Microstructural evolution and tensile mechanical properties of thixoformed high performance aluminium alloys

Abstract

Commercial thixoforming is generally based on conventional aluminum-based casting alloys such as A356 and A357. However, these alloys do not give such high mechanical properties as the alloys that are conventionally wrought such as 2000, 6000 and 7000 series or the high-strength aluminium casting alloys such as 201. In this paper, 201, 2014, 6082, 7075, 7010 and ‘modified’ versions of 2014 (3.87 wt.% Cu, 0.6 wt.% Mg, 0.14 wt.% Si), 6082 (1.29 wt.% Si, 1.21 wt.% Mg) and 7075 (6.01 wt.% Zn, 3.99 wt.% Mg, 2.06 wt.% Cu, 0.12 wt.% Si) were thixoformed. The recrystallisation and partial (RAP) melting route and the cooling slope (CS) casting route were used to obtain the semi-solid feedstock for thixoforming. Microstructural evolution during partial melting was studied at temperatures where the amounts of liquid were comparable for the various alloys. Tensile mechanical properties of thixoformed parts in the T6 condition were determined and compared with those of conventionally wrought alloys and thixoformed high performance aluminium alloys from the literature. The tensile properties of the thixoformed alloy 201 show a significant improvement over permanent mould casting and the tensile properties of the thixoformed 2014, with an optimised heat treatment schedule, were close to those of forged 2014 but with lower elongation to fracture. Good elongation to fracture (19.6%) was obtained for thixoformed ‘modified’ 2014 in the T6 condition, with a yield strength of 270 MPa and a tensile strength of 408 MPa. This yield strength is significantly short of the target wrought 2014 value and the tensile strength is a little short. The elongations to fracture (24.4 and 12.3%) are satisfactory for alloy 6082 and ‘modified’ 6082 but yield strength and tensile strength need to be increased to achieve the wrought 6082 target. The tensile properties for 7075 thixoformed from starting material produced by the recrystallisation and partial melting route are a little better than those of 7075 thixoformed from starting material produced on the cooling slope. Both routes give values close to the wrought target 7075. The yield strength and tensile strength for ‘modified’ 7075 and 7010 are quite close to the wrought target.