| The complex aircraft components such as the impellers are difficult to manufacture because of their complicated shapes and high requirements of mechanica l properties. Hot isostatic pressing technology can take the advantage of the densification of the powders to form the complex components to solve forming problem of impeller. Besides, those components usually need a functional coating layer on the surface to improve the reliability, which adds an extra production cycle after manufactured. However, all the presented studies just used HIP process to form a single material part. The part fabricated needs to be done with the subsequent coating process, which increases the manufacturing cycle and cost.In this paper, an integrated forming process was proposed to manufacture a complex-shaped impeller together with the surface coating layer using hot isostatic pressing. For forming the Ti6Al4 V impeller with a wear-resistance surface coating, this paper studied the materials chosen of the cores and the design of the HIP mold in this new process, and successfully fabricated the impeller with the surface coating, and then revealed the inner principle of this new process through the performance evaluation of the impeller. The paper mainly focused on the following aspects and achieved corresponding achievements:(1) Through the simulated data and the analysis of the shape-controlling between the graphite core and metal core(H13/T8/Cr12), it was found that the shape-controlling effect was influenced by the relationship between the high-temperature yield characteristics and the core/powder interaction forces: the higher the high-temperature yield characteristic of the core, the better of the shape-controlling effect. This provides the theoretical foundation for hot isostatic pressing of high-precision components.(2) In order to verify the shape-controlling effect of the graphite core and the feasibility of the integrated forming process, a HIP mold with the screw graphite core was designed. The results showed that the screw has a precise shape without any flaws, which verified the validity of the graphite core from the living example. Besides, the Ni coating which was pre-deposited on the surface of the core uniformly diffused to the surface of the screw.(3) According to the structure of the impeller, a HIP mold was designed to form the impeller. And then the HIP experiment was conducted by using the integrated process. T he impeller had a relative density of 99.1%. By using the graphite cores to control the shape, the three main dimensions of the blades have a 0.15 mm precision. Besides, the relationship between mold design and component precision was concluded from three aspects: mold materials, mold structure and HIP process.(4) The microstructural analysis showed the Ni coating layer deposited on the surface of the graphite core has successfully diffused to the surface of Ti6Al4 V impeller, and formed a Ti2Ni/TiNi coating layer with a thickness of 140μm which has a function of wear resistance. The result of the tensile experiment shows that the HIP Specimens have a yield strength of 930 MPa and a tensile strength of 1025 MPa, which are better than the same sized parts manufactured using forgings. The hardness and wear loss were 690 HV and 112 mg, respectively. The hardness and wear resistance have an obvious improvement after forming a Ti2Ni/TiNi coating layer. The surface of the impeller has a roughness with Ra=1.86μm. |
PDF Full Text Request