Effect of heat input on the microstructure and mechanical properties of TIG welded-brazed magnesium / steel joints

A tungsten inert gas(TIG) welding-brazing technology using an Mg-based filler was developed for joining Mg alloys to steel. The effects of heat input on the microstructure and mechanical properties of Mg/steel joints were investigated. The results indicate that the joining strength of the joints is poor with low heat input,owing to insufficient metallurgical reaction between the Mg alloy and steel. But with excessive heat input,the growth of the brittle second phase in the seam occurs,resulting in the decrease of mechanical properties. The joining strength of the joints increases first and then decreases with the increase of welding current and welding speed. An acceptable joint is obtained with a welding current of 70 A,representing 88.7% joint efficiency relative to AZ31B base material. With this parameter,fracture occurs at the fusion zone and the fracture surface is characterized by equiaxed dimple patterns accompanied with lamellar tearing. The fracture surface embodies the characteristics of plastic fracture.