A study of the bonding of aluminium alloys to mild steel prepared by an overcasting process

Abstract

Fabrication of aluminium/steel bimetallic components has been extremely useful in automobile and aerospace industries for the development of lightweight structural and functional applications. This research investigates the possibility of using overcasting of liquid aluminium to solid steel to bond aluminium to steel. Specifically, the bonding of steel to aluminium-tin based alloy can enable the use of steel as a reinforcement in soft aluminium-tin bearings to improve their life expectancy. This project is concerned with the understanding of the effect of overcasting processing conditions (eg. cooling rate and melt holding time) and alloy composition on resultant bond microstructure, kinetics of interaction layer and mechanical properties of aluminium/steel joint. Two cooling rates of 0.63 K/s and 5 K/s were used during overcasting process with sand and steel moulds, respectively. Steel substrates coated with Zn, Ni and Ni-Zn layers of thickness ranging from 2 μm to 12 μm were used for this study. A range of aluminium alloy compositions were studied. They include pure Al, binary Al- (1-12.2 wt%) Si, Al-20wt%Sn-7wt%Si and Al-Si-Mg alloys. The microstructure of overcast sample was characterised using a combination of scanning electron microscopy (SEM), X-ray Diffraction (XRD) and Transmission Electron Microscopy (TEM). The bond strength of overcast sample was determined using tensile testing method. This research studied the effect of zinc coating in a very systematic way on the Al/steel joints. Different cooling rates and holding times used to study the behaviour of microstructure and strength of joints between overcast aluminium and steel substrates with and without coatings of zinc, gallium-zinc, nickel and nickel-zinc. Interaction between pure aluminium and steel in different processing conditions were studied. No interaction was found between pure aluminium and zinc coated steel overcast at a high cooling rate of 5 K/s (750/500 ºC). However, an interaction was detected in samples prepared at a lower cooling rate of 0.63 K/s (750/500 ºC). to provide a metallurgical bond between Al and steel. After this experiment, all samples were made inside sand mould in the same cooling rate of 0.63 K/s (750/500 ºC). Intermetallic compound (IMC) interaction layer was found between aluminium and steel with finger-like features of the IMC were directed towards steel. Al13Fe4 phase was detected within the IMC next to the aluminium and Al5Fe2 phase was detected within IMC layer adjacent to steel. The kinetics of formation of Al5Fe2 IMC was demonstrated to be controlled by a mass diffusion process. It is confirmed that growth of IMC layer followed a parabolic law. TEM studies confirmed the presence of Al13Fe4 with the IMC next to aluminium and Al6Fe2 within IMC next to steel. From all bond strength tests, the fractures were found to propagate alongside the IMC and perpendicular to the direction of the force. Interaction of pure aluminium/gallium-zinc coated steel was similar to the pure aluminium/zinc coated steel and no difference in the joint microstructure was detected. Interaction between pure aluminium and uncoated steel joint occurred after holding at750 ºC for 1 minute. The joint microstructure consisted of isolated islands of IMC discontinuous layer located between the aluminium and steel regions. These islands didn’t touch each other during their growth. Interaction between pure aluminium and nickel coated steel, resulted in the formation of Al13Fe4 and Al5Fe2 IMC layers between aluminium and steel. However, the kinetics of formation of the IMC layer was slower than the interaction of pure aluminium with zinc coated steel. The reason can be lower melting temperature of zinc coating compared with nickel coating, 420 ºC vs. 1455 ºC. As the low melting point coating of zinc melts faster in overcasting process, interaction layer forms quickly in the bond between aluminium and zinc coated steel. Interaction between pure aluminium/NiZn coated steel gave very similar interaction between pure aluminium and zinc coated steel, however the kinetics of interaction was slower in NiZn coated steel because the nickel coating reduced the kinetics of interaction between steel and aluminium. Similar IMC layers were formed on NiZn coated steel that were found previously in zinc coated steel. Measurement of bonding strength revealed that generally the bond strength increased by increasing holding time from 0 to 10 minutes. However, uncoated steel showed very small or zero bond strength in bimetallic joints with different aluminium alloys. All of the measured bond strengths are related to the formation of IMC layers. Under processing conditions with no IMC, no bond was formed. However, increasing holding time gave rise to increased IMC thickness and stronger bond strength. For example, at high cooling rate of 5 K/s, IMC did not have enough time to form in pure aluminium/Zn coated steel joint, so no bond formed in the joint. Generally, it was found that different coatings on the steel substrate can change the kinetics of interaction between aluminium and steel, even though IMC phases remained unchanged for all the coatings used in this study. Two types of cracks were detected in the bimetallic joints. The first type was found between Al13Fe4 phase and aluminium. The second type was found alongside the Al5Fe2 phase. The first type of crack was formed because of the difference in diffusion rates of elements between the IMC and the aluminium, while the second type of crack was formed because of the difference in thermal expansion coefficients. Kirkendal effect led to the formation of pores within the Al5Fe2 layer in the samples prepared after holding the aluminium melt at 750 ºC for 30 minutes and more. They formed feasibly because the rate of diffusion of one element was higher than the other element. Addition of silicon to aluminium reduced the kinetics of formation of IMC layer by reducing the atomic diffusion rate. Therefore, lowering of atomic diffusion and reaction kinetics can be the reasons of reduction in thickness of IMC layers after addition of silicon in the aluminium melt. Thickness of the interaction layer in Al-20Sn-7Si/nickel coated and zinc coated steel joints was increased by increasing holding time. The interaction layer thickness in Al-20Sn-7Si/zinc coated steel sample was higher than those in the Al-20Sn-7Si/nickel coated steel. Bond strength of Al-20Sn-7Si/zinc coated steel after holding for 5 minutes in 750 ºC was measured to be 30 MPa, the maximum bond strength of the bimetal joints. Kinetics of interaction layers was compared in all of the processing conditions using parabolic law. The Al/Zn coated steel joint had the highest rate of formation of IMC, and Al-7Si/Ni coated steel joint had the lowest rate of formation of IMC. Thanks to the high activation energy of pure aluminium and low melting temperature of zinc coat, the Al/Zn coated steel joint accelerated the interaction between aluminium and steel while the low activation of Al-7Si and high melting temperature of the nickel coat reduced the kinetics of interaction in the Al-7Si/Ni coated steel joint. Finally, it was concluded that careful control of processing conditions and material composition is required to achieve improvement in the strength of the bimetal joint without excessive degradation of the strengths of the primary materials.