Objective Traditional forging and machining technologies, which are used to produce titanium alloy parts, often involve long lead times and considerable material waste. It is much more effective to repair titanium alloy parts which are damaged due to wrong machining or are worn after long service than to simply discard them. As an advanced repair technology, laser repairing is often adopted to repair damaged titanium alloy parts. However, due to significant differences in microstructures and mechanical properties between the repair zone (RZ) and the titanium alloy parts, the mechanical properties of the titanium alloy parts that have been repaired by laser repairing are usually unwanted. This study proposes a novel type of repair technology that combines point-mode forging (PF) and laser repairing (LR) (called PF-LR) to repair TA15 titanium alloy forgings.
Methods The PF-LR experiment was conducted using the in-house PF-LR system, which consists of a 3300 W fiber laser, a powder feeder, a coaxial powder delivery nozzle, and a four-axis computerized numerical control (CNC) PF-LR working table. The powder size of the TA15 titanium alloy is approximately 150 μm. The TA15 titanium alloy forging is 80 mm long, 20 mm wide, and 6 mm thick. An argon-purged chamber with oxygen content of less than 6×10 -6 was used to prevent oxidation of the molten pool. In the PF-LR process, first, a 0.5 mm thick layer of TA15 titanium alloy was deposited on the top surface of the forging. Next, the laser cladding layer of TA15 titanium alloy was forged point-by-point. Both LR and PF were performed alternatively until completion of the repair task. The LR and PF processing parameters are as follows: laser power (1500 W), spot size (3 mm), laser scanning speed (120 mm/min), LR overlapping ratio (30%), powder feed rate (8 g/min), reduction (0.2 mm), and PF overlapping ratio (20%).
Results and Discussions The RZ of TA15 titanium alloy consists of equiaxed grains with an average size of approximately 200 μm. The microstructure of the RZ consists of basket-weave microstructure and transformed β. The microhardness of the wrought substrate zone (WSZ) is approximately 365 HV, which is lower than that of the RZ (405 HV). The microhardness in the heat affected zone rises sharply from the WSZ to the RZ, which means that the interface strength between the WSZ and the RZ is greater than that of the WSZ. Because of the smaller equiaxial grains and fine microstructures, the yield strength, tensile strength, and ductility of the RZ are 20.5%, 23.3%, and 93.7%, respectively greater than the minimum standard of aero-tensile mechanical properties of TA15 titanium alloy forging. The mechanical properties of TA15 titanium alloy forging, which contains 10% volume fraction of the RZ, are superior to the minimum standard of aero-tensile mechanical properties of forging. With the increase of the volume fraction of the RZ, the mechanical properties of forging repaired by the PF-LR technology increase gradually. Due to the coarse grain size and Widmanstatten structure, the tensile fracture mechanism of the WSZ exhibits a transgranular model with quasi-cleavage feature. The fracture morphologies of the forging containing 30% volume fraction of RZ showed a gradual transition model from the brittle fracture of the WSZ to the ductile fracture of the RZ.
Conclusions PF-LR technology can be used effectively to repair damaged titanium alloy parts. This novel technology can produce a RZ of equiaxial grains in the forging of TA15 titanium alloy, whose grains are equiaxed. Because of the excellent mechanical properties of the RZ and the strong interface strength between the WSZ and RZ, all forgings with 10%, 30%, and 50% volume fraction of RZ reach and exceed the standard of aero-tensile mechanical properties of TA15 titanium alloy forging. This indicates that the PF-LR technology is completely appropriate for the repair of damaged TA15 titanium alloy forgings, which have flaws of different sizes.
