最少量潤滑(MQL)應用於NAK80模具鋼高速銑削之研究

Abstract

本文之目的係探討最少量潤滑(MQL)對於NAK80模具鋼高速銑削之影響。在使用TiAlN鍍層碳化物刀具及低黏度油之情況下，不論在較低或較高之切削速度，MQL均能改善乾切削之刀具壽命與表面光度，濕切削則因為劇烈熱震造成刀具的裂痕而導致刀刃的崩裂，以致刀具壽命最差，不適用於高速切削。在切削速度較低時，使用高黏度油可得到較好的刀具壽命。當切削速度較高時，低黏度油因為具有較多的低分子量成分，比較容易被蒸發而帶走熱能，可以發揮較佳的冷卻效果，同時也可延緩刀具黏屑的發生，有助於刀具壽命的改善。此外，油滴可增進MQL的冷卻效果，幫助改善刀具壽命。然而噴油量過多時，熱應力變化變大，使刀具容易發生裂痕，反而不利於刀具壽命。MQL還可在氧化物層的生成上扮演促進劑(promoter)的角色，因為MQL可以提供豐富的氧來促進切屑材料和TiAlN鍍層氧化作用之進行以形成氧化物層，該層氧化物可以發揮保護刀具之作用，有延長刀具壽命之效果。 基於上述之研究，本文提出MQL於NAK80模具鋼高速銑削的刀具壽命影響機制，除了一般所認知的冷卻效果可減少刀具軟化以延長刀具壽命之機制外，還包括過量的冷卻所造成熱疲勞會導致裂痕容易發生而對刀具壽命產生負面效應，以及氧化物保護層的形成對刀具壽命之正面效果，上述三項機制的綜合效應決定刀具之壽命。因此，以碳化物刀具高速銑削NAK80模具鋼時，若是使用TiAlN鍍層碳化物刀具及低黏度油，並適當的選擇切削速度及噴油量，刀具壽命將可因為MQL的適度冷卻效果及氧化物層的保護作用而獲得最顯著的改善。

The influence of the minimum quantity lubrication (MQL) in high speed milling (HSM) of NAK80 mould steel was studied in this thesis. The TiAlN coated carbide tool and the lower viscous oil were adopted. It is found that cutting under flood cooling condition results in the shortest tool life due to severe thermal cracks. While the use of MQL, in comparison with dry cutting, is beneficial to tool life and surface funish both in the lower speed cutting and the higher speed cutting conditions. The oil with a higher viscosity is preferable to increase the tool life in the lower speed cutting. In the higher cutting speed, the less viscous oil containing a higher fraction of low molecular weight components that could volatilize more easily is essential so that cooling effect can be effective and welding of chips can be delayed. It in turn leads to the enhancement of tool life. The oil droplets are proved to increase the cooling effect of MQL so as to improve the tool life. However, the tool life will be decreased due to cracks resulted from the large fluctuation of temperature on tool face when there is too much flow rate of oil in MQL. MQL can provide extra oxygen to activate the oxidation of the chip and TiAlN coating, as a result to generate the oxide layer. The cutting tool can be protected by this layer, which in turn leads to an increase of tool life. Based on this study, the mechanism dominating the tool life of HSM of NAK80 mould steel with MQL is proposed below. Firstly, the cooling effect by MQL can retain the strength and wear resistance of cutting tool, so as to enhance the tool life. Secondly, thermal fatigue induced by the excessive supply of oils would reduce the tool life. Thirdly, the formation of the protective layer can prolong the tool life. The tool life is determined by the combined effect of the aforementioned three factors. Hence, it is concluded that the tool life can be significantly improved due to the appropriate cooling effect by MQL together with the protection of oxide layer in HSM of mould steels with the use of TiAlN coating tool and lower viscous lubricant when cutting speed and flow rate of oil are chosen properly.