Powder Metallurgy PDF

1 POWDER METALLURGY MFGA 4358 / MANU 4347 ASSOC. PROF. DR. TASNIM FIRDAUS ARIFF  OBJECTIVE 2  To know the definition of powder metallurgy  To know the powder production methods  To acknowledge the steps involved in powder metallurgy  To be able to distingu...

1 POWDER METALLURGY MFGA 4358 / MANU 4347 ASSOC. PROF. DR. TASNIM FIRDAUS ARIFF  OBJECTIVE 2  To know the definition of powder metallurgy  To know the powder production methods  To acknowledge the steps involved in powder metallurgy  To be able to distinguish between different types of powder metallurgy processes  Introduction 3  P / M –compacting metal / non-metallic powders in suitable dies and sintering them  Sintering – heating without melting  E.g. gears, cams, cutting tools, piston rings  Pure metals, alloys, mixtures of metallic and non- metallic materials can be used on powder metallurgy  Video on powder metallurgy  Why Powder Metallurgy is 4 Important?  PM parts can be mass produced to net shape or near net shape, eliminating or reducing the need for subsequent machining  PM process wastes very little material - ~ 97% of starting powders are converted to product  PM parts can be made with a specified level of porosity, to produce porous metal parts  Examples: filters, oil-impregnated bearings and gears  More Reasons Why PM is Important 5  Certain metals that are difficult to fabricate by other methods can be shaped by powder metallurgy  Tungsten filaments for incandescent lamp bulbs are made by PM  Certain alloy combinations and cermets made by PM cannot be produced in other ways  PM compares favorably to most casting processes in dimensional control  PM production methods can be automated for economical production  Limitations and Disadvantages 6  High tooling and equipment costs  Metallic powders are expensive  Problems in storing and handling metal powders  Degradation over time, fire hazards with certain metals  Limitations on part geometry because metal powders do not readily flow laterally in the die during pressing  Variations in density throughout part may be a problem, especially for complex geometries  Introduction (cont) 7  Most common metals:  Most common alloys:  Iron  Brass  Copper  Bronze  Aluminum  Steel  Tin  Pre-alloyed powders  Nickel where the powder  Titanium particle is an alloy  Refractory metals  PM Work Materials 8  Largest tonnage of metals are alloys of iron, steel, and aluminum  Other powder metals include copper, nickel, and refractory metals such as molybdenum and tungsten  Metallic carbides such as tungsten carbide are often included within the scope of powder technology  Powder Metallurgy Products 9  Gears, bearings, sprockets, fasteners, electrical contacts, Go to fullsize image Go to fullsize image cutting tools, and various machinery parts  Advantage of PM: parts can be made to near net shape or net shape Go to fullsize image  When produced in large quantities, gears and bearings Go to fullsize image are ideal for PM because:  The geometry is defined in two dimensions  There is a need for porosity in the part to serve as a reservoir for lubricant  Introduction (cont) 10  Produces good dimensional accuracy  Sizes range from tiny ball point pens to parts weighing about 50kg  P/M process:  Powder production  Blending  Compaction  Sintering  Finishing operations  Introduction (cont) 11  Finishing operations  The process depends include: on:  Coining  Shape  Sizing  Size  Forging  Porosity  Machining  Chemical purity  Infiltration  Bulk characteristics  Resintering  Surface characteristics  Powder Metallurgy Parts 12 Figure 16.1 A collection of powder metallurgy parts (photo courtesy of Dorst America, Inc.).  Engineering Powders 13 A powder can be defined as a finely divided particulate solid  Engineering powders include metals and ceramics  Geometric features of engineering powders:  Particle size and distribution  Particle shape and internal structure  Surface area  Measuring Particle Size 14  Most common method uses screens of different mesh sizes  Mesh count - refers to the number of openings per linear inch of screen  A mesh count of 200 means there are 200 openings per linear inch  Since the mesh is square, the count is equal in both directions, and the total number of openings per square inch is 2002 = 40,000  Higher mesh count = smaller particle size  Screen Mesh 15 Figure 16.2 Screen mesh for sorting particle sizes.  Interparticle Friction and Powder Flow 16  Friction between particles affects ability of a powder to flow readily and pack tightly  A common test of interparticle friction is the angle of repose, which is the angle formed by a pile of powders as they are poured from a narrow funnel Figure 16.4 Interparticle friction as indicated by the angle of repose of a pile of powders poured from a narrow funnel. Larger angles indicate greater interparticle friction.  Powder particle 17  Size of the powder particle depend on:  Temperature of metal  Rate of flow  Nozzle size  Jet characteristics  Methods of powder production 18 1. Atomization  Producing liquid metal stream by injecting molten metal through a small orifice  The stream is broken down by jets of inert gas, air or water  3 types of atomization:  Melt atomization  Atomization with a rotating consumable electrode  Gas atomization  Gas Atomization Method 19 High velocity gas stream flows through expansion nozzle, siphoning molten metal from below and spraying it into container Figure 16.5 (a) gas atomization method  Methods of powder production (cont) 20 2. Reduction process  Reduction of metal oxides-FeO, Fe2O3, or Fe3O4, are reduced in the presence of a reducing atmosphere. In addition, the carbon within the particles is removed via the formation of CO and CO2.  H2 and O2 are used as reducing agents  Very fine metallic O2 are reduced to its metallic state  Spongy and porous  Uniformly sized spherical or angular shapes  Methods of powder production (cont) 21 3. Electrolytic deposition 4. Carbonyls  Either aqueous solution or  Metal carbonyls fused salts  Fe(CO)5 – Iron carbonyl  Powder among the purest  Ni (CO)4  Fe /Ni react with CO  Small, dense, uniform spherical particles of high purity  Methods of powder production (cont) 22 5. Communition 6. Mechanical alloying  Pulverization process  > 2 metals are mixed  Involves crushing and in a ball mill milling in a ball mill  Powders weld together and grinding brittle or less ductile metals into by diffusion, forming small particles alloy powders  Brittle materials – angular shapes  Ductile materials – flaky shapes  Iron Powders for PM 23 Figure 16.6 Iron powders produced by decomposition of iron pentacarbonyl (photo courtesy of GAF Chemical Corp); particle sizes range from about 0.25 - 3.0 microns (10 to 125 -in).  PM Materials – Pre-Alloyed Powders Each particle is an alloy comprised of the desired chemical composition  Common pre-alloyed powders:  Stainless steels  Certain copper alloys  High speed steel 24 Pre-alloyed NiTi powder Pre-alloyed Ti alloy powder Cobalite powder-Co-Cu-Fe  Blending 25  Powders are mixed for uniformity Go to fullsize image  Impart special physical properties and characteristics  Lubricants are mixed to improve the flow characteristics Go to fullsize image  Less friction : Go to fullsize image  Longer die life  Better flow characteristics  Blending (cont) 26  Lubricants are stearic acid / zinc stearate (0.25 - 5% weight)  Deterioration / contamination:  caused by excessive mixing which may alter the shape of the particles & work harden them  Compacting has more difficulty  Hazards:  larger surface area/volume  Metal powders are explosive  Particularly – Al, Mg, Ti & Zr  Some common geometries of blenders a) cylindrical b) rotating cube c) double cone d) twin shell 27 28 Figure 16.7 Conventional powder metallurgy production sequence: (1) blending, (2) compacting, and (3) sintering; (a) shows the condition of the particles while (b) shows the operation and/or workpart during the sequence.  Compaction 29  Blended powders are pressed into shapes in dies either hydraulically / mechanically  Purpose:  To obtain the required shape, density and part to part contact  To make the part strong enough to be processed Go to fullsize image  Green compact:  Pressed powder  Not fully processed  Must flow easily  Room temperature  Can be done at high temperature  Compaction (cont) 30  Produces powder with:  High density  High strength  High modulus of elasticity, E  Requires:  High pressure  High resistance to external force  Compaction to form bushing 31 Compaction (cont) 32  Compaction (cont) 33  The higher the velocity of compression – the more air is trapped in the die cavity  This prevents proper compaction  Press for Conventional Pressing in PM 34 Figure 16.11 A 450 kN (50-ton) hydraulic press for compaction of PM parts (photo courtesy of Dorst America, Inc.).  Isostatic Pressing 35  Cold Isostatic Pressing (CIP)  Metal in flexible rubber mold  Either neoprene, urethane, PVC  Pressurized hydrostatically in a chamber with water (usually)  400MPa – 1000MPa  Larger size – more complex shapes 36  Isostatic Pressing (cont) 37  Hot Isostatic Pressure (HIP)  Container is made of high melting point sheet metal  Pressurized by inert gas / vitreous gas  100MPa at 1100ºC  Produce compacts with 100% density  Good metallurgical binding among part & good mechanical properties  Expensive – superalloy components for aerospace  Pressure is uniform – no die wall friction  Uniform grain structure & density  Produces product: ◼ Uniform strength ◼ Toughness ◼ Good surface details  HIP & Hot Pressing 38 View Image  Other compacting & shaping process 39  Rolling  Powder rolling / Roll compaction  Powder fed into the roll gap into a two high rolling mill  Then compacted into a continuous strip at speeds up to 0.5 m/s  Room temperature and at high temperature  Other compacting & shaping process 40  Extrusion  Pressureless compaction  After sintering,  Die filled with metal preformed P/M parts powder by gravity may be reheated &  Powder sintered directly forged into closed die in the die for final shape  Low density compaction  Superalloy powders:  Porous parts like filters ◼ Hotextruded for are produced improved properties  Sintering 41  Compressed metal powder is heated in a controlled atmosphere furnace at temperature with low melting point  Prior to sintering, compact is brittle and its strength (green strength) is low  Nature & strength of the bond depend on:  Mechanisms of diffusion  Plastic flow  Evaporation of volatile materials  Recrystallization  Grain growth  Pore shrinkage  Sintering 42 Heat treatment to bond the metallic particles, thereby increasing strength and hardness  Usually carried out at between 70% and 90% of the metal's melting point (absolute scale)  Generally agreed among researchers that the primary driving force for sintering is reduction of surface energy  Part shrinkage occurs during sintering due to pore size reduction  Sintering (cont) 43  Important factors:  Temperature: 70 -90% of the melting point  Time : 10 min – Fe, Cu 8 hrs – Tungsten & tantalum  Atmosphere: Oxygen free – no oxidation  Sintering time increases – conductivity of the metal powder increase  Vacuum –stainless steel & alloys  Temperature increases – diffusion occurs 44  Sintering (cont) 45  High sintering temperature:  Reduces porosity  Increases strength  Improves solidness  Improves ductility  Improved thermal & electric conductivity  Sintering Sequence 46 Figure 16.12 Sintering on a microscopic scale: (1) particle bonding is initiated at contact points; (2) contact points grow into "necks"; (3) the pores between particles are reduced in size; and (4) grain boundaries develop between particles in place of the necked regions.  Sintering Cycle and Furnace 47 Figure 16.13 (a) Typical heat treatment cycle in sintering; and (b) schematic cross section of a continuous sintering furnace.  SINTERING METHODS 48  Vacuum furnace  SINTERING METHODS 49  Hot Isostatic Pressing (HIP)  SINTERING METHODS 50  Microwave furnace  Impregnation and Infiltration 51  Porosity is a unique and inherent characteristic of PM technology  It can be exploited to create special products by filling the available pore space with oils, polymers, or metals  Two categories: 1. Impregnation 2. Infiltration  Impregnation 52 The term used when oil or other fluid is permeated into the pores of a sintered PM part  Common products are oil-impregnated bearings, gears, and similar components  Alternative application is when parts are impregnated with polymer resins that seep into the pore spaces in liquid form and then solidify to create a pressure tight part  Infiltration 53 Operation in which the pores of the PM part are filled with a molten metal  The melting point of the filler metal must be below that of the PM part  Involves heating the filler metal in contact with the sintered component so capillary action draws the filler into the pores  Resulting structure is relatively nonporous, and the infiltrated part has a more uniform density, as well as improved toughness and strength  Spray casting (Osprey process) 54  Molten metal is sprayed over a tube or pipe which is placed over a rotating mandrel  Suitable for axisymmetric parts  Design considerations 55  Compact should be simple shaped & uniform  No sharp corners  No thin section  Uniform thickness  High length – diameter ratio  Holes or recesses should be parallel to the axis of punch – chamfers should be provided  Widest tolerances, longer tool & die life  Saves production cost  Design Guidelines for PM Parts - I 56  Economics usually require large quantities to justify cost of equipment and special tooling  Minimum quantities of 10,000 units are suggested  PM is unique in its capability to fabricate parts with a controlled level of porosity  Porosities up to 50% are possible  PM can be used to make parts out of unusual metals and alloys, cermets for cutting tools- materials that are difficult if not impossible to produce by other means  Design Guidelines for PM Parts - II 57  Part geometry must permit ejection from die  Part must have vertical or near-vertical sides, although steps are allowed  Design features like holes and undercuts on part sides must be avoided  Vertical undercuts and holes are permissible because they do not interfere with ejection  Vertical holes can have cross-sectional shapes other than round without significant difficulty  Side Holes and Undercuts 58 Figure 16.17 Part features to be avoided in PM: side holes and (b) side undercuts since part ejection is impossible.  Design Guidelines for PM Parts - III 59  Screw threads cannot be fabricated by PM  They must be machined into the part  Chamfers and corner radii are possible in PM  But problems occur in punch rigidity when angles are too acute  Wall thickness should be a minimum of 1.5 mm (0.060 in) between holes or a hole and outside wall  Minimum recommended hole diameter is 1.5 mm (0.060 in)  Chamfers and Corner Radii 60 Figure 16.19 Chamfers and corner radii are accomplished but certain rules should be observed: (a) avoid acute angles; (b) larger angles preferred for punch rigidity; (c) inside radius is desirable; (d) avoid full outside corner radius because punch is fragile at edge; (e) problem solved by combining radius and chamfer. 61  THE END