Processing, As-Cast Microstructure and Wear Characteristics of a Monotectic Al-Bi-Cu Alloy
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Processing, As-Cast Microstructure and Wear Characteristics of a Monotectic Al-Bi-Cu Alloy Rodrigo V. Reyes, Vitor E. Pinotti, Conrado R.M. Afonso, Luiz C. Casteletti, Amauri Garcia, and Jose´ E. Spinelli (Submitted July 31, 2018; in revised form October 24, 2018) Ternary Al-based monotectic alloys have a good combination of wear resistance and mechanical strength. While self-lubricating soft elements guarantee an adequate wear resistance, the modification with third elements can increase the ability to support load. In the present investigation, a collection of microstructures is generated through transient directional solidification of the Al-3.2wt.%Bi-3.0wt.%Cu alloy. Samples with different Bi spacing have been subjected to micro-adhesive wear ball tests. A relationship linking the wear volume, V, the microstructural spacing and the test time is proposed for Bi spacing higher than 16 lm, according to which V decreases with the decrease in Bi spacing. It is observed that wider and deeper grooves emerged on the surface of the samples related to more refined Bi and Al2Cu phases, that is, associated with Bi spacing and Bi diameter lower than 16 and 2.4 lm, respectively. A reverse trend is noted for these finer microstructures, for which V increases with further decrease in Bi spacing. This can be caused by the detachment of the very fine and less cohesive Al2Cu lamellas as the Al2O3 oxide breaks up forming debris, with the presence of these lamellas as loose debris at the interface acting as third-body abrasives. Keywords
Al alloys, microstructure, monotectics, solidification, wear
1. Introduction The monotectic reaction can be understood as a phase change in which one liquid (with a certain composition) forms another liquid of a different composition and a solid phase (Ref 1-4). A homogeneous liquid will be formed with a dispersion of droplets at a first cooling stage. Very complex interactions may occur with droplets colliding with each other, promoting coagulation and aggregation. For lower temperatures, the molten volume is separated, at the monotectic temperature, into two liquid phases in equilibrium with a solid phase. It is well established that the application properties of monotectic alloys are affected by the microstructure, which is quite reliant on the solidification cooling conditions (Ref 5-9). Al-Bi monotectics permit an even distribution of the soft element in the harder metal matrix (Ref 10). Also, the ability to form core-shell structure in a large-scale component is reported in ternary Al-Bi-Sn alloys (Ref 11-13). This kind of microstructure has also been reported to occur within the droplets of directionally solidified Al-Bi-Sn samples (Ref 14). These droplets are constituted by a combination of two soft phase regions, i.e., either Bi-rich or Sn-rich isolated areas, which are Rodrigo V. Reyes, Vitor E. Pinotti, Conrado R.M. Afonso, and Jose´ E. Spinelli, Department of Materials Engineering, Federal University of Sa˜o Carlos, Sa˜o Carlos, SP 13565-905, Brazil; Luiz C
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