Investigation On Microstructures And Mechanical Properties Of Joints Of E36 And 304 Prepared By Laser Welding

With the progress of science and technology, the requirements of different mechanical workpiece are also getting higher and higher. It requires materials owing not only good strength but also good corrosion resistance, wear resistance and low temperature toughness. The welding of dissimilar materials has become the inevitable trend because the lower performance of single material.It is different to obtain welds with little defects using traditional welding as different physical and chemical properties of different materials. Laser welding offers good condition for dissimilar materials because constrained energy and lower influence to beam of different material physical and chemical properties.The optical fiber laser welding of the high-strength steel E36 and austenitic stainless steel 304 was treated as the research object in this paper. Systematical tests of the welded joints were performed to analyze the morphology, chemical composition, fracture morphology, microstructure, micro hardness, tensile properties and corrosion resistance of the welded joint, by means of optical microscopy, scanning electron microscopy, X- ray diffraction, micro hardness tester, tensile testing machine, electrochemical instrument. The main works were to research the macro morphology, the microstructure of heat affected zones, primary microstructure, secondry microstructure, chemical composition, hardness, tensile strength and corrosion resistant of the joints under the three conditions of the different laser power and welding speed, the same line energy, and the different laser beam offset.The laser weld of the E304 and stainless steel 304 was executed, the experiments results indicating that desired welded joint were obtained at different welding speeds/laser power. In the position of 304 fusion zone, δ-ferrites are gradually decreased with the increase of welding speed. The microstructure of weld closed to stainless steel 304 is planar crystal while that closed to E36 and welding center is columnar crystal. With the increase of welding speed, the quantity of cellular crystal of weld closed to stainless steel 304 is increasing and so does to cellular crystal of welding center, but there has no significant change in weld microstructure near the E36. Weld zone is mainly consisted of martensites and a small amount of carbides. Microstructural evolution caused by welding speed is not obvious. Composition of welded joint is homogeneous. However, there is composition gradient on both sides of the fusion zone, which is influenced significantly by welding speed. Lath-shaped martensites, ferrites and pearlites emerge in the heat affected zone of E36. The highest hardness value appears at the welding center. The hardness of weld zone and E36 heat affected zone is increasing with the increase of welding speed. The fracture positions of tensile simples are at the base metal on both sides at different welding speeds/ laser power. In the same condition of the line energy, experimental results showed that both heat input and laser power density play a crucial role in macro-morphology of welded joint. Welding velocity has a significant impact on grain morphology. When the laser power is 1 k W, the hardness of the seam is lower than the others situations and fracture position occurs at the welded joint of the tensile simple. While the fracture positions of the others situations occur at the base metal E36. When the laser beam offset is set of different value, study results suggested that when the offset ranges from-0.1mm to 0.2mm, materials welded well and the fracture occurs at the base metal E36; when the offset are-0.3mm, 0.3mm and 0.4mm, poor fusion exist in welded joint. The fracture occurs in welded joint. Moreover, the fracture is mainly quasi cleavage fracture in the condition of negative offset, while it is mainly plastic fracture in the condition of positive offset. When the offset is negative(or 0mm), the welded joint is mainly composed of martensite and a small amount of carbide, where the highest hardness occurs. When the offset is positive, the welded joint is mainly composed of austenitic and a small amount of δ-ferrite, while the highest hardness occurs in the heat affected zone of E36. The weld corrosion resistance is increasing when the offset is positive, while it is decreasing when the offset is opposite. When the offset is 0.2mm, the weld owns the most superior corrosion resistance.Above all, the offset of 0.2 mm to stainless steel 304 can not only guarantee the tensile strength of the welded joint but also improve its corrosion resistance in laser welding of E36 and stainless steel 304.