Precipitation of Al3(Sc,Zr) particles in an Al-Zn-Mg-Cu-Sc-Zr alloy during conventional solution heat treatment and its effect on tensile properties

Influences of pulsed current tungsten inert gas welding parameters on the tensile properties of AA 6061 aluminium alloy

Effects of Sc and Zr microalloying additions and aging time at 120 °C on the corrosion behaviour of an Al–Zn–Mg alloy

Effect of matrix alloy and influence of SiC particle on the sliding wear characteristics of aluminium alloy composites

Dry sliding wear behaviour of cast high strength aluminium alloy (Al–Zn–Mg) and hard particle composites

It has been reported that small additions of scandium (Sc) can improve the weldability and mechanical properties of some aluminium aerospace alloys that are normally considered to be ‘unweldable’. In order to determine the mechanisms by which these improvements occur, and more rapidly arrive at optimum Sc addition levels, small wedge-shaped castings have been used to simulate the cooling rates found in MIG/TIG welds. Using this technique, a range of Sc addition levels have been made to two typical Al-aerospace alloys, 2024 and 7475. It has been found that when the Sc level exceeds a critical concentration, small Al 3 Sc primary particles form in the melt and act as very efficient grain nucleants, resulting in simulated fusion zone grain sizes as fine as 15 μm. This exceptional level of grain refinement produced an unusual grain structure that exhibited no dendritic, or cellular, substructure and a large increase in strength and ductility of the castings. Sc also produced changes in the alloy's freezing paths, which cannot yet be fully explained, but led to the appearance of the W phase in the 2024 alloy and, in both alloys, an overall reduction in the amount of eutectic formed during solidification. When coupled with the high level of grain refinement, this behaviour could be used to explain the increased strength and ductility of the castings. In 2000 and 7000 series aluminium alloys, it is therefore, anticipated that optimised Sc bearing filler wires will significantly improve the mechanical properties of the weld metal, as well as reducing the tendency for solidification cracking.

S Thompson

Papers

Examination of the effect of Sc on 2000 and 7000 series aluminium alloy castings: for improvements in fusion welding