超音波輔助鑽孔碳化矽之研究

Abstract

碳化矽(SiC)是一種機械性質表現優異的先進陶瓷材料，其性質包括高硬度、耐高溫與化性穩定，常使用在處於嚴苛條件下之零組件。但其優異之機械性質同時使其擁有難加工之特性。本研究目的為探討超音波輔助加工(UAM)應用於碳化矽之材料移除機制，瞭解超音波振動所扮演之角色，並討論在超音波輔助下各種加工條件之影響，最後提出如何得到較佳材料移除率與工件品質之策略。 本研究使用之超音波主軸是由主軸內部提供一垂直方向(工具機Z軸)之縱振動共振於刀具。根據研究結果，超音波輔助鑽孔在加工時因振動使磨粒軌跡呈現波浪狀，加上磨粒間的交互影響，在振動週期的任何瞬間，磨粒與材料皆呈現部分分離、部分接觸的狀況；超音波振動亦在工件上產生許多小坑洞與尖點。當磨粒接觸材料時，材料尖點局部溫度上升導致局部抗壓強度降低，材料容易移除；同時由於振動持續作用，使表面繼續產生尖點。此兩程序持續循環，因此超音波輔助加工之切削力顯著降低。研究結果顯示，鑽石磨棒配合超音波輔助應用於碳化矽微孔加工可得到很好的效果，包含降低切削力、增加材料移除率與得到較少的缺陷。使用ψ1鑽石磨棒時，高主軸轉速與高超音波振幅之加工條件會得到較低的切削力。在進給0.1 μm/rev之條件下進行ψ1鑽孔加工，超音波輔助使推力由20 N降低至2 N，推力僅為傳統加工的10%；孔口崩落亦由139.5 μm變為無崩落。超音波輔助可使進給由0.1提高至0.3 μm/rev而不出現刀具損壞，使材料移除率變為3倍，且僅有少量的缺陷。超音波輔助加工在高進給時(0.3 μm/rev)會在孔底中央處產生約10 μm之小崩落，而低進給時(0.1 μm/rev)可得到無崩落之工件；若一開始以高進給速率加工，在最後接近孔底時轉換成低進給速率將中央之小崩落去除，即可兼顧材料移除率與工件品質。

Silicon carbide (SiC) is an advanced ceramic material with great mechanical properties, including high hardness, endurance of high temperature and chemical stability. Therefore, SiC often uses in extreme condition. But its great mechanical properties lead to difficulties of machining. The first purpose of this study is to investigate the material removal mechanism of ultrasonic-assisted machining (UAM) of SiC and understand what role is ultrasonic vibration. And second one is to discuss influence of various machining parameters. Last one is to bring up a strategy for better material removal rate (MRR) and workpiece quality. In this study, the ultrasonic spindle provides longitudinal vibration which is automatically resonating with tools and in the Z-axis direction of machine tool. According to studying results, abrasives path is wavy in drilling process due to the vibration of UAM. Because of the interactive influence between abrasives, the relationship between abrasives and material becomes part separation and part contact in any moment of vibration period. Vibration also generates many cusps and tiny cavities. When abrasives contact with material, the local temperature of cusps increases which decreases material’s local compressive strength; at the same time, vibration continuously generates cusps. This two process revolve repeatedly so the cutting force of UAM decreases significantly. According to studying result, drilling small holes in SiC with UAM could get many good effect, including reducing cutting force, increasing MRR, less defects. High spindle speed and high amplitude get better effect when using ψ1 diamond tool. In ψ1 hole drilling with 0.1 μm/rev feed, the thrust force decreases from 20 to 2 N with UAM, only 10% of original value; chipping in orifice also decreases from 139.5 μm to no chipping. UAM can increase feed from 0.1 to 0.3 μm/rev without tool breakage, so the MRR also increases to 3 times with only few defects. UAM with high feed (0.3 μm/rev) would cause small defects about 10 μm at the center of bottom while UAM with low feed (0.1 μm/rev) would get no defects. If the machining process starts with high feed rate and transfers to low feed rate in the end, there will be good MRR and also workpice quality.