Squeeze cast aluminium reinforced with mild steel inserts

Abstract

The bonding of a mild steel insert to an Al-7Si alloy during squeeze casting has been studied for a range of processing conditions. Assessment of the mild steel/Al-7Si alloy interface shear strength has been made with a push-out test, and the results have been correlated with microstructural observations and residual stress calculations. Uncoated inserts do not exhibit any significant reaction with Al-7Si because of rapid cooling of the melt during squeeze casting, giving a low interface shear strength of ∼ 30 MPa. Preheating the inserts to 900°C slightly improves the interface shear strength to ∼ 45 MPa, but reaction between the steel and Al-7Si is prevented by the formation of an Fe₃O₄ magnetite layer on the steel surface. Inserts hot-dipped in molten Al-10Fe before squeeze casting have a much greater interface shear strength of ∼ 110 MPa, with failure in the Al-7Si matrix rather than at the steel/Al-7Si interface. Inserts vacuum plasma spray coated with titanium have the greatest interface shear strength of ∼ 130 MPa, without any interface reaction, because of mechanical keying of the rough splat-quenched titanium surface combined with high residual stresses in the Al-7Si matrix.