Liquid Phase Sintering of Al—Cu Compacts

Abstract

The sintering of AI-Cu compacts made from powder mixtures has been studied by dilatometry and metallography in an attempt to elucidate the mechanism of dimensional changes occurring in this alloy system. Sintering kinetics are governed by the formation and spreading of eutectic liquid; this has been found to be influenced by the formation of intermetallic phases during heating to,.the eutectic temperature.Liquid phase sintering has made aluminium an attractive material for powder metallurgy processing. 1 -4 During formation of the liquid phase the thin but highly stable oxide film which is always present on the surface of aluminium particles is disrupted and good particle bonding is achieved by subsequent neck formation during which the liquid phase facilitates material transport. 5 ,6 Eutectic liquid can be formed by reaction of Al with elemental powders such· as Cu, Mg, Si, and Zn,7,8 which also convey improved strength directly or upon heat treatment. In the Inain, alloys of type AI-Cu-Mg (typically 4-6%Cu, up to 1%Mg) or AI-Mg-Cu (typically O' 5-1' 5%Mg, up to O' 5%Cu) are preferred in powder metallurgy.9During sintering, compacts of elemental powder mixtures of these alloy types exhibit a sudden swelling upon reaching and surpassing the eutectic temperature.The extent of this dimensional change depends on alloycomposition, particle size, sintering atmosphere, and temperature. lO The swelling is followed by a rapid contraction. lO • 11 These strong dimensional changes are undesirable in precision parts. It is the objective of this study to throw some light on their mechanism.Swelling of a similar type occurs in Fe-Cu alloys.12.13Its mechanism has been elucidated in terms of penetration of the melt into the grain boundaries of the iron particles and diffusion of the copper into the iron. The amount of swelling has been found to depend critically on the wetting conditions between the melt and the solid phase, which can be affected by carbon additions. A quantitative understanding of the swelling process and of the ways in which it can be controlled has been developed. 17 -19The swelling of AI-Cu compacts has been studied less extensively. On the basis of dilatometric measurements at high rates of heating, Esper and Leuze ascribed the swelling to the penetration of Al grain boundaries by the melt. lO Sundaresan and Ramakrishnan interpreted dilatometric observations on the sintering of Al-Sn and Al-Si-Cu-Ni alloys in terms of particle rearrangement and solutionreprecipitation. 20 * Liquid phase .• sintering is usually described in terms of three mechanisms which are dominant during successive stages of the process 12 ,16.(i) particle rearrangement with an almost linear time dependence of shrinkage (ii) dissolution and reprecipitation which removes material from the contact zones of particles, thereby diminishing the cente-to-centre distances. This stage produces shrinkage at a less than linear rate, and produces the shape adjustment of adjacent grains which is characteristic of liquid-phase sintered structure...

Disruption of an Alumina Layer During Sintering of Aluminium in Nitrogen

“…Researchers investigating the sintering mechanisms of aluminium and its alloys tend to agree [2,3,[17][18][19][20][21][22][23][24][25] that successful sintering requires a liquid phase that penetrates the stable, but spalled, aluminium oxide film covering the powder particles. For any aluminium based pre-alloyed powder or elemental powder mixture the appearance of a liquid phase during sintering results from the presence of alloying elements, like Cu, Mg, Zn, Sn, forming low melting point alloys with Al.…”

“…Hence, the surface liquid will promote further disruption of any remaining oxide film. Furthermore, liquid aluminium may penetrate the interface aluminium/alumina and also in this way destroy the oxide film, similarly to that shown for the Al-Cu system [18]. With regard to the greater volume expansion coefficient of the melt than of solid aluminium, this melt can also affect oxide crust degradation as a result of its local expansion.…”

An Evaluation of Severe Plastic Deformation on the Densification Behaviour of Powder Metallurgy Aluminium Alloy Al-Mg-Si-Cu-Fe and Al-Zn-Mg-Cu

“…It is well-known that for an effective liquid phase sintering, a wetting liquid represents an essential requirement. Authors (Kehl & Fischmeister, 1980) suggested that the Al-CuAl 2 eutectic can wet Al 2 O 3 at 873 K. However, not even magnesium additions (to melt aluminium) reduce the contact angle sufficiently to produce wetting (Martín et al, 2004); (Danninger, 1987); (Martín & Castro, 2007). This is possibly the main reason why sintering Al-Zn-Mg-Cu and Al-Mg-Si-Cu-Fe aluminium alloys still cannot be considered that easy.…”

“…In particular, the effect of copper in the alloys seems to be efficacious and therefore the sintering behaviour of Al-Zn-MgCu alloys needs to be developed properly. Authors (Kehl & Fischmeister, 1980) suggested that the Al-CuAl 2 eutectic can wet Al 2 O 3 at 873 K. However, magnesium additions to molten aluminium reduce the contact angle sufficiently to produce wetting (Ip et al, 1993); (Liu et al, 1992). The work of adhesion of liquid metals on oxide surfaces increases with the free energy of formation of the metal oxide.…”

TEM Observation of Interfaces between Particles in Al-Mg Alloy Powder Compacts Prepared by Pulse Electric Current Sintering

“…[1][2][3] In order to achieve effective sintering, the surface oxide layer needs to be removed and/or reduced. The previous investigations have shown some effective methods to remove and/or reduced the surface oxide film, for example, the use of liquid phases in the solid-state sintering, 4) mechanical breakdown by friction between surfaces, 5,6) ion beam bombardment treatment in a vacuum chamber, 7,8) the use of active alloying elements such as magnesium (Mg) and lithium (Li), [9][10][11][12] and so on.…”