Control of intermetallics in the interface in directed energy deposition of titanium on stainless steel 316L

Application of a single metal or alloy is often restricted by its properties from optimal combination of performance and cost. Therefore, there is a vast need of joining dissimilar metals for various applications in biomedical, aerospace, automobile and many other fields. However, for many dissimilar metals, brittle intermetallic compounds (IMCs) are formed at the joint. Plus due to the often mismatching coefficients of thermal expansion (CTE), material failure easily happens at the joint. This problem has long existed for conventional joining techniques. Titanium(-alloys) and stainless steel (SS) 316L is a representative material pair that suffers from this problem. Brittle IMCs such as Fe2Ti and FeTi are formed in the joint upon mixing of the base materials. Many joining techniques like diffusion bonding, explosive welding, laser welding, friction welding, etc. have been investigated to produce a sound joint between Ti(-alloys) and 316L. Either the joint exhibits poor mechanical strength, or the form of the base materials is largely limited. To the best of the author's knowledge, a convenient and reliable solution to the joining of Ti(-alloys) and SS 316L remains to be developed.Additive manufacturing (AM) is a solid freeform manufacturing technology that builds three-dimensional (3D) parts by progressively adding thin layers of materials in pre-determined geometry. Directed energy deposition (DED) is an AM technology with unique advantages in building up dissimilar metals directly within a single work piece. Thus, gradient materials or steep transitions between two base metals can be realized.The present research is focused on joining Ti and 316L by DED with a 500 W maximum power continuous wave (CW) 1068 nm laser beam. Several results have been achieved in this PhD work.With the DED of Ti single tracks in the conventional process window, the deposition geometry, microstructures, chemical composition transition and phases have been studied. From the 316L side to the Ti side, four possible interfacial regions have been observed in the deposition with decreasing Fe concentration. Three types of composition transitions were found. Low melt depth and dilution ratio are found to lead to complete transition from 316L to pure Ti, with the IMCs constrained within two thin interfacial bands at the bottom of the solidified melt pool. The transition distance, namely the total thickness of the interfacial bands, is found in a range of 20~70 ÎŒm.DED with high powder feedrate, high nozzle velocity and high laser power is found to be beneficial for reducing the total thickness of the IMC-containing interfacial bands while forming acute contact angles on the lateral sides of the single track. Powder feedrate of 21 g/min, nozzle velocity of 13000 mm/min and laser power of 500 W is a representative parameter set. A transition layer thickness down to around 11 ÎŒm has been achieved. With Ti cuboids deposited on top of layers printed of such parameter sets, the ultimate shear strength (USS) of the interface is measured in a range of 45~153 MPa.Thermal management further improved the USS of the interface. Preheating to 520℃, holding the temperature for an hour after deposition finished and cooling down at 25℃/5min increased the USS to a range of 173~381 MPa. The highest USS achieved is 381±24 MPa, exceeding the weaker base material pure Ti (371 MPa) and below the stronger base material 316L (517 MPa).