The Special Micro Forming Process Of Cu-based Metal Powder And Perfomance Evaluation

Micro parts play an important role in micro-electromechanical systems(MEMS), and have been widely applied in biomedical, aerospace, telecommunications and other fields. With the increasing demand for micro parts, its performance and manufacturing processes have been put forward higher requirements. Because of the unique manufacturing process characteristics of powder metallurgy technology make it possible to fabricate micro parts. In view of the problems of some special powder metallurgy technology in actual application, such as the long production cycle, the complex process, and the narrow application scope, the micro ultrasonic powder moulding(micro-UPM) was applied to fabricate micro parts by high melting point alloy powder firstly. A mixture of a high-melting point alloy powder and a low-melting alloy powder in a certain proportion was used as the raw material. The thermal energy produced by the strong friction and squeeze among the powder particles under ultrasonic vibration quickly melted the low melting powder. Then, the micro parts were fabricated via micro-UPM in a special semisolid forming process. However, the micro-UPM process has strong selectivity for metal powder and poor adaptability. Therefore, micro resistance weld moulding(micro-RWM) was proposed to prepare micro parts, which use the resistance heat generated when the current flow though metal powder to achieve the forming of micro parts. Due to the good forming quality, low energy consumption, short cycle of the process, and significantly improvement of the thermal and mechanical properties of micro parts, which not only propose a new method for the preparation of micro parts, but also provide new ideals on theoretical and practical application. The main content of this dissertation include:(1) The micro-UPM process was applied to fabricate Sn42Bi58/Cu composite micro parts. The combined mold was designed, the experimental procedure and process parameters were determined. A mixture of Sn42Bi58 alloy powder, copper powder and copper fiber was used as raw material to fabricate different specification micro gears successfully by micro-UPM, utilizing rapid melting features of low melting Sn-Bi alloy. The micro gears had a clear contour, good forming quality, thus the feasibility of fabricate micro parts with high melting point powder via micro-UPM was verified.(2) The strengthen mechanism of micro parts was investigated. Two groups Sn42Bi58/Cu samples were prepared with the same process parameters, the samples in one group were sintered while samples in another group without any treatment. Through the differential scanning calorimetry(DSC) analysis, after sintering the melting point of samples was increased from 139.65? to 791.34?, which increased by 447.62 % and its thermal performance was significant increased. The results of metallurgical analysis of samples showed the microstructure of sample became more density after sintering, while the copper powder and copper fiber wrapped together by the melted Sn-Bi alloy, constituting an integral microstructure of the sample before sintering. In addition, the Cu-Sn composition was detected in the X-ray diffraction(XRD) indicated that the alloying occurred in the samples during sintering. The conclusion was verified in the following tensile test, compared with the tensile strength of the samples before sintering the value increased by 99.17%, reaching 179.09 MPa, the breaking elongation also increased to 36.83% while the contraction rate was 1.21% after sintering. These results indicated that the alloying process between componets of the sample caused by sintering can effectively improve the thermal and mechanical properties of micro parts.(3) In order to solve the problem of strong selective for raw material of micro-UPM result from the low energy, the micro-RWM was applied to fabricate micro parts with metal powder. The experimental platform was build and the forming molds were designed. A CuSn33 alloy powder was used raw as materials to fabricate micro parts with good quality, different contour and size by micro-RWM. Thus the feasibility of micro-RWM was verified.(4) The micro-RWM process principle and the effect of different process parameters on the properties of the sample were studied. Samples were prepared with different voltage values to study its impact on the performances of the sample. In addition, the possibility of improve the performances of sample was discussed by using the same process parameters to prepare an array of samples at different sintering temperature. DSC, metallographic analysis, EDS, XRD and micro-hardness testing were used to characterize the performance of the samples. The samples exhibited the same thermal performance before and after sintering but possessed better hardness prior to sintering. Increasing the diameter of the sample decreased the hardness prior to sintering. The hardness of the sintered samples with the same diameter increased before decreasing as the temperature increased. The size of micro parts suitable for micro-RWM includes a horizontal size of less than 4 mm and a height of less than 2 mm.