低溫冷卻用於Ti-6Al-4V微銑削之研究

Abstract

本論文針對微銑削時的表面黏屑、刀具磨耗和毛邊等問題，提出了以液態二氧化碳來輔助微銑削Ti-6Al-4V，並將結果與乾切削、高壓空氣輔助、最少量潤滑和濕切削的加工結果比較。 在乾切削環境下，使用刀徑0.5 mm銑削時，表面粗糙度會隨著切削速度的增加而減低，符合一般的切削趨勢；但在使用刀徑0.3 mm乾銑削時，在切削速度高於40 m/min時會有切屑黏附於加工表面，使得表面粗糙度變差，推測這是因為切屑細小且破碎，加上切削時的高溫使之黏附於表面上。使用液態二氧化碳輔助銑削因為能有效降低切削溫度，故能改善表面黏屑的情形，於切削速度70 m/min時，能將乾切削的Ra 值從0.09 μm減至0.04 μm；進給4 μm/tooth，切深20 μm，切削長度1200 mm之後並沒有刀腹磨耗，但乾切削在同樣實驗參數下卻有70 μm的刀腹磨耗；因為低溫降低材料延性，毛邊發生的現象也獲得改善，毛邊高度比乾切削時少了77 %。此外，為了增進以液態二氧化碳輔助加工的潤滑效果，本文提出以液態二氧化碳輔以最少量潤滑來銑削Ti-6Al-4V，實驗結果顯示切削速度70 m/min時，切削力較單純使用液態二氧化碳輔助低了12 %，且沒有發現表面黏屑的情形，顯示此策略既保有低溫冷卻的效果，也可增進潤滑能力，表面粗糙度從0.04 μm降至0.034 μm，改善了15 %，毛邊高度從32 μm減至16 μm，減低了50 %。

The use of liquid carbon dioxide to assist cutting so as to improve the encountered difficulties in micro end milling Ti-6Al-4V titanium alloy was proposed in this study. To confirm the effects of the proposed approach, experimental results were compared with those under dry cutting, high pressure air jet, minimum quantity lubrication (MQL), and wet conditions. It was found that the machined roughness decreased with the cutting speed as observed in the conventional cutting when a larger diameter end milling cutter (D = 0.5 mm) was used. On the contrary, the chips produced were tiny and broken when a smaller diameter cutter (D = 0.3 mm) was used. Under high cutting speed condition these chips could adhere on the machined surface and led to an increase of surface roughness.The application of liquid carbon dioxide assisted micro milling can effectively decrease cutting temperature, which in turn reduce the amount of chips adhering on the machined surface and lower the tool flank wear. The surface roughness Ra at a cutting speed of 70 m/min was improved from 0.09 μm under dry cutting to 0.04 μm under the proposed liquid carbon dioxide assisted cutting. And there were no flank wear at feed of 4 μm/tooth, depth of cut 20 μm and cutting length of 1200 mm. In addition, there were very few burrs left on the machined surface. The height of the burr was only 23% of that under dry cutting. In the end, the use of liquid carbon dioxide and MQL to assist cutting to improve the lubrication in the use of liquid carbon dioxide. The experimental results show that this method provided the lowest cutting force, burr height and surface roughness among all the cooling/lubrication conditions employed. And there were no chips adhere on the machined surface.