Microstructure Control And Process Optimization Of Aluminum Alloy And Steel EBW Joints

In this paper, electron beam welding between 5A02 aluminum alloy and 0Cr18Ni9 stainless steel dissimilar metal joint was investigated. Microstructure control was attempted by beam offset and bringing in interlayer, aiming to constrain the brittle intermetallic compounds in the weld, process optimization was also researched. The microstructure of the joints under different parameters was analyzed by means of optical microscope, SEM and XRD. The mechanical properties were estimated by tensile strength and micro hardness experiments.The aluminum/steel assembly was welded directly at first. The centered joining was unfeasible because of large quantities of brittle Fe-Al IMC in the weld. The amount of Fe-Al IMC could be reduced by moving the beam spot for a certain distance from the touching face to the aluminum side, and thus improve the microstructure. An IMC layer composed of FeAl, FeAl3 and FeAl6 was formed on the interface of the joint. The tensile strength of the joint was up to 136MPa, 62.7 percent that of the 5A02 body material. The optimal thickness of the IMC layer was about 0.9~1.55μm. Fracture defect was found on the IMC layer or the aluminum weld near it when the beam current when the beam current or the amount of beam offset was not proper.The effect in terms of microstructure control was compared when bringing in the three interlayer materials: AlSi7, Cu and Ag. The results showed that in the joints with AlSi7 and Cu as interlayer materials, it was difficult to constrain the brittle IMC, and the fracture defect couldn't be removed, resulting in bad .mechanical properties of the joints. In the joints with Ag as interlayer material, fracture defect was not found in the Ag-Al compounds, which indicated that the brittleness of Ag-Al compounds was comparatively low, and the microstructure control effect of Ag interlayer was better than that of others.The process optimization was investigated with 1.0mm thick Ag as the interlayer material. The key was to remove the consecutive Fe-Al IMC layer on the touching face of steel and Ag interlayer, on which fracture was often observed. 1.0mm thick Ag interlayer proved to block and dilute Al atoms effectively under proper parameters, and helped to reduce the touching and reacting of Fe and Al atoms. The heat input distribution of the two interfaces could be adjusted properly by moving the beam spot to the Ag interlayer for a certain distance, resulting in the improvement of the joint morphology, elimination of the air hole and good combination of both the interfaces. The optimal tensile strength of the joint was up to 193MPa, 88.9 percent that of the 5A02 aluminum alloy, with the beam current 11mA, moving velocity 360mm/min and beam offset 0.2mm from the silver-steel interface to the silver side. Fe-Al IMC layer was found to be formed when the beam offset was too much, resulting in the reduction of mechanical propertiesof the joints, so the beam offset was to be constrained to a certain scale.