Study On Spray Forming For Hot Die

Spray forming was a major innovation in metallurgy in the twentieth century, which could overcome the shortcomings of the traditional casting process, such as coarse grains, carbide network and so on. Compared with powder metallurgy, spray forming not only had its advantages,such as homogeneity of structure, eliminating macrosegregation and so on,but also overcame the interface oxidation, complicated production process and other shortcomings. Spray forming was successfully applied in the high-alloy materials, tool steel etc. Precision spray forming combined with rapid manufacturing technologies,which could achieve high performance, complex shapes, near net shape forming mold,and became the latest development in the field of spray forming.This paper described a self-designed small and general precision spray forming device,and discussed melting installations, atomization devices and deposition apparatus of it.Precision spray forming mode of engine valve was studied,as-deposited and as-cast micro structure of HM1 alloy was analysed, using optical microscopy, X-ray diffraction, scanning electron microscopy and energy spectrum analysis, compression testing machine.And its density, hardness and mechanical properties also was tested and analyzed, and discussed the fracture mechanism of the compression fracture.A self-designed small and general precision spray forming device was designed,and its melting equipment with a 20 Kg crucible did not use tundish,result in higher heating speed, lower power consumption. Atomizer with secondary porosity could effectively reduce the phenomenon of anti-spray. Transmission device with 3 degrees of freedom could spray forming various kinds small die with rectangular cavity and the rotary cavity.Working about technological parameters of decision spray forming, the optimal technological parameter was obtained. It was:the best length of diversion pipe was out 3mm, descent speed of deposion was 0.5mm/s,rotation velocity of substrate was 40 rpm, spray gas pressure was 0.6MPa,the diameter of the delivery tube was 4mm, melt temperature was about 1580°C,spray heiht was 135-145mm. Micro structure observation showed that the grain size of as-cast alloy was about 100μm,and carbide network was distributed in the grain boundary. The grain size of as-deposited alloy was about 20μm, and lots of small carbides which were distributed in the grains and grain boundaries,improved the mechanical properties of materials.The results of mechanical properties showed that as-cast and as-deposited materials both had the higher compressive strength of materials. The compressive strength of as-cast material was 2650 MPa,and its compression ratio was 40.8%. Because holes existed in the as-deposited material, especially angular holes,which reduced the compressive strength of as-deposited material. The compressive strength of as-deposited material was 2200 MPa, and its compression ratio was 42%.X-Ray, SEM results showed that the phases of as-cast and as-deposited materials both were martensite, residual austenite and Cr7C3 carbide. As-deposited rockwell hardness was between 41 and 45,when test samples tempered 2 hours at the 600°C, martensite was resolved into tempered martensite and residual austenite was also changed into tempered martensite, which leaded to increase hardness in HRC 3 5, reaching HRC46-48. Fracture mechanism of as-cast and as-deposited materials was shear fracture and microvoid coalescence fracture,and transgranular fracture were dominant. And as-deposited material had more dimples than as-cast condition.Spray forming technique of the mould of engine valve was preliminary studied,and the diameter of die was 78mm, height was 12mm. The experimental results demonstrated that outline of the mold cavity was clear,and the micro structures of the depodition were composed of fine grains,density was above 95%,hardness was between 41 and 45. Although the surface quality still could not reach requirements of the net form shape, after a slightly process,it could be used in the production of forging, which basically reached net-shape forming standards.The study showed that the manufacturing of valve forging die using jet mold was achieved by making further efforts to improve the forming technology and enhance the surface quality.