Investigation On Dissimilar Materials Bonding Process Of TC4 Titanium Alloy And 316L Stainless Steel With Selective Laser Melting

A titanium-steel dissimilar metal joint integrates the excellent properties of all the components and cover the shortages of the single phase.It is widely used in many fields such as petrochemical engineering,marine engineering and medical applications for its outstanding overall performance.Traditionally titanium-steel dissimilar material structure is often fabricated by welding,but all kinds of welding methods have some problems in common,such as high welding temperature,large welding deformation,large internal stress,welding cracks and other issues.So it is in the urgent need of a new technology to overcome the above problem.The rise of the selective laser melting technology provides a new idea for the bonding of titanium-steel dissimilar materials.Because of the small melting pool,high cooling speed during the fabrication process,dissimilar metal parts made by selective laser melting technology have some advantages such as high manufacturing precision,narrow fusion zone and heat affected zone,which can effectively refine the grain,reduce the matrix material deformation and improve the bonding strength,so as to obtain dissimilar metal joints with good metallurgical bonding and high mechanical properties.In this paper,the dissimilar materials bonding process of TC4 titanium alloy and 316L stainless steel with selective laser melting was investigated.We started with the research on process parameters of single materials following the steps:powder particle size analysis,single track experiments and multi-layer experiment.Then as-fabricated specimens were characterized by optical microscopy（OM）,scanning electron microscopy（SEM）,etc.According to the characterization result,we revealed the influence rule of process parameters such as laser power,layer thickness,scanning speed and scanning hatch on the surface quality,density,internal structure and mechanical properties of the molded parts.Five optimized combinations of process parameters were obtained for both materials as well.Based on the above results,we studied the bonding process of TC4 titanium alloy and316L stainless steel with selective laser melting.The oxygen content was strictly controlled during the test and the substrate was preheated to 200°C to reduce the accumulation of thermal stress.By observing the microstructures of interface,it can be found that intermetallic compound layer mainly consist of equiaxed grains,while in the 316L stainless steel region,the microstructure showed a transformation from columnar grains to equiaxed grains.At the same time,the X-ray diffraction method was employed to analyze the phase of the joint.The results showed that the intermetallic compounds were Fe₂Ti,FeTi,FeAl₂,TiC,Ti₃Al,NiTi,σ-（FeCr）.The presence of these intermetallic compounds is detrimental to the mechanical properties of the joint.Finally,this paper analyzed the mechanism of crack defects in depth.There are three main reasons for the generation and extension of cracks:the loss of low melting metal elements,the presence of intermetallic compounds,and the residual stresses due to differences in the thermo-physical properties of the two materials.