Question

Why are the parts manufactured using powder metallurgy of near net shape? For mixtures of dissimilar metal powders, explain how (a) blending/mixing, and (b) compaction are carried out before sintering. Explain how (i) blending/mixing and (ii) compaction steps influence the (a) sintering step and (b) porosity development in the processed part.

Answer 1

A) Powder mixing and compaction: Blending refers to mixing of same kind of metal powders whereas, mixing refers to the uniform mixing of powders of different kind. Mixing or blending of elementary powders is carried out in mixers or ball mills or attritor mills under inert or controlled atmospheres. Oxidation and explosion of powders should be prevented during mixing by ensuring inert atmosphere in the mixer. Powder size could be controlled using high speed ball milling of the powders. Compaction of metal powders is the process of pressing green powders inside closed die-punch assembly. Powders are compacted by applying suitable pressure through punches. Lubrication is necessary to reduce inter-particle friction and particle-die wall friction. Lubricants such as graphite powder, zinc stearate powders are usually blended with metal powders before compaction. Density of compacts is important from the point of view of handling the green compacts. In compaction the particles come closer and establish contact. A heterogeneous mix of particle shape and size will promote uniform density of compacts. Softer powders promote higher green density. Similarly, higher compaction pressure leads to higher density of the compact. Admixed lubricant also promotes higher green density. The density of the compact is not uniform along the compact height due to non-uniform transfer of compact pressure from the punch to the compact. Compaction die and punch are often manufactured from die steel, though Compacting pressure varies for different metal powders. Alloy powders require higher compaction pressures. Normally the pressure varies from 300 to 800 MPa for iron and from 50 to 300 MPa for aluminium powders. Hydraulic presses of capacity upto 300 tons is commonly used for compaction. Usually the maximum density achieved in uniaxial compaction is limited to 95%. In order to achieve near full density in compaction cold or hot isostatic pressing could be employed. Cold isostatic pressing involves the application of hydrostatic pressure using a hydraulic medium on the powder contained inside a flexible container such as rubber or leather bag. Pressures upto 1000 MPA are applied. Hot isostatic pressing is carriedout in a container made of sheet. The metallic container with the powder is heated to high temperature – upto 1300 K, uniform pressure of 100-200 MPa is applied, in order to obtain 100% density. Uniform densification is achieved in isostatic pressing. Powder rolling in another compaction process, in which the powder is fed into the roll gap of a pair of rolls. The compacted strips are passed onto a sintering furnace. Thin sheets long and continuous can be produced by powder rolling. Powder extrusion – both hot and cold extrusion can be done at room temperature or at elevated temperature. Higher extrusion pressures are normally required for powder extrusion. In powder injection molding, the metal powder is mixed with 30 – 40% polymer binder, squeezed inside a molding die, at 400 to 500 K, similar to plastic injection molding, debinding is done at 400 K and sintered. PIM process is commonly used for making intricate parts out of metals such as steels, titanium, copper, tool steel etc. It has high productivity, suitable for mass production and is a competitive process as against forging or casting. Warm compaction is carried out at slightly higher temperatures but below temperatures on-half the melting temperatures. Warm compaction is often used for hard, abrasive powders, as the higher temperature of compaction softens the powders.

B)Green compacts do not have sufficient strength and they may collapse even under small loads, because the particles are loosely bonded with each other. They also contain porosity between the particles. In order to eliminate porosity and to establish metallurgical bond between powder particles, sintering is carried out after compaction. Sintering is the process in which the compact is heated to a temperature below melting point in order to achieve chemical bonding of particles by the process of interparticle diffusion, plastic deformation, grain formation and grain growth. If the sintering temperature is higher than the melting temperature of one of the metallic components of the compact, then liquid phase sintering occurs. Liquid phase sintering enhances the density of the compact. Sintering is to be performed in controlled atmosphere in order to prevent oxidation of the surface of the compacts. For ferrous materials, nitrogenhydrogen gas mixture is used as sintering atmosphere. Cracked ammonia is often used as source of nitrogen and hydrogen. Other gases such as argon, helium, nitrogen are also used as sintering atmosphere. During sintering, the following stages are known to occur: Particle bonding by necking Neck growth Pore channel closure Pore shrinkage Various factors influence the sintering process. Some of the factors are: sintering temperature, sintering time, sintering atmosphere. Sintering temperature is often within 90% of the melting temperature of the metal. Higher sintering temperatures promote higher densification. Larger sintering time often leads to higher sintered density. Similarly, higher nitial compact density results in higher sintered density. Vacuum sintering of stainless steels leads to better densification.

1)Tensile strength, ductility and fatigue strength of powder metal parts are considerably degraded by the presence of porosity in sintered material. Porosity may be present in the form of interconnected pores or individual voids. Porosity is helpful for some applications such as bearings and filters. Pores retain the lubricating oil in sintered bearings. High strength applications require pore-free p/m parts. Additional operations may be necessary after sintering in order to eliminate porosity in sintered parts. Some of the important post-sintering operations which help improve the density of the sintered material are: Forging, extrusion, repressing, infiltration.

2)Plastic deformation behavior of porous preforms differs significantly from conventional wrought materials. Poisson’s ratio of porous preforms is a function of density. There is volume change of the sintered preform during plastic deformation, unlike conventional materials. Similarly, the yield criterion for porous materials is also a function of density.

3)Infiltration: In this process, a molten low melting metal is made to flow into the pores of a porous metal by capillary action, so that the pores get filled. Copper is often used for infiltrating iron based porous alloys. Some of the bearings are formed by infiltration process. Similar process is oil impregnation of porous alloys for bearing applications. The evacuated porous alloy is immersed in oil so that the pores get impregnated with oil. Universal joints used in machines and automobiles are often made by this process in order to avoid application of grease lubricant. Surface treatment can be done on powder metal parts in order to improve their surface wear resistance and hardness. One common process is steam treatment of the surface of sintered ferrous alloys. Exposure of the surface to steam results in formation of thin oxide layer on the surface, which imparts high surface hardness and wear resistance.