本研究主要探討放電加工參數與材料特性對放電加工表面裂紋敏感性的影響。實驗中先分析放電電流、放電持續時間以及衝擊因子對碳素工具鋼JIS SKS93表面裂紋敏感性影響，界定出臨界裂縫曲線後，再針對常用鋼料：SNCM439、SCM440、SKS2、SK2、SKH9和SKD11進行放電加工，並將表面裂紋型態做歸納，次探討其成因。
綜合七種鋼料之放電裂紋型態，可歸納為下列五種：穿孔裂紋、交界裂紋、輻射狀裂紋、網狀裂紋與積碳裂紋，其中穿孔裂紋、交界裂紋以及網狀裂紋為最基本的裂紋擴展型態，而輻射狀裂紋與網狀裂紋多見於長持續時間之加工表面，積碳裂紋則大多於衝擊因子較大時，不穩定放電下所形成。
研究結果顯示，藉由提高放電電流不但可有效改善材料移除率，並能降低材料裂紋敏感性，然而提高放電電流雖能有效降低表面裂紋密度，但當電流超過8A以上時，所產生之裂紋寬度往往較寬，且深入至熱影響區的比例亦較高。而當持續時間為70μs以下時，總裂紋面積則有隨放電電流增加而減少的現象。此外，衝擊因子越大，表面裂紋密度越高，但本實驗改變衝擊因子並未造成白層厚度及組織的不同，因此裂紋寬度並無顯著的差異存在。鋼料裂紋敏感性除了隨放電電流的增加與持續時間的縮短而降低外，其合金元素所換算之碳當量值越小，或熱傳導係數越高時，亦能有效降低其裂紋敏感性。綜合所述，鋼料裂紋敏感性由小至大依序為：低合金鋼、碳素工具鋼、高速鋼、冷作工具鋼。

The research is to evaluate the crack susceptibility of EDMed surface by changing the parameters of EDM and material properties. In the beginning of this research, the surface crack susceptibility of JIS SKS93, a high carbon tool steel, was analyzed with EDM parameters：peak current, pulse duration, and duty factor, and then marked a crack critical line (CCL) curve. Furthermore, the type of EDMed surface crack of JIS SNCM439, SCM440, SKS2, SK2, and SKH9, the steel in common use, was generalized and discussed as its relation.
According to the former results, the EDMed surface cracks could be categorized as Pore Crack, Contour Crack, Network Crack, Radiate Form Crack and Arcing Crack. Among them, the former three cracks are assumed as basic types. Radiate Form Crack and Network Crack are thought to occur under long pulse duration processing, and Arcing Crack usually appears due to unstable discharge machining.
The results revealed that a higher peak current could not only improve the material removal rate, but also reduce the surface crack susceptibility. Although the peak current may restrain the density of surface crack, it promoted the cracks to become wider and made the cracks extend into the Heat-Affected Zone as it exceeded 8A. When pulse duration time is below 70μs, the total area of crack reduces with the raise of peak current. Moreover, the larger Duty Factor is, the higher the density of surface crack. The crack widths of various Duty Factors are alike due to the similarity of thickness and the analogy of microstructure of white layer in all conditions. The results also indicate that not only increasing the peak current and shortening the pulse duration time, but also a lower carbon equivalent and a higher thermal conductivity of steel could reduce and improve the surface crack susceptibility. As a conclusion, the steel sequence from low crack susceptibility to high one is low alloy steel, carbon tool steel, high speed steel, cold work steel.

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