Substitution of Nickel by Combined Addition of Cobalt and Zirconium in Alloy A 332

Abstract

Due to the increasing international competition and the resulting pricing pressure it is imperative to avoid the use of expensive alloying elements during the production of aluminium castings. The piston alloy A 332 shows an optimum combination of mechanical and casting properties and an attractive cost‐performance ratio whereas nickel is the most expensive alloying element. A substitution of nickel by a combined addition of low contents of cobalt and zirconium has the potential capacitiy to increase the mechanical properties and reduce the costs of the alloy. At Clausthal University of Technology Thermo‐Calc simulations and casting experiments were carried out to investigate the effect of the nickel subtitution. Thermo‐Calc‐simulations were made to analyze the intermetallic phases in these alloys. These simulations were evaluated by observations under optical microscope and SEM of specimens poured into permanent moulds. The size and morphology of the intermetallic phases and the primary silicon was analyzed by the use of image analysis software. The mechanical properties of the alloys were determined by tensile tests at room temperature, 250° C and 350° C. The tensile specimens were tested in as‐cast and pre‐aged condition. The effect on the castability was characterized by determining the flow length and the susceptibility to form shrinkages and hot cracks. The standard alloy A 332 and the new piston alloy with cobalt and zirconium were compared. The new alloy AlSi12,6Cu1Mg1CoZr exhibits a yield strength of 115 MPa and a tensile strength of 171 MPa at 250° C in pre‐aged condition (250° C/100 h). At 350° C the new alloy displays a yield strength of 57 MPa and a tensile strength of 85 MPa in pre‐aged condition (350° C/100h). Compared to the reference alloy AlSi12,6Cu1Ni1Mg1 the yield strength at 250° C was improved about 25% and the yield strength at 350° C about 7%. The favorable castability of the reference alloy is not affected by the substitution of nickel. These results prove the development of a heat‐resistant, thermally stable alloy without nickel. The mechanical properties and the cost‐performance ratio are superior to the standard alloy. The competitiveness of the new piston alloy was improved significantly.