Linear contraction and residual stress of aluminium alloy sand castings

Abstract

Variations in the linear contraction of unconstrained and constrained rectangular and round bar castings of aluminium alloys have been studied. The processing variables included test bar design and dimensions, alloy composition and mould and core materials. The results show that the largest effects on linear contraction were due to constraint by enclosed cores in round bars and/or the flanges at the ends of bars. The results also indicate that the size of the constraint effects can be numerically related, for a given mould material, to the geometry of the bar and its constraint. The contraction was found to be a function of the relative modulus and relative areas of the bar and its restraining flange.Measurements of AlSi5Cu3 and AlSi11 castings in sand moulds indicate that residual stress in these light alloys does not seem to vary significantly with alloy type. However, it is sensitive to (i) mould restraint, and thus is affected by the type of binder, greensand giving much higher stresses than furan, and (ii) the geometrical constraint provided by the shape of the casting. Residual stress is reduced by shaking out the casting from the mould at an early stage after casting. It was found that the residual stress could vary between values as low as 0 to 10 MPa and as high as 110 MPa. The high level of stress appears to approximate to yield stress of the alloys, indicating that the castings are in a plastic yielding condition when emerging at near room temperature from the mould.