Study On The Crack Formation Mechanism And Control Methods Of Laser Micro-welded Joints Of NiTiNb And TC4 Dissimilar Alloys

NiTiNb shape memory alloy and TC4 titanium alloy have a broad application prospect in aviation, aerospace, automotive, biomedical materials, and other fields, but when NiTiNb same material and NiTiNb/TC4 dissimilar materials are welded, the joints are easy to crack. It restricts the application of NiTiNb and the dissimilar materials welded structures with TC4 titanium alloy severely. This paper studied the crack sensitivity, crack formation mechanism and control measures of the same material of NiTiNb, and NiTiNb/TC4 dissimilar materials of laser micro-welded, then obtained the following main results:When laser butt weld of the same material of hot rolled state of NiTiNb, surface impurities are responsible for the cracks. Cracks are obsevered on the joint which don’t remove surface impurities, but the joint forming is good when the surface impurities are removed. On this basis, the rule of the effects of the welding process to the joint performance is studied, and the welding process is optimized by orthogonal method. When laser welding parameters are: the average power is 16.8 W, pulse width is 4.5 ms and frequency is 4.5Hz, the maximum tensile strength of the welded joint is 880 MPa, which is up to 95% of the parent metal’s tensile strength. Shape memory performance of the joint annealed at 500℃ up to 88% of the matrix.When laser welding of dissimilar materials of NiTiNb and TC4, the cracks are found in the weld when the laser power is small and the weld is just penetration. But the cracks don’t eliminate by changing the parameters of the laser process. When the laser speckle biases to the TC4 side, the number of cracks reduce significantly. The sources of transverse cracks are near the fusion line of Ti2 Ni phase. Among the three kinds of compound of Ti2 Ni, NbNi3 and TiNi, Ti2 Ni has the minimum formation enthalpy, and the minimum Gibbs free energy, which is unavoidable in the welding. Longitudinal crack is due to liquid membrane composed of low melting point eutectic enrichment in the grains boundary, then the crystallization cracks initiating under the action of welding stress.By filling Ni、Ti and Nb, may inhibit the formation of cracks during laser micro-welding of dissimilar materials of NiTiNb/TC4 in varying degrees. For lap joints of NiTiNb/TC4 dissimilar materials, when put TC4 in upper and put 50μm or 100μm thickness of thin plate of Ni in the middle, the joint has large crack sensitivity, but when NiTiNb places in the upper, put 50μm thickess of thin plate of Ni in the middle, no crack is found in the weld. However, brittle Ti2 Ni phase is fonud at the bottom with parent metal. For Ni TiNb/TC4 dissimilar materials butt joints, when filling in a pure Ni wire with diameter of Φ300μm, there is a crack near the fusion line at the side of TC4. A Ti2 Ni phase area about 20μm wide is found at the side of the TC4, while the combination is very good at the side of NiTiNb. The maximum tensile strength of the joint is 322 MPa. Fill in a pure Ti wire with the diameter of Φ300μm, micro cracks are found in the surface of the weld. A Ti2 Ni phase zone about 10μm wide is observed at the side of NiTiNb. The maximum tensile strength of the joint is 447 MPa. Fill in a pure Nb wire with the diameter of Φ300μm, the forming of the weld surface is very good, and no crack defect in the joint. The maximum tensile strength of the joint is 740 MPa, which is up to 82% of the base material of annealed state of TC4.