool exits out of the joint end, and tools can have or not retractable pin (if we don’t have a retractable pin we are talking of monolithic solution). Friction stir spot welding (FSSW) In friction stir spot welding, individual spot welds are created by pressing a rotating tool with high force onto the top surface of two sheets that overlap each other in the lap joint. We have two different solutions to avoid the presence of the keyhole: - Refill FSSW The refill friction stir spot welding is a more recent process in which we don’t have the presence of the keyhole and is performed through a proper tool made of different moving parts. The absence of the keyhole improves the mechanical strength of the joint but it’s a very expensive process. In the first step (clamping and tool rotation) the sleeve is rotating into the material and the clamping ring is pressed against the sheets. Then in the third step, when we retract the middle parts, we push the central part. - Pinless FSSW It’s a cheaper process with respect to refill one. We have deeper weld and so is not suitable when welding thick sheets (>2mm). It’s a simple operation since it uses a flat shoulder.  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 Brazing and soldering Only the filler metal is melted and then by capillarity, it passes through the material acting like a glue. Both, brazing and soldering processes use filler metal, but there is no melting of base metal. the main question is: when to use brazing or soldering instead of fusion welding? Well factors are different : - Metal with poor weldability - Joining of dissimilar metals ( for example, in some case for steel and Al solid fusion welding is not possible due to the creation of britlle material) - Intense heat of welding may damage components Brazing Brazing is a joining process in which a filler metal is melted and distributed by capillary action between surfaces of the parts to be joined; no melting of base metal occurs; only filler metal is melted because its Tm is lower than Tm of base metal(s). with brazing process we can join any metals, also dissimilar ones (this is not possible with welding operations); multiple joints can be brazed at the same time permitting high production rates. In this process the amount of heat required is lower than welding and so we have less problems related to the HAZ. But at the same time we have some disadvantages such as the joint strength which is lower than a welding joint; also from an aesthetic point of view color of brazing metal may not match color of base metal parts example of furnace brazing : the filler metal is a shaped wire and moves into the surfaces by capillary action with the application of heat. Some features of a desirable brazing metal: - Melting temperature of filler metal compatible with base metal - Low surface tension il liquid phase - High fluidity for penetration into interface - Avoid chemical and physical interactions with base metal  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 Soldering Soldering is a joining process in which a filler metal with Tm less or equal to 450°C is melted and distributed by capillary action between surfaces of metal parts. There is no melting of base metals, but filler metal wets and combines with base metal to form metallurgical bound. Is similar to brazing but here the filler metal is called solder, and this process is most closely associated with electrical and electronics assembly. This is a process that requires lower energy than brazing or fusion welding, it’s easy to repair and rework and also soldered joints have good electrical and thermal conductivity. It’s not a process that we use for high joint strength. The filler metal is called solders, usually we use alloys of tin (Sn) and lead (Pb): - Lead is poisonous and its percentage is minimized - Tin is chemically active and promotes wetting action for successful joining - Copper and tin form intermetallic compounds that strengthen bond Another aspect of soldering is the presence of fluxes. Functions of soldering fluxes: - be molten at soldering temperature - remove oxide films and tarnish from base part surfaces - prevent oxidation during heating - promote wetting surfaces - be readily displaced by molten solder during process - leave residue that is non-corrosive and non-conductive Weld quality, testing and inspection In order to obtain an acceptable weld joint that is strong and absent of defects we have to analyse residual stresses and distortion.  ASSEMBLY TECHNOLOGIES Automotive Engineering- A.Y. 2022-2023 When we melt parts they can be externally undeformed nut at the same time we could have internal stresses, that must be detected especially during the service because these regions are more subjected to corrosion. The internal residual stress can be located near the HAZ and also near the unaffected zone due to the volumetric expansion due to the grains transformation; in zones where the stress is larger we can have fracture and we have to predict this behaviour because we have some critical changes in mechanical properties. On the other side, distortion depends on initial geometry and on the location in which the welding operation is done; the parts due to thermal gradient are subjected to distortion. Summing up, we talk about residual stresses and distortion when we have rapid heating and cooling in localized regions during FW. We have different types of distortion caused by differential thermal expansion and contraction of different regions of the welding assembly we can avoid angular distortion (a) reducing heat input or using laser welding or cold metal operation. Or in case of corner weld (c) we must be careful to the fact that when we melt just one side at time we are deforming the piece. The defects during welding operation are different: - cracks - cavities - solid inclusion - imperfect shape or unacceptable contour - incomplete fusion - miscellaneous defects let’s study a butt joint: - Lack of fusion cap → a sort of step which is more sensitive to nucleation of cracks - Zone 1→ related to the head of the weld part: we can have two defectes, which are lack of fusion or undercut -Copper/ slag inclusion→ oxidation of material or external inclusion - Solidification crack→ appear in last region that solidifies because we have a reduction of volume of the liquid part ( V solid A.Y. 2019/2020 02 – FUDAMENTALS OF MECHANICAL PROPERTIES OF MATERIALS 2.1 – INTRODUCTION Manufacturing is a process in which the material is transformed. The behaviour of the material changes depending on the forces applied, the temperature and other parameters of the process, so the study of materials’ properties is of paramount importance for the success of the operations. Mechanical properties determine the material’s behaviour when subjected to mechanical stresses. The dilemma is that mechanical properties which are desirable from the point of view of the mechanical designer usually make manufacturing more difficult, so the manufacturing engineer and the design engineer should be aware one of the other’s point of view. Three types of mechanical stresses are involved in manufacturing processes: a) Tensile stress, which tends to stretch the material. b) Compressive stress, which tends to squeeze the material. c) Shear stress, which tend to cause adjacent portions of material to slide against each other. Tensile and compressive stress are the most relevant. 2.2 - TENSILE TEST The tensile test is the most common test for the study of the stress-strain relationship. It is performed by applying a force that pulls the material, elongating the specimen and reducing its cross-section area up to fracture. The machine is basically formed by a base in which there is an actuator that is responsible for the movement of the moving crosshead along the two columns. The upper crosshead is usually the fixed one. The test is performed keeping constant the speed of the moving crosshead, with the velocity that is usually very low (1-10 mm/min). The specimens have standardized shape, usually with circular (round or bulk sample) or rectangular (sheet sample) cross section. The force is applied at the extremities of the specimen, which are bigger than the central part. In the central part the cross- section area is smaller than at the extremities in order to make sure that the fracture will happen there, where is present the so called calibrated length (identified by the two Gauge marks). The stress-strain curve of metals follows a typical path. At the beginning of the test we have a linear relation between the applied stress and the elongation, up to when we get to the yielding point, that represents the border between the elastic region (linear) and the plastic region (non-linear). The elongation and the reduction of the cross-section area are homogeneous through the specimen up to when we reach the maximum load, then the necking phenomena begins (localized reduction of the cross-section area). The plastic deformation starts only when we have overcome the yielding point, but must be noticed that in the plastic region the elastic deformation keeps growing, so in the plastic region we have both plastic and elastic deformation at the same time. YIELDING POINT point at which corresponds a permanent deformation equal to 0,2 % Due to the localized reduction of the area caused by the necking, the force required to deform the specimen decreases progressively up to when fracture occurs. After that fracture occurred, we can measure the final length by putting back together the two halves. Must be said that after fracture the two halves of the specimen immediately recover the elastic deformation. A very important thing to be noticed is that even though the length increases and the cross-section area decreases, the volume remains constant. The applied force can be accurately measured by a load cell, while we can have a very precise measure of the elongation by means of an extensometer. The engineering stress-strain curve is obtained interpolating the values of engineering stress and engineering strain, calculated from the experimental data supposing the cross-section area to be always equal to the original one: 𝐹 𝑬𝒏𝒈𝒊𝒏𝒆𝒆𝒓𝒊𝒏𝒈 𝒔𝒕𝒓𝒆𝒔𝒔 𝜎𝑒 ≝ (2.2.1) 𝐴0 𝑙 − 𝑙0 𝑙 𝑬𝒏𝒈𝒊𝒏𝒆𝒆𝒓𝒊𝒏𝒈 𝒔𝒕𝒓𝒂𝒊𝒏 𝑒≝ = − 1 (2.2.2) 𝑙0 𝑙0 pag. 3 Elia Grano A.Y. 2019/2020 We can obtain the true stress-strain curve by interpolating the values of true stress and true strain, which are computed starting from the experimental data. The main difference between the two is that the true values are calculated with respect to the instantaneous area and length, while the engineering ones are computed with respect to the initial area and length. 𝐹 𝑻𝒓𝒖𝒆 𝒔𝒕𝒓𝒆𝒔𝒔 𝜎≝ (2.2.3) 𝐴 𝑙𝑓 𝑙 𝑑𝑙 𝑑𝑙 𝑙 𝑻𝒓𝒖𝒆 𝒔𝒕𝒓𝒂𝒊𝒏 𝑑𝜀 ≝ ⇒ 𝜀 = ∫ 𝑑𝜀 = ∫ = ln ( ) (2.2.4) 𝑙 𝑙0 𝑙0 𝑙 𝑙 0 Remembering that during the test the volume remains constant, we can easily derive the expression for true stress and true strain with respect to the cross-section area: 𝑙 𝐴0 𝑉 = 𝑐𝑜𝑛𝑠𝑡 ⇒ 𝐴0 𝑙0 = 𝐴 𝑙 ⇒ = 𝑙0 𝐴 We can use this substitute the ratio between the lengths with the one between the areas. We can do so also in order the expression of the true stress and of the true strain on the basis of the engineering stress and strain: 𝐹 𝐹 𝐴0 𝐹 𝑙 𝜎= = = = 𝜎𝑒 (1 + 𝑒) ⇒ 𝜎 = 𝜎𝑒 (𝑒 + 1) (2.2.5) 𝐴 𝐴0 𝐴 𝐴0 𝑙0 𝑙 𝑙 − 𝑙0 ε = ln ( ) = ln ( + 1) = ln(1 + 𝑒) ⇒ 𝜀 = ln(𝑒 + 1) (2.2.6) 𝑙0 𝑙0 The equality of the ratios is valid only up to the necking point, so we can’t use the expression written above for the computation of the true strain starting from the engineering one. These equation are very important because in the practice we measure the force, the initial area and the engineering strain, so the process of elaboration of the experimental data starts from the computation of engineering stress and strain and then goes on with the evaluation of true stress and strain on the basis of the engineering ones up to the necking point. After the necking point we notice a reduction in the force applied and a in the cross section area. It can be demonstrated that this leads to a progressive increment of the true stress up to fracture. In the neck we have a 3D pressure field, so a correction is applied (Bridgman correction) in order to obtain the equivalent value for in the case of tensile pressure field. The two curves are very similar one to the other for small elongations, while for large deformations the use of the engineering curve leads to unrealistic results, with a strength higher than the real one. Due to this The engineering stress-strain curve is used in the field of mechanical design, where pieces does not significantly change their shape during their operative life. The true stress-strain curve is used in the field of field of manufacturing design, where we have to deal with very high deformations. Engineering strain 𝑒 True strain 𝜀 1. It’s easier to calculate. 1. It’s the exact value, not an approximation. 2. It’s preferred in engineering analysis of materials that 2. It’s used to characterize materials that deform by experience only elastic strain. large amounts. 3. It’s geometrically symmetric. 3. Sequential strains can be added. 4. It’s geometrically symmetric. There are many characteristics of the material that can be obtained from the analysis of stress-strain curves. The most important are the ultimate tensile strength (TS or UTS) and the ductility. TS is the highest value of stress in the engineering curve and is the point at which starts necking. It is defined by means of the initial area 𝐴0 as follows: 𝐹𝑚𝑎𝑥 𝑼𝒍𝒕𝒊𝒎𝒂𝒕𝒆 𝒕𝒆𝒏𝒔𝒊𝒍𝒆 𝒔𝒕𝒓𝒆𝒏𝒈𝒉𝒕 𝑈𝑇𝑆 ≝ (2.2.7) 𝐴0 pag. 4 Elia Grano A.Y. 2019/2020 The ductility can be measured as elongation percentage at fracture or as cross-section area reduction at fracture: 𝑙𝑓 − 𝑙0 𝐴0 − 𝐴𝑓 𝑫𝒖𝒄𝒕𝒊𝒍𝒊𝒕𝒚 𝐷 ≝ 100 = 100 𝑒𝑓 ; 𝐷 ≝ 100 (2.2.8) 𝑙0 𝐴0 We classify as brittle materials the ones that have very low ductility, whose plastic deformation at fracture is almost null. For this category is not possible to identify the yielding point due to the absence of plastic deformation. It is worth to mention the fact that the word malleability in some cases is used as synonymous of ductility, even though it actually expresses the ability of the material to be shaped into wires. The malleability is quantified by means of the slope of the plastic region of the stress-strain curve. The last relevant property that is represented is the toughness, which is a measure of the ability of the material to avoid cracks to widespread and is defined as the energy to be spent to cause the fracture. Let now spend some words for the analysis of the relation between strain and stress in the different regions of the curve. The elastic region, as already said before, is the region prior to the yielding point. Here the deformation is reversible and the relation between stress and strain is linear, as described by the Hooke’s law: 𝑯𝒐𝒐𝒌𝒆′ 𝒔 𝒍𝒂𝒘 𝜎𝑒 = 𝐸 𝑒 (2.2.9) 𝐸 is called elastic modulus or Young’s modulus and is a measure of the inherent stiffness of the material. The higher 𝐸, the stiffer the material. Worth to mention that for Fe-based alloys 𝐸 ≈ 210 𝐺𝑝 while for Aluminium alloys 𝐸 ≈ 70 𝐺𝑝, so steel alloys are more or less 3 times stiffer than aluminium alloys. The yield point Y (also called elastic limit) represents the border between the elastic and the plastic regions. It is identifiable by the change in slope at the upper end of the linear region, but very often it’s not easy to do identify it in this way, so as a convention we consider the yield point as the one at which we have a permanent 0.2% offset. The plastic region is the one that begins at the yielding point. Here the deformation is not only plastic but elastic-plastic, even though the elastic component is negligible with respect to the plastic one. The plastic deformation is irreversible. Due to mechanisms involving its microstructure, the material increases its resistance as the deformation increases. This phenomena is called strain- hardening and is responsible for the increment in the force required to further deform the material despite the progressive reduction of the cross-section area. In the plastic region the relation between stress and strain is no longer linear and so a different law is needed to describe the material’s behaviour: 𝑯𝒐𝒍𝒍𝒐𝒎𝒐𝒏′𝒔 𝒆𝒒𝒖𝒂𝒕𝒊𝒐𝒏 𝜎 = 𝑘 𝜀𝑛 (2.2.10) 𝑛 is the strain-hardening exponent and 𝑘 is the strength coefficient (is equal to the stress acting when 𝜀 = 1). Worths to mention that the Hollomon’s equation can be interpreted as a generalization of the Hooke’s low, in facts if 𝑛 = 1 we find 𝜎 = 𝑘 𝜀, which corresponds to the Hooke’s law. The evaluation of 𝑛 and 𝑘 is made on the basis of the experimental data, which once plotted on a log-log scale result to be aligned in a straight line: 𝑠𝑖𝑚𝑖𝑙𝑎𝑟 𝑡𝑜 𝜎 = 𝑘 𝜀 𝑛 ⇒ ln 𝜎 = ln(𝑘) + 𝑛 ln(𝜀) → 𝑦=𝑐+𝑛𝑥 For the evaluation of 𝑛 and 𝑘 a tensile test is therefore needed. Is important to underline that the Hollomon’s equation is an approximation that doesn’t fit 100% the true strain-stress curve, especially in the elastic region. That’s why we don’t consider it in the elastic region but only in the plastic one. FLOW STRESS 𝝈𝒇 instantaneous value of stress required to continue deforming the material. As a rule of thumb, we can say that the flow stress is a function of temperature and strain rate at high temperatures, and of strain rate only at low temperatures. pag. 5 Elia Grano A.Y. 2019/2020 𝑭𝒍𝒐𝒘 𝒔𝒕𝒓𝒆𝒔𝒔 𝜎𝑓 = 𝑘 𝜀 𝑛 𝜎𝑓 = 𝑓(𝜀̇; 𝜀; 𝑇) 𝑎𝑡 ℎ𝑖𝑔ℎ 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 ; 𝜎𝑓 = 𝑓(𝜀; 𝜀̇) 𝑎𝑡 𝑙𝑜𝑤 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 If the temperature is high, the capability of dislocations to move through the crystal lattice of the material is improved, resulting in an higher ductility. Moreover at high temperatures recrystallization occurs while deforming, so there is no strain-hardening because the number of dislocation does not grow up. This leads to the fact at high temperature metals behave approximately like a perfectly plastic material (𝑛 tends to zero, so the flow stress is nearly constant with respect to 𝜀). If the temperature increases: The stiffness 𝐸 decreases. Yielding strength 𝑌and tensile strength 𝑈𝑇𝑆 decrease. Ductility and toughness increase. Strain-hardening exponent 𝑛 decreases. As we said, the mechanical response of the material depends also on the strain rate, so we have to properly define it: 𝑙 − 𝑙0 𝑑𝑒 𝑑 ( 𝑙0 ) 1 𝑑𝑙 𝑣 𝑬𝒏𝒈𝒊𝒏𝒆𝒆𝒓𝒊𝒏𝒈 𝒔𝒕𝒓𝒂𝒊𝒏 𝒓𝒂𝒕𝒆 𝑒̇ = = = = (2.2.11) 𝑑𝑡 𝑑𝑡 𝑙0 𝑑𝑡 𝑙0 𝑑𝜀 1 𝑑𝑙 1 𝑑𝑙 𝑣 𝑻𝒓𝒖𝒆 𝒔𝒕𝒓𝒂𝒊𝒏 𝒓𝒂𝒕𝒆 𝜀̇ = = = = (2.2.12) 𝑑𝑡 𝑑𝑡 𝑑𝑡 𝑙 𝑑𝑡 𝑙 So the engineering strain rate is the velocity of deformation dived by the initial length, while the true strain rate is the velocity of deformation divided by the instantaneous length, coherently with the definitions of engineering strain and true strain. As the strain rate increases the resistance of the material to the deformations gets higher and higher, phenomena that is called strain rate sensitivity and is characterized by the strain sensitivity factor 𝑚. This behaviour is due to the fact that if the strain rate is high the dislocation do not have all the time they need to move, resulting in an higher resistance to deformation. At high temperature we have several process that improves the mobility of dislocations, so the effect of strain rate sensitivity is very evident, while at low temperatures the capability of dislocations to move is very low and so the strain rate sensitivity is much less relevant. The dependency of the flow stress from the strain rate and the temperature can be expressed as follows: 𝜎𝑓 = 𝐶 𝜀̇𝑚 𝑤ℎ𝑒𝑟𝑒 𝐶 = 𝐶(𝑇) ; 𝑚 = 𝑚(𝑇) (2.2.13) In practical operations is very difficult to evaluate the strain rate because this operation is complicated by the geometry of the workpart and by the variation in strain rate in different regions of the part. 2.3 – COMPRESSION TEST In several processes deformation occurs through compression along with very high deformation (𝜀 > 1). In these cases the mechanical behaviour of the material can’t be described through a tensile test, because in this type of test the material is subjected to a tensile state of stress and necking limits the maximum achievable deformations, therefore a compression test must be performed. The sample undergoes a controlled uni-axial compressive load. The sample is usually cylindrical, with the load that is applied at its ends. During the test the sample is shortened, consequently enlargingthe cross-section due to the constancy of volume. Just like the tensile test, we can plot the engineering curve and the true curve. Startig from the engineering sress and strain we have: 𝐹 𝑖𝑔𝑛𝑜𝑟𝑖𝑛𝑔 𝑡ℎ𝑒 𝑠𝑖𝑔𝑛 𝐹 𝑬𝒏𝒈𝒊𝒏𝒆𝒆𝒓𝒊𝒏𝒈 𝒔𝒕𝒓𝒆𝒔𝒔 𝜎𝑒 ≝ − → 𝜎𝑒 = − (2.3.1) 𝐴0 𝐴0 𝑙 − 𝑙0 𝑙 𝐴0 𝑖𝑔𝑛𝑜𝑟𝑖𝑛𝑔 𝑡ℎ𝑒 𝑠𝑖𝑔𝑛 𝑙0 − 𝑙 𝑙 𝐴0 𝑬𝒏𝒈𝒊𝒏𝒆𝒆𝒓𝒊𝒏𝒈 𝒔𝒕𝒓𝒂𝒊𝒏 𝑒≝ = −1= −1 → 𝑒= =1− =1− (2.3.2) 𝑙0 𝑙0 𝐴 𝑙0 𝑙0 𝐴 pag. 6 Elia Grano A.Y. 2019/2020 Since the height reduces, the values of stress and strain are negative, but in the common practice the sign is ignored in order to work with positive quantities. The definition of true stress is more complicated in the case of compression test because of the sample barrelling phenomena. It is induced by the frictional forces at the interfaces between the sample and the plates, which arise an opposition to the lateral enlargement of the material. The frictional force is minimum at the centre of the specimen and maximum at the ends, resulting in the sample barrelling. Let start defining true stress and true strain in the ideal condition of no friction, in order not to consider the barrelling effect: 𝐹 𝑖𝑔𝑛𝑜𝑟𝑖𝑛𝑔 𝑡ℎ𝑒 𝑠𝑖𝑔𝑛 𝐹 𝑻𝒓𝒖𝒆 𝒔𝒕𝒓𝒆𝒔𝒔 (𝒊𝒅𝒆𝒂𝒍 𝒄𝒐𝒏𝒅𝒊𝒕𝒊𝒐𝒏𝒔) 𝜎≝− → 𝜎= (2.3.3) 𝐴 𝐴 𝑑𝑙 𝑙 𝑖𝑔𝑛𝑜𝑟𝑖𝑛𝑔 𝑡ℎ𝑒 𝑠𝑖𝑔𝑛 𝑙0 𝑻𝒓𝒖𝒆 𝒔𝒕𝒓𝒂𝒊𝒏 (𝒊𝒅𝒆𝒂𝒍 𝒄𝒐𝒏𝒅𝒊𝒕𝒊𝒐𝒏𝒔) 𝑑𝜀 ≝ ⇒ 𝜀 = ln ( ) → 𝜀 = ln ( ) (2.3.4) 𝑙 𝑙0 𝑙 For ductile materials the stress-strain curve obtained with the compression test is nearly identical to the tensile one up to the necking point. The main difference between the two curves is that in the case of compression we have no necking due to the fact that the area increases. Considering the increment in the cross-section area and in the strength of the material because of strain-hardening, it is easy to understand why in the plastic region the curve becomes steeper as strain increases. The difficulty is now to deal with the sample barrelling, because the data that we collect during the test are affected by the presence of the friction, that can’t be neglected. An appropriate lubrication of the contact surfaces helps to notably reduce barrelling, but does not eliminate the phenomena. Let now imagine to have a body with infinite length and to use it for a compression test. Being the sample infinitely long, the portion of it which is subjected to relevant friction forces is infinitely smaller than the rest of the body and so the effect of the friction on the experimental data is negligible. This approach is adopted in the Watts-Ford, which allows us to extrapolate the behaviour of the ideal body from the results obtained with real tests. Watts-Ford test uses a set of different specimens with equal diameter and different lengths, remaining in the range 0,5 ≤ 𝑑0 ⁄ℎ0 ≤ 3 in order to avoid the problems related to buckling. A load 𝐹1 (the same for each specimen) is applied to the samples and the deformation (∆ℎ = ℎ0 − ℎ) is measured in each one of them. Then a second load 𝐹2 > 𝐹1 is applied and the deformations are measured and so on. The contact surfaces of the samples are lubricated each time a load is about to be applied, in order to reduce the effect of the friction. The experimental data are elaborated calculating the ratios: ∆ℎ𝑖𝑗 ℎ0𝑖 − ℎ𝑖𝑗