放電加工於純鈦金屬染白研究

Abstract

鈦的氧化層生成傳統上以陽極處理為主，近年來也有學者成功以放電加工生成之，但無論生成方法為何，除非以染料染色，否則至今罕有方法可直接在鈦金屬表面生成白色氧化層。 本研究使用一般商用的放電加工機，以石墨為電極，自來水為加工液，在純鈦工件上加工生成白色氧化層並探討放電加工主要參數對該層生成的影響。研究結果顯示極性是決定白色氧化層生成與否的首要條件，白色氧化層只有在純鈦工件接陽極時才有可能生成；純鈦工件接負極時，生成的反而是塊狀結晶物。經由SEM觀察、EDS分析、XRD晶格結構判別及AES縱深分析後得到純鈦工件接陽極時所生成的白色氧化層乃是由從純鈦工件表面析出的二氧化鈦（TiO2，anatase）晶粒所組成，析出高度約5∼6 μm；純鈦工件接陰極時所產生的塊狀結晶物是氧化鈣（CaO）結晶，邊長約1∼2 μm。 另外以開路電壓、on time、off time、加工電流、加工時間、放電時間、電極拉起距離及間隙電壓等八個參數配合田口方法中的直交表來探討白色氧化層的生成條件。發現影響白色氧化層生成的重要因子依序為開路電壓（貢獻度50.85％）、加工時間（貢獻度14.49％）、on time（貢獻度11.42％）及off time（貢獻度8.06％）。白色氧化層的生成，必頇在特定條件下，即開路電壓要大（220 V）、加工時間要足夠（2 min），on time與off time的比例要適當，否則白色氧化層的寬度將減小，甚至沒有生成。最佳化條件為開路電壓 = 220 V、加工時間 = 2 min、on time = 400 μs、off time = 4 μs、電極拉起距離 = 1.5 mm、放電時間 = 5 sec、間隙電壓 = 60 V及加工電流 = 1.5 A，此條件下白色環的寬度可達1900 μm。

The main process to fabricate titanium oxide layers is traditional anodizing although some researchers developed EDM process to fabricate it. So far there are few methods to fabricate white titanium oxide layers on the titanium workpiece surface directly except for dyeing. A commercial EDM machine was used in the study. Graphite was used as electrode, pure titanium was used as workpiece and dielectric fluid was tap water. The study investigated the influence of EDM parameters including open voltage, on time, off time, peak current, machining time, discharging time, electrode pulled distance and gap voltage on fabricating white titanium oxide layers. Taguchi method was applied in the experiments to find the growth conditions of white titanium oxide layers. The result reveals the polarity is the main key to fabricate white titanium oxide layers. The pure titanium workpiece must be connected to anode for fabricating white titanium oxide layers. If The pure titanium workpiece was connected to cathode, the products were cubic crystals instead. According to the analysis of SEM, EDS, XRD and AES depth profiling analysis, The white titanium oxide layers consist of titanium dioxide (anatase) grains which precipitated from the surface of pure titanium workpiece. The height of the grains is about 5 ~ 6 μm. When the pure titanium substrate connected to cathode, the produced cubic crystals are crystalline calcium oxides (CaO) with a side length of 1 ~ 2 μm. The degree of significant factors influencing the width of white titanium oxide layers from large to small as follows: open voltage (50.85% contribution), machining time (14.49% contribution), on time (11.42% contribution) and off time (8.06% contribution). Larger open voltage can form wider width of white titanium oxide layers. had better set to be be large (220 V). The machining time to be long enough (2 min). The ratio of on time and off time must to be appropriate to obtain there were seldom white titanium oxide layers.