Key words: aluminium, powder metallurgy, sintering, nitrogen. ...... ASM (1998), 'Conventional aluminium powder metallurgy alloys', in ASM Handbook,. Vol.
Part III Sintering of advanced materials
289
1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 20 1 2 3 4 5 6 7 8 9 30 1 2 3 4 5 6 7 8 9 40 1 2 43X
12 Sintering of aluminium and its alloys M. QIAN and G. B. SCHAFFER, The University of Queensland, Australia
Abstract: The conventional press-and-sinter powder metallurgy (P/M) technique is a unique cost-effective method for net shape or near net shape fabrication of complex aluminium parts. The chapter begins by providing a historical account of aluminium P/M and its application in North America, where the industry originated. It then reviews key issues of the press-and-sinter aluminium P/M technique and the science of sintering aluminium and its alloys under nitrogen, including the distinctive roles of magnesium and tin. Key words: aluminium, powder metallurgy, sintering, nitrogen.
12.1
Introduction
This chapter is about the sintering of aluminium and its alloys. The scope is restricted to the conventional press-and-sinter powder metallurgy (P/M) routes and to where aluminium differs from other sintering systems. Section 12.2 provides a historical account of aluminium P/M in North America, where the industry originated, and an overview of aluminium P/M applications. Section 12.3 discusses green shape formation from aluminium powder and the importance of the use of internal lubricants. The effect of the sintering atmosphere and dew point control is considered in Section 12.4. A critical review of the surface characteristics of airatomised aluminium powder and the oxidation behaviour of aluminium powder is presented in Section 12.5, which constitutes a basis for understanding the sintering complexity of aluminium. This is followed by Section 12.6, which is concerned with the disruption of the oxide film by powder compaction and the amorphous-tocrystalline transformation of the oxide. Section 12.7 discusses the unique sintering response of aluminium under nitrogen. The thermodynamics is analysed first, followed by the effects of Mg, AlN and Sn. A summary of the commercial grade aluminium P/M alloys and their mechanical properties is given in Section 12.8, and compared to those of ferrous and copper P/M alloys. The future directions for aluminium P/M are considered in Section 12.9.
12.2
Aluminium P/M and its application
The first major attempt to manufacture P/M parts with aluminium powder as an important constituent dates back to the 1930s (Howe, 1942), following the invention of the Al-Ni-Fe permanent magnet alloys (ALNICO) by Dr Tokushichi 291
1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 20 1 2 3 4 5 6 7 8 9 30 1 2 3 4 5 6 7 8 9 40 1 2 43X
292
Sintering of advanced materials
Mishima of Tokyo Imperial University in 1931 (Cullity and Graham, 2008). The intention was to utilise P/M’s advantages to produce small Al-Ni-Fe magnet products of intricate design with a dense and fine-grained structure. However, the practice of sintering Al-Ni-Fe green bodies made from blended elemental powders encountered considerable difficulties due to the inherent Al2O3 film on each aluminium powder particle, which are not reducible by hydrogen (Schwarzkopf, 1947). The sintering method which finally proved successful was through the use of an Al-50%Fe master alloy, which can be readily disintegrated into fine powder (Howe, 1942; Schwarzkopf, 1947). The introduction of aluminium in this fashion practically eliminated the ruinous oxidation problem with aluminium powder. A detailed bibliography of the work on aluminium P/M prior to 1949 can be found from Bickerdike’s paper (Bickerdike, 1947) and Goetzel’s Treatise on Powder Metallurgy (Goetzel, 1952). The earliest was that by Sauerwald and Elsner in 1925, followed by Kikuchi in 1937 (Bickerdike, 1947). Systematic trials were made to sinter aluminium and its alloys in air, vacuum and ammonia in the 1940s (Kempf, 1940; Bickerdike, 1947; Goetzel, 1950), where ten different binary Al-X systems (X = Mg, Zn, Cu, Fe, Ni, Si, Pb, Sn, Tl and C) and a variety of their combinations were sintered. Table 12.1 provides a snapshot of some of these early efforts; useful properties were attained from pressing and sintering aluminium in air or vacuum. A variety of factors that affect the attendant mechanical properties of the sintered aluminium alloys were identified. These include: (i) the compaction pressure; (ii) alloy composition; (iii) heating rates and atmosphere; and (iv) sintering temperature in relation to the solidus of the alloy Table 12.1 Pressed-and-sintered aluminium alloys in the 1940s (Kempf, 1940; Bickerdike, 1947) Alloy chemistry (wt pct)
Compaction pressure (MPa)
Sintering temperature (°C) and time
Sintering medium
Tensile strength (MPa)
Source
Al-10Mg Al-10Mg Al-10Zn Al-5Cu Al-7Zn-3Mg
275 768 552 552 552
427; 24 hr 427; 24 hr 510; 24 hr 549; 4 hr 510; 24 hr
37 172 107 223 276
[1] [1] [1] [1] [1]
Al-6Cu Al-6Cu Al-6Cu Al-6Cu Al-6Cu
207 207 689 689 689
590; 20 hr 590; 20 hr 500; 20 hr 500; 20 hr 590; 20 hr
Air furnace Air furnace Air furnace Air furnace Air furnace (quenched) Air furnace Vac. furnace Air furnace Vac. furnace Vac. furnace
218 277 210 244 331
[2] [2] [2] [2] [2]
Note: [1]: Kempf, 1940; [2]: Schwarzkopf, 1947. Most data were converted from their English units.
Sintering of aluminium and its alloys
293
(Kempf, 1940; Bickerdike, 1947; Goetzel, 1950). These technical factors are still largely characteristic of today’s aluminium P/M. The SAP material, which denotes ‘Sinter-Aluminium-Pulver’ (sintered aluminium powder), is a related development disclosed in 1949 (Irmann, 1949). The name is deceptive as SAP is in fact a dispersion-strengthened Al-Al2O3 composite fabricated from aluminium powder (Irmann, 1952; Grant et al., 1967; Blakeslee, 1971). Containing up to 21 vol.% of Al2O3, SAP was made by extruding or pressing superfine aluminium flakes (< 1 µm) at 500–600 °C (Irmann, 1952). The high oxide content stems from the fine particle size. For example, the oxide content of an aluminium particle with a diameter of 100 nm is about 20 vol.% (Irmann, 1952). Owing to the extraordinary oxide dispersion strengthening (ODS) effects, the compositionally simple (Al and O) SAP materials were superior to any other aluminium material at temperatures above 200 °C, including even those precipitation-hardened (Blakeslee, 1971). The ODS-SAP approach is still pursued today with micrometer-sized (
