Cu/SiCp複合材料之製程、熱膨脹與放電加工/研磨特性研究

Abstract

放電加工技術為非傳統加工技術中最為業者普遍採用之一種機械加工技術，尤其在模具製作領域中，佔有極重要的角色；放電加工性能之良窳，實質影響到加工速度及精度。 對於放電加工方面之研發，通常係從三方面著手: (一)、機械本體精度及控制之改良。(二)、放電加工參數最佳化之選擇，及在各種難切削材料之放電加工測試。(三)、複合放電加工法之導入。本論文係針對第三項，提出一種結合放電加工與研磨加工之新穎加工方法，將粉末冶金法製作之銅基複合材料電極應用到放電複合加工，而大大改善放電加工速度緩慢之缺失。 為實踐前述新加工方法，必需研發新電極材料來達成，本論文於是選擇具有高導電性和能夠在高溫環境下操作之銅基複合材料進行相關性研究；此種複合材料的製作方法則是選用能夠有效增加基材和加強相間之鍵結及適合大量生產之粉末冶金加工技術。為得到較佳之SiCp和Cu間之鍵結效果，本論文在SiCp表面事先進行無電鍍處理，所製得試片並與未鍍銅之試片進行分析比較。 本論文共分六章，涵蓋了Cu/SiCp 的製程、材料特性分析及在放電研磨複合加工之應用。第一章在介紹本論文之目標及方向。第二章文獻探討部分詳細解說複合材料之發展、分類、製備方法以及銅基複合材料之應用情形，以做為研發新素材之基礎知識；另外對放電加工(EDM)、放電研磨(EDG)以及放電及研磨加工(EDMG)亦加以介紹。第三章則對Cu/SiCp 複合材料製備方法、機械性質分析、以及在放電加工之應用進行探討，以製得具有高強度、高導電性、高放電加工速度及低消耗之電極材料。第四章則對Cu/SiCp 複合材料之微結構及熱膨脹特性進行分析與學理探討，以製備SiCp分散性優良、熱膨脹係數低之精密加工用電極。其中對所製得的複合材料進行從50到550℃間之熱膨脹特性量測及分析，這些熱膨脹特性並和各種理論模式進行比較。第五章則對Cu/SiCp 複合材料電極在放電及研磨加工之應用結果進行探討，並提出該放複合放電加工之應用模式。第六章則為整個論文之結論及未來可繼續進行之研究主題。 經由相關之實驗及研究，本論文之具體結論如下： 一、粉末冶金法是一種製作Cu/SiCp複合材料的極佳方法。 二、SiCp強化粒子經無電鍍銅膜後所製作之Cu/SiCp複合材料，可以在Cu和SiCp間得到極佳之鍵結效果。 三、SiCp強化粒子經無電鍍銅膜後所製作之Cu/SiCp複合材料比未鍍銅膜者有較佳之機械強度、較高密度與較低之熱膨脹係數，而且SiCp在銅基材中之分佈均勻性較佳。熱擠製過程可使複材密度達到理論密度之95%以上。拉伸強度、密度隨SiCp含量之增加而降低，電阻係數則隨SiCp含量之增加而變大。 四、Cu/SiCp複合材料隨SiCp含量之增加而產生較大內應力，經過受熱冷卻後，會因為熱應力釋放而產生較大熱膨脹係數。複材 熱膨脹係數會隨Cu和SiCp間之鍵結改善而降低。 五、Cu/SiCp複合材料之熱膨脹係數和熱遲滯應變通常隨SiCp顆粒增大而變大，而且從高溫降溫到室溫時均呈正向熱遲滯行為(thermal hysteresis behavior) ，熱應變大小則為電極中含碳化矽體積分率量和熱循環次數之函數。 六、以Cu/SiCp複合材料製作之放電加工用電極進行放電加工測試，其材料去除率會隨SiCp含量之增加而變大，亦即加工速度變快，但電極消耗速度亦會增加。 七、以Cu/SiCp複合材料電極應用在放電及研磨複合加工，在適當之電極旋轉速度、SiCp顆粒大小以及放電電流條件下，可以有效的增大加工速度，而確立EDMG之新加工機制。 八、模具鋼經EDMG後之表面比放電加工後之表面，其表面粗度有相當程度之改善，但兩者加工面表面形貌也有極大之差異。

On mold manufacturing industry, the electrical discharge machining (EDM) has been adopted for years; it is the most important machining tool for this industry. For progressing the EDM, three fields are always concerned; (1) to improved the machine set accuracy and update the control system, (2) to determine the optimum machining parameters for selected materials, (3) to composite EDM with others machining method to broaden the utility of EDM. This thesis aims at the third field, and tries to develop a new method, termed electrical discharge machining and grinding (EDMG), which increased material removal rate effectively. To fulfill the EDMG operation, a Cu/SiCp composite material electrode with high electrical conductivity and capable of operating at high temperature was fabricated and studied. Although there are various methods for fabricating metal matrix composite materials, the powder metallurgy method, feasible to enhance matrix/reinforcement bonding and mass production, is selected to fabricate the Cu/SiCp composite in this thesis. In order to obtain optimal bonding between SiCp and Cu particles, the electroless plating steps were introduced. The thesis is composed of six chapters and they cover all fundamental aspects of Cu/SiCp, from the fabrication and characterization of Cu/SiCp, to the application in EDM and EDMG. The first chapter provides the objective of this thesis. Chapter 2 provides definition information and the development of composite materials and metal matrix composite materials. The principles of EDM, EDG (electrical discharge grinding) and EDMG are also introduced in this chapter. Chapter 3 describes the fabrication and characterization of the Cu/SiCp composite and its application in EDM, The objective of this chapter is to investigate the mechanical properties of Cu/SiCp and the optimum composition of an electrode for EDM usage. Chapter 4 focuses on the microstructure and the thermal expansion property of Cu/SiCp. Thermal expansion property of as-formed product was measured in the temperature range from 50℃ to 550℃. The composites exhibited positive thermal hysteresis behavior when cooled down from the peak temperature to room temperature. The magnitude of this strain was a function of the SiCp volume fraction and the number of thermal cycles. The thermal expansion property of composites was compared with those predicted from various theoretical models. The feasibility of using a copper matrix composite electrode to perform the EDMG operation on mold steel is discussed in chapter 5. Cu/SiCp electrode with a rotating device was made and employed to study the EDMG technology. It was found that 3-7 times the normal EDM material removal rate could be achieved in EDMG under suitable conditions of electrode rotating speed, SiCp particle size and current. Thus a new revised machining methodology, termed electrical discharge machining and grinding, based on conventional EDM is formally proposed. Finally, discussion and general conclusions are in Chapter 6. Overhauling the present work, the following conclusions can be drawn. 1. The powder metallurgy method is a feasible fabricating process for SiC particulate reinforced copper composites. 2. Improved bonding can be achieved by activation of SiC powder surface through electroless-coating of Cu on to SiC powder. 3. The density of composites can be improved to a certain extent by coating a copper film on SiCp. The hot extrusion process can improve the density of the composites up to 95% of the theoretical density value. The promoted bonding between SiCp and Cu by the copper coating method lead to higher hardness. Both tensile strength and density of composite decrease with increasing amount of SiCp. The electrical resistivity increases with a higher SiCp content. 4. A larger SiCp content in the composite would induce more stress accumulation in the matrix, and more stress releasing would occur during the heating and cooling cycle, causing a larger CTE value. The CTE of metal matrix composite could be decreased effectively through good bonding between the reinforcement phase and metal matrix. 5. The CTE and thermal hysteresis strain generally increases with increasing SiCp particle size. The composites exhibited positive thermal hysteresis behavior when cooled down from higher temperature to room temperature. The magnitude of this strain was a function of the SiCp volume fraction and the number of thermal cycles. 6. In EDM, a higher material removal rate can be achieved at moderate SiCp content. However, tool wear ratio increases with increasing SiCp content. 7. The effectiveness of EDMG with a Cu/SiCp electrode is confirmed from the observed results, leading to a much higher material removal rate under suitable conditions of electrode rotating speed, SiCp particle size and current. 8. The morphology of EDMGed surfaces of mold steel differ greatly from that of EDMed surface and the surface roughness processed by EDMG is apparent improved.