Powder Metallurgy Methods and Applications
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ASM Handbook, Volume 7, Powder Metallurgy Copyright # 2015 ASM InternationalW P. Samal and J. Newkirk, editors All rights reserved asminternational.org Powder Metallurgy Methods and Applications W. Brian James, Hoeganaes Corporation, retired Powder metallurgy (PM) is the production ferrous PM structural parts are used). Powder and screw machining. The industry comprises and utilization of metal powders. Powders are metallurgy parts are used in engine, transmis- powder suppliers and parts makers, plus the defined as particles that are usually less than sion, and chassis applications. Sometimes it is a companies that supply the mixing equipment, 1000 nm (1 mm) in size. Most of the metal unique microstructure or property that leads powder handling equipment, compacting particles used in PM are in the range of 5 to to the use of PM processing: for example, porous presses, sintering furnaces, and so forth. 200 mm (0.2 to 7.9 mils). To put this in context, filters, self-lubricating bearings, dispersion- Powder metallurgy processing offers many a human hair is typically in the 100 mm (3.9 mils) strengthened alloys, functionally graded materi- advantages. The PM process is material and range. als (e.g., titanium-hydroxyapatite), and cutting energy efficient compared with other metal The history of PM has already been outlined tools from tungsten carbide or diamond com- forming technologies. Powder metallurgy is in the article “History of Powder Metallurgy” in posites. Captive applications of PM include cost effective for making complex-shaped parts this Volume. This article reviews the various materials that are difficult to process by other and minimizes the need for machining. A wide segments of the PM process from powder pro- techniques, such as refractory metals and reac- range of engineered materials is available, and duction and powder processing through to the tive metals. Other examples in this category are through appropriate material and process selec- characterization of the materials and their prop- special compounds such as molybdenum disili- tion the required microstructure may be devel- erties. It will cover processing methods for con- cide and titanium aluminide, or amorphous oped in the material. Powder metallurgy parts solidating metal powders including options for metals. have good surface finish and they may be heat processing to full density. The metal powder industry is a recognized treated to increase strength or wear resistance. Powders have a high ratio of surface area to metal forming technology that competes dir- The PM process provides part-to-part reproduc- volume and this is taken advantage of in the ectly with other metalworking practices such ibility and is suited to moderate-to-high volume use of metal powders as catalysts or in various as casting, forging, stamping (fine blanking), production. Where necessary, controlled micro- chemical and metallurgical reactions. While porosity can be provided for self-lubrication or this article focuses on the use of powders to filtration. While dimensional precision is good, make functional engineering components, many it typically does not match that of machined metal powders are used in their particulate parts. form. This aspect of PM is covered in the arti- In the case of ferrous PM parts, they have cle “Specialty Applications of Metal Powders” lower ductility and reduced impact resistance in this Volume. compared with wrought steels. Powder technologies are exciting to engi- The majority of PM parts are porous and con- neers because processing options permit the sideration must be given to this when performing selective placement of phases or pores to tailor finishing operations. the component for the application. The capabil- ity of press and sinter processing or metal injec- tion molding (MIM) processing to replicate Metal Powders parts in high volumes is very attractive to design engineers. The ability to fabricate complex Metal powders come in many different shapes shapes to final size and shape or to near-net and sizes (Fig. 2). Their shape, size, and size dis- shape is particularly valuable. Powder metal- tribution depend on the manner in which they lurgy offers the potential to do this in high were produced. Metal powder production is cov- volumes and also for applications where the ered in depth in various articles in the Section, volumes are not so large. “Metal Powder Production” in this Volume. The three main reasons for using PM are eco- There are three main methods of powder nomic, uniqueness, and captive applications, as production: shown in Fig. 1 (Ref 1). For some applications that require high volumes of parts with high Mechanical, including machining, milling, precision, cost is the overarching factor. A good Fig. 1 Three main reasons for choosing powder and mechanical alloying metallurgy shown in the form of a Venn example of this segment is parts for the auto- diagram. The intersection of the three circles represents Chemical, including electrolytic deposition, motive industry (where approximately 70% of an ideal area for applying PM techniques. Source: Ref 1 decomposition of a solid by a gas, thermal 10 / Introduction to Powder Metallurgy limited plasticity. Rigid die compaction is not suitable for consolidating such powders, and they must be processed by other means such as hot pressing, extrusion, or hot isostatic pressing (HIP), described subsequently in this article. Highly reactive metal powders are also not suitable for rigid die compaction. They generally need to be vacuum hot pressed, or encapsulated and extruded, or HIPed. Rigid die compacted parts and MIM parts are thermally treated to increase their strength in a process known as sintering. The parts are heated, generally in a reducing atmosphere, to a temperature that is below the melting point of the primary constituent of the material, in 5 μm 5 μm order to form metallurgical bonds between the compacted metal powder particles. Sintering Fig. 2 Example of the different particle shapes possible with metal powders is a “shrinkage” process. The system tries to reduce its overall surface area via various diffu- sion processes. Metallurgical bonds (micro- decomposition, precipitation from a liquid, chemical analysis of the metallic elements in scopic weldments) form between adjacent precipitation from a gas, solid-solid reactive PM materials are provided in MPIF Standard metal particles (after oxides have been reduced synthesis 67 (Ref 2). on the surface of the powder particles), pore Physical, including atomization techniques Complex, multilevel PM parts compacted surfaces become less irregularly shaped, and in rigid dies will not have the same green larger pores grow at the expense of the smaller Most metals are available in powder form. density throughout. While the objective is gen- pores. Sintering is generally carried out using Some may be made by many different methods, erally to achieve a density as uniform as pos- continuous mesh-belt furnaces. For higher while for others only a few options are possible. sible throughout the compacted part, taller temperatures (>1150 C, or 2100 F), pusher, The characteristics of the powder are deter- parts and parts with multiple levels are subject roller hearth, or walking-beam furnaces may mined by the method by which it is produced. to the presence of density differences between be used. Batch furnace processing is used for The shape, size, size distribution, surface area, adjacent regions. This is due to frictional effects special applications (e.g., pressure-assisted apparent density, flow, angle of repose, com- and compacting tool deflections. Taller parts will sintering). More information on sintering may pressibility, and green strength depend on the have a neutral zone or density line—the region befoundintheSection“SinteringBasics”in powder production method. In-depth coverage of the compact that has experienced the least this Volume. of the sampling and testing of metal powders relative movement of powder. The position of is presented in the articles in the Section “Metal the neutral zone may be adjusted by varying Powder Characterization” in this Volume. the pressure exerted by the upper and lower Powder Metallurgy Material punches. Properties Compaction in rigid dies is limited to part Powder Processing shapes that can be ejected from the die cavity. The majority of PM parts contain pores (see Parts with undercuts, reverse tapers, threads, options for processing metal powders to full For the production of PM parts in high and so forth, are not generally practical. Such density later in this article). This is an advan- volumes, compaction is carried out in rigid features are formed by postsintering machining tage when metal powders are used to make dies. In most instances, the metallic powders operations. self-lubricating bearings in which the surface- are mixed with a lubricant (e.g., ethylene bis- There are two main types of compacting connected pores of the parts are impregnated stearamide) to reduce interparticle friction dur- press: mechanical and hydraulic. Some hybrid with oil. When the bearing surface heats up ing compaction and to facilitate ejection of presses offer features of both. A detailed treat- due to frictional heat, oil is released from the the compacted parts by reducing friction at the ment of compaction is provided in the Section pores. When the bearing cools, the oil is sucked die-wall and core-rod interfaces. “Metal Powder Compaction” in this Volume. back into the pore channels by capillary action. The metal powders may be elemental pow- Some PM parts are molded (shaped) rather The porosity in PM parts has an effect on the ders; mixtures of elemental powders; or mix- than compacted. Fine-particle-size metal pow- physical, mechanical, magnetic, thermal, wear, tures of elemental powders with master alloys ders (5 to 20 mm, or 0.2 to 0.8 mils) are mixed and corrosion properties of the parts. or ferroalloys, prealloys, diffusion alloys, or with binders and plasticizers and processed to Thermophysical properties of sintered steels, hybrid alloys.