Friction Stir Welding Of AZ31 Magnesium Alloy And DC01 Steel

Magnesium alloy has a strength-to-weight ratio 1/3 higher than aluminum alloy,which enables it to promote automobile lightening by replacing steel.As a result,the joining between these two dissimilar and immiscible materials becomes a critical question.This study realized the friction stir welding of AZ31 magnesium alloy and DC01 mild steel in three different modes: spot welding,short-distance welding and lap welding.An appropriate welding parameter window and the best parameter combination were obtained by controlling variables in all three modes.The maximum load force of the welds are 3463 N,6184N and 2956 N respectively.The influence of key parameters on the joint strength were summarized.In addition,a new method of Z-load control was adopted during the lap welding and created better and stable results under suitable parameter conditions.Two mechanisms were attributed to the good bonding in the friction stir weld between magnesium and steel: mechanical interlocking and chemical bonding.Ear-shaped structure bears the tensile stress in the macro-scale and micro-interlockings of various formations were found in the interface.An 0-20 ?m thick Al-rich layer exists on DC01 side near the interface in all three modes.Refined characterization results proved it to be a multi-layer structure with different thickness and aluminum content in each layer.The one nearest to the interface contains most probably AlFe new phase after cross referencing the EDS results with phase diagram.On the AZ31 side of the interface,a Al-depletion layer(Al% < 1%)with thickness of a few hundred microns can be found.There exists two fracture mode for the lap-welded specimen: fracture along the interface and fracture through AZ31 magnesium in the thickness direction,which mainly correspond to low-strength and high-strength specimens respectively.Fracture originated from the weak-bonded interface at the root of ear-shaped structure.