Optimisation of DC cast microstructure of Aluminium alloys containing immiscible elements

Abstract

Free machining alloys containing soft immiscible phases in the aluminium (Al) matrix, like lead (Pb) and bismuth (Bi), are of great industrial interest. Typical applications in automotive industry are components requiring very high machinability, such as braking pistons and antiblocking system (ABS) housings. Presence of soft immiscible phases is giving their machining properties to this class of alloys. These phases melt due to localised heat build-up generated by machining process and induce chips breaking. Such type of alloys offers best in class performance when the soft phase is uniformly distributed in the Al matrix. The main objective of this work was to develop a method to tailor the distribution of the immiscible phase particles in the final solidified structure of DC cast billets in order to provide enhanced machinability while keeping low levels of Pb and/or Bi additions. As a consequence, another objective of this study was to improve recyclability of such alloys as well as to reduce their environmental impact. Three categories of Al-Pb alloys and different solidification paths were studied: hypermonotectic Al-3Pb, monotectic Al-1.2Pb and industrial hypo-monotectic free machining alloy containing both Pb and Bi. A newly developed melt conditioning combines mechanical, thermal and chemical treatments to obtain a very fine and uniform distribution of the immiscible phase droplets and eliminate compositional heterogeneities. The effect of these new melt treatments on microstructure was evaluated. For the soft phase droplets size was reduced and distribution becomes finer and more homogeneous under the individual effect of each of the treatments and optimum results obtained with the combination of them. These new melt treatments affect not only the nucleation of the Pb/Bi droplets, enhancing their heterogeneous nucleation but reduces considerably the Marangoni motion and Stokes sedimentation reducing therefore the droplet coalescence and restricting their growth. As a consequence of this improved microstructure, mechanical properties and machining performance were enhanced considerably. The results from this study provide a promising new microstructure with a fine and uniform distribution of droplets.