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Abstract
In an effort to improve the properties of Al-based alloys, microalloying with select elements has proven to be a viable method. Despite the broad variations in alloys and microalloying additions considered, almost all studies have relied on the use of traditional ingot metallurgy practices. Consequently, any influence from the means of alloying is seldomly addressed. With this in mind, an alternative technique termed “core/shell” processing was developed based on powder metallurgy principles.
Prior to initiating “core/shell” experiments, the sintering response of two alloy powders (ternary Al-4Cu-0.5Mg and a commercial P/M version of 2014 Al-4.4Cu-0.8Si-0.8Mn-0.5Mg) was assessed. Both alloys responded well to sintering and reached sintered densities that facilitated final swaging to near theoretical density.
Numerous samples microalloyed with Li, Sn or Ag as well as alloy standards were prepared using a variety of sintering times. Mechanical and physical properties of the “core/shell” processed samples were then characterized and compared to those of alloy standards. The properties evaluated included age hardening response/TEM, tensile behaviour, dry sliding wear resistance, density and the corrosion rate in salt water.
Results of age hardening experiments indicated that Ag promoted a substantial increase in peak hardness in the ternary alloy, but was far less effective in commercial P/M 2014. Similar experiments were conducted with Sn-bearing samples of P/M 2014. It was discovered that Sn promoted a decrease in both hardness and tensile strength; this became more acute as sintering time was prolonged. Since Sn accumulated in the intergranular regions, the soft malleable nature of this metal was then imparted to P/M 2014. Consequently, the reduction in hardness and tensile strengths resulted.
In wear tests of P/M 2014 standards, a trend of reduced resistance to wear with increased sintering time was noted and postulated to be the result of microstructural coarsening. Despite the occurrence of similar coarsening in Sn modified samples, wear resistance was found to increase with sintering time and in turn Sri content. This resistance was found to surpass all P/M samples considered as well as a wrought version of 2014. Although Ag appeared to promote a similar response, the extent of improvement was relatively minor. It was postulated that Sn would melt through the frictional heat developed during sliding, thus reducing the direct metal to metal contact and consequently, wear rate.
To determine the corrosion resistance of samples based on P/M 2014, polarization resistance curves were measured. Relative to a wrought counterpart, P/M samples exhibited a superior resistance to corrosion. Due to the relatively minor extent of Sn and Ag additions, the majority of samples exhibited a similar rate of attack.
Through the completion of this work, several of the benefits that P/M processing offers over ingot metallurgy were explored. Those included were near net shape formability, reduced machining, as well as enhanced mechanical properties and corrosion resistance. Using the fundamental principles of P/M, a novel technique for microalloying P/M products was developed. Relative to ingot metallurgy techniques the process offers the versatility of P/M and is specifically advantageous through its ability to (i) alloy selected regions of a given component, (ii) vary the extent of alloying (macro/micro) and (iii) vary the depth of alloying (surface/through thickness). Furthermore, since the process is potentially feasible for several alloying additions and alloy systems it may well prove to be a viable industrial practice.
