Powder Metallurgy PDF

PROD1334 - Powder Metallurgy Tuesday, November 5, 2024 10:39 AM Powder metallurgy is a metal forming & fabrication technique that competes with casting, forging & machining. It consists of three major stages: - Primary material is physically powdered (divided into many small individual par...

PROD1334 - Powder Metallurgy Tuesday, November 5, 2024 10:39 AM Powder metallurgy is a metal forming & fabrication technique that competes with casting, forging & machining. It consists of three major stages: - Primary material is physically powdered (divided into many small individual particles) - Powder is injected into a mold or passed through a die to produce a green structure (pressure welding) very near the true dimensions of the object ultimately to be manufactured - End part is formed by applying pressure, high temperature, long setting times (during which-self welding occurs) or any combination of these. Powder Metallurgy Applications - Solid products such as gears, cams, bushings, rotors can be made - Porous parts such Oilite bushings, filter elements & pneumatic exhaust fittings - Taps & Dies for cutting threads are made from powder metallurgy - Tungsten-carbide ball-point pen balls - Piston rings, brake pads - Surgical instruments & implants Powder Metallurgy Advantages/Disadvantages Typical Sequence for Powder Part Production 1. Production of powder (either elemental particles or pre-alloyed particles) 2. Blending of the powder, particularly elemental powders. This step may include adding binder particles for metal injection molding 3. Compaction (consolidation) to form a compact, or green compact. This has sufficient strength for handling, but needs further processing to improve properties 4. Sintering (controlled, sub-melting temp heating) to bind, densify & strengthen the compact 5. Finishing operations such as re-pressing, forging, sealing, plating, stream treating, heat treating, machining, joining. Powder Production - Atomization: Molten metal is sprayed into a jet of water or argon - Reduction: chemical removal of oxygen from metal oxides, leaving a spongy, porous metallic powder - Pulverizing: crushing of metal in a ball mill or other type of mill to make rough powders Pulverizing Techniques - Rolling Mills - Ball Mills - Hammer Mills Particle Shapes in Metal Powders Compaction of Metal Powders Powder can be compacted (or consolidated) into shape by techniques - Mechanical Pressing - Cold isostatic pressing (CIP) - Hot isostatic pressing (HIP) - Metal injection molding (MIM) - Other processes such as powder extrusion, powder rolling, pressureless compaction/sintering, & others Consolidation by Pressing - Similar to forging - Typically done at room temp - Longitudinal holes can be formed by compacting the powder around a core rod - High pressures are needed, typically 10 ksi for aluminum, 120 ksi for steel - Part strength & stiffness increases with density - Density is increased by using high pressure & also a variety of particle sizes Density & Compacting Pressure Density & Mechanical Properties Density Variations & Die Design Isostatic Pressing - Compaction by pressing provides a unidirectional application of compacting force - Isostatic pressing is used to provide compacting force in all directions by using a pressurized fluid. ○ More uniform compaction ○ Higher density ○ Improved mechanical properties ○ Reduced density variation within the part & between parts Cold Isostatic Pressing - CIP applied compacting pressure using a pressurized fluid, usually water at 60 ksi - CIP is followed by sintering - Powder part must be isolated from the pressurized fluid typically using a rubber mold with the powder inside Hot Isostatic Pressing - HIP combines compaction & sintering into one operation - High pressure fluid is usually inter gas (zero valence) at 15 ksi @ 1200C - Heating is provided by resistive heating coils inside the pressure vessel - Powder part must be isolated from the pressurized fluid, typically using an evacuated sheet metal  mold (can) around the powdered part - HIP is largely used in super-alloys for aerospace, military, medical & chemical applications - It is also used to improve properties (reducing porosity) of titanium casting P/M tool steel parts & cutting tools - Can also be used for cladding of dissimilar materials Sintering Involves heating the green compact at a controlled temperature in a controlled atmosphere - Sintering is done at elevated temperatures but below melting temp of the powder metal - Since sintering depends on solid-state diffusion (and other mechanisms) to produce bonding & densifying - Diffusion requires time, so the sintering temperature rise time, dwell time & cooling are tightly controlled - Shrinkage typically occurs during sintering as densification occurs (voids are filled with metal) Sintering Parameters Critical parameters for sintering include:  - Heating rate - Sintering temperature - Time at sintering temperature (dwell) - Cooling rate - Sintering atmosphere (has to be oxygen-free to prevent oxidation of the metal. May be chemically active or inert or a vacuum) Powder Metal Injection Molding PMIM is most suitable for small complex parts made of steel, stainless, copper, bronze & titanium - During blending, very fine metal powder is mixed with binders (polymers or wax) to provide green strength of the compact - Powder mixture is loaded into an injection molding machine, which is similar to die casting - After compacting the part undergoes debinding or debinderizing to remove most of the binder by evaporation or chemical removal - This is followed by sintering, which shrinks the part to the final desired size, & removes remaining binders Advantages of P/M injection molding include: - Applications include watch parts, electrical connector bodies, valve spools, surgical knives, auto parts - Mechanical properties are excellent, similar to wrought products (forged or stamped) - High production are possible - Close tolerances & complex shapes are achievable, including transverse holes & grooves Powder Metallurgy Process Selection Powder Metallurgy Definitions Atomization - involves a liquid-metal stream, produced by injecting molten metal through a small orifice, whereby the stream is broken up by jets of inert gas, air or water known as gas or water atomization respectively. Comminution - Also known as pulverization involves either crushing, milling in a ball mill or grinding brittle or less ductile metals into small particles. Green Compact - Powder that has been pressed to result in powder adhesion & compaction is known as green compact, because the part has low strength just as in green parts in slip casting. Green Strength - Hot Isostatic Pressing - Combines compaction & sintering into one single operation using a high pressurized fluid usually inert gas at 15ksi and at elevated temperatures of 1200C Cold Isostatic Pressing - Applies compacting pressure using a pressurized fluid usually water at a much higher pressure than HIP around 60ksi. CIP does not combine sintering and compaction into one operation, sintering is done after compaction at much lower temperatures. Sintering - heats the green compacts in a controlled-atmosphere furnace to a temperature below the melting point of the metal but sufficiently higher to allow bonding (atom fusion) of the individual particles to impart strength to the part. Powder Metallurgy Review Questions 17.1 Describe briefly the production steps involved in making powder-metallurgy parts. a. Production of powder (either elemental particles or pre-alloyed particles) b. Blending of the powder, particularly elemental powders. This step may include adding binder particles for metal injection molding c. Compaction (consolidation) to form a compact, or green compact. This has sufficient strength for handling, but needs further processing to improve properties d. Sintering (controlled, sub-melting temp heating) to bind, densify & strengthen the compact e. Finishing operations such as re-pressing, forging, sealing, plating, stream treating, heat treating, machining, joining 17.4 Describe the methods used in metal powder compaction. Cold Isostatic Pressing - metal powder is placed in a flexible rubber mold, pressurized hydrostatically in chamber using water and to a pressure of 60ksi. Hot Isostatic Pressing - the container is typically made of a high-melting point sheet metal, the pressurizing medium is high temp inert gas or a vitreous (glasslike) fluid to a pressure of 15ksi at elevated temperatures. Sintering is combined with the compaction operation due to the elevated temperature. Powder-Injection Molding - very fine metal powders are blended with 25 to 45% polymer or wax- based binders, the mixture undergoes similar processes to die casting where it is injected into the mold between 135 to 200C. Parts generally have runners & sprues like with injected molded parts. Powder-Injection Molding is also referred to as Metal-Injection Molding (MIM) 17.8 Describe the wet-bag & dry-bag techniques. Cold Isostatic Pressing is divided into two types: Wet-bag & dry-bag. In wet-bag, powder is filled in a forming mold and sealed airtight outside the high-pressure vessel before direct immersion into a pressure medium. Then, isostatic pressure is applied to the outer surfaces of the mold to compress the powder into a shape. In dry-bag process is a method to mold powder filled in a forming rubber mold, as shown in the figure below, by transmitting pressure through a pressing rubber mold in the high-pressure vessel. 17.11 What is roll densification? Why is it done? Roll densification is the operation of a small-diameter hard roll pressed against a PM part and it causes sufficiently high contact pressures to cause plastic deformation of its surface layers. Powder metallurgy gears and bearing races are generally treated by roll densification, since the surface layer is more fatigue resistant and better able to support higher contact stresses than untreated components. 17.13 What is the osprey process?  converts a stream of molten metal into a particulate spray by gas atomization, which then produces a preform shape by impingement directly on to a collector. 17.21 Explain why the mechanical & physical properties depend on their density. Mechanical properties rely on their density to maximize metal particle contact and cohesion which affects a metal's strength, ductility, elastic modulus, and toughness.