Microstructure and mechanical properties of aluminum/steel bimetal using compound casting with electroless nickel plating

Abstract

The surface of steel substrate was pretreated by nickel plating and Aluminum/nickel-plated steel bimetal was prepared by compound casting. After liquid aluminum was casted on the surface of the nickel-plated steel, it was placed in a heat preservation device and temperature set at 760 °C. In the compound casting process of molten aluminum and nickel-plated steel, good metallurgical bonding is formed between aluminum and steel by means of diffusion. Subsequently, the microstructure of the aluminum/nickel-plated steel bimetal was examined using Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) techniques. The phase composition of the intermediate layer was analyzed by x-ray diffraction (XRD). Further, the bonding properties and fracture mechanism of the intermediate layer were analyzed by measuring the corresponding Vickers microhardness and shear strength. The results show that the intermediate layer is divided into two layers. In addition, intermetallic compounds appear in the intermediate layer; Al5Fe2, near the steel side, and Al8Fe2Si, near the aluminum side. The main phases in the intermediate layer are α-Al, α-Fe, Al5Fe2, Al5FeSi and Al8Fe2Si. The microhardness of the intermediate layer of the aluminum/steel bimetal is higher than that of both the aluminum matrix and steel matrix. It is observed that the thickness of the intermediate layer of the aluminum/nickel-plated steel bimetal increases with the increase of the holding time. When the holding time was 15 min, the intermediate layer of the aluminum/nickel-plated steel bimetal exhibits the highest shear strength, i.e. 13.4 MPa. The fracture of the aluminum/steel bimetal is a brittle fracture, and starts from the intermediate layer. The results show that nickel plating on steel substrate can obviously improve the casting properties of an aluminum/steel bimetal.