【正文】 cuiting current. This electric explosion usually cause a fine spatter which influence the forming quality.. Features of the CMT welding process The welding features of the CMT process was investigated via beadonplate welding on pure aluminium plates of thickness 1 mm.. The bead macroscopic appearance Though the metal transfer is a shortcircuiting process, the fixed direction and the magnitude of the backdrawing force guarantee the process stability and can realize nospatter welding when welding by CMT power source.Fig. 4. The shortcircuiting metal transfer process during conventional MIG process (frame/1 ms). Three aspects of the surface quality of the weld beads are evaluated: the uniformity of the bead along the direction of welding, the smoothness of the bead surface and the severity of . 5 shows the weld bead produced CMT process. It can be clearly seen that the process has significantly improved the surface quality of the weld beads. The bead is very uniform and regular.Fig. 5. The macroscopic appearance of the bead.. Low heat input and decreased deformation According to the previous analysis, CMT requires less current for the same amount of material deposition in parison with conventional pulsed MIG welding. The backdrawing force cancels the metal transfer demanding for the current. So the heat input is decreased abruptly. The deflection deformation was investigated via beadonplate welding on pure aluminium plates of thickness 1 mm. Two clamps were used to fix the plates near the arc start and arc end points. The clamps were removed immediately after welding to allow the plate to deform freely. The beadonplate is about 150 mm long and the original plate is about 200 mm. After beadonplate welding test, the deflection deformation of the plate about 10 mm is very small pared with the conventional MIG welding process.. High gap bridging ability A lower heat input also contributes to the high gap tolerance as shown in Fig. 6. The gap is readily bridged because the melting of the upper sheet of the lap joint is reduced.Fig. 6. The figure of a lap joint with a gap about 2 mm between two sheets.4. Conclusion A current and voltage waveforms sensing system and a metal transfer visual sensing system were constructed to examine the process characteristics of the CMT. The results show that the metal transfer process is very stable based on the special wave control features and a assistant backdrawing force. Beadonplate tests were done on the thin pure aluminium sheets, a good appearance can be obtained owing to the nospatter welding process. Meanwhile, the low heat input of CMT can not only decrease the deflection deformation of the thin sheets when welding pure aluminium,but also promote the gap bridging ability.References[1] Haraga K. Strength properties of aluminum/aluminum and aluminum/steel joints for light weighing of automotive body. Weld World 2000。44(4):23–7.[2] Larsson JK. Overview of joining technologies in the automotive industry. Weld Res Abroad 2003。49(6/7):29–45.[3] Anon. Lightweight body construction. Steel or aluminium. Steel Times 1994。222(3):2.[4] Winandy CD. Light metals in the automotive industry. Age 1994。52(9–10):46–54.[5] Hermans MJM. Process behavior and stability in short circuit gas metal arc welding. Weld J 1999。78(4):141–73.[6] Schierl A. The CMT – process – a revolution in welding technology. Weld World 2005。49(9):38.41附錄BCMT短路金屬及其使用薄鋼板焊接鋁轉(zhuǎn)移過程1 介紹為了保護(hù)環(huán)境，使用輕型材料，在汽車行業(yè)變得越來越廣泛。由于鋁合金輕強(qiáng)度高，并且可以回收利用，采用輕薄的鋁合金生產(chǎn)新型汽車正在發(fā)展，一些產(chǎn)品已經(jīng)在市場(chǎng)上出現(xiàn)[14]。那么促進(jìn)鋁合金的使用以及保證采用鋁合金制成的汽車的價(jià)值的關(guān)鍵是鋁合金的焊接。焊接薄鋁板由很多問題，例如燒穿和變形。傳統(tǒng)的MIG焊通常用于鋁的焊接，但是當(dāng)加入薄鋁材料的時(shí)候，缺乏控制滲透往往限制了它的使用。短路過渡由于其熱輸入小的特征是加入薄鋁材料的合適的方法[5]。但是在焊接過程中產(chǎn)生的大量飛濺也給生產(chǎn)者帶來很大麻煩。最近的一次焊接技術(shù)的發(fā)展冷金屬合金焊因其在焊接過程中的小飛濺和較低的熱輸入是焊接鋁的理想選擇。我們稱之為CMT的冷金屬過渡過程是由福尼斯公司發(fā)明的一個(gè)修復(fù)MIG焊的過程。主要的創(chuàng)新在于焊絲的運(yùn)動(dòng)集成到焊接過程整體控制的這個(gè)過程。每次發(fā)生短路，數(shù)字程序控制中斷供電進(jìn)而控制線收縮。線收縮運(yùn)動(dòng)幫助在短路過程中的熔滴分離，這樣，即使沒有電磁力的援助金屬也可以轉(zhuǎn)移到熔池中去。然后可以大大減小熱輸入和飛濺問題[6]。本文主要目的是檢查焊接電流電壓的波形和CMT金屬過渡的過程。于此同時(shí)，焊接純鋁是為了評(píng)估適合薄鋁板焊接的過程。2 試驗(yàn)裝置及程序在CMT焊接工藝過程中，通過電弧感應(yīng)焊接電流和電弧電壓和高速成像的液滴轉(zhuǎn)移相結(jié)合彼此捕獲脈沖波形和金屬轉(zhuǎn)移的過程。圖1為焊接裝置的示意圖。它由四個(gè)部分組成：（1）工業(yè)PC作為中央控制計(jì)算機(jī)，（2）焊接系統(tǒng)包括CMT電源及其配件，（3）焊接電流和電弧電壓的傳感系統(tǒng)，（4）金屬過渡的成像系統(tǒng)。高倍相機(jī)（CAD60256W）用來拍攝熔滴成像，955幀的圖像輸出第二個(gè)相機(jī)的在線計(jì)算機(jī)分析，光線通過左邊的氙燈投射到熔滴上，然后圖像平面成為一組平行光。被熔滴阻擋的光不會(huì)到達(dá)正確的鏡頭和圖像平面。其余的光會(huì)照亮像平面并且在照相機(jī)中會(huì)被觀察到。因此金屬轉(zhuǎn)移過程能被獲取。過濾系統(tǒng)用來減少光的強(qiáng)度及提升像的清晰度。電流和電壓波形是由相應(yīng)的傳感器捕獲然后寫入計(jì)算機(jī)中心的?；谏鲜龅挠布刂栖浖脕硖岣吆附酉到y(tǒng)、圖像捕獲系統(tǒng)、電流和電壓采樣系統(tǒng)的同步運(yùn)行。熔焊是在采用1mm厚的純鋁材料和純氬氣的焊接環(huán)境下進(jìn)行。圖1 焊接試驗(yàn)裝置示意圖3 結(jié)果及討論典型的CMT焊接電信號(hào)的周期如圖2所示。一個(gè)周期可以被定義為需要熔化的電極的熔滴進(jìn)入熔池的過程。由圖可知，我們可以將周期分為三個(gè)階段：（1）峰值電流階段。它代表了一個(gè)持續(xù)的電弧電壓所對(duì)應(yīng)的高脈沖電流。高脈沖的焊接電流使焊接電弧引弧容易然后加熱焊絲電極形成液滴。（2）背景電流階段。這個(gè)階段代表著較低階段的電流。經(jīng)過峰值電流階段，在焊絲頂端形成小液滴，為了抑制球狀轉(zhuǎn)移，電流突然降低，這個(gè)階段就是背景電流階段。然后再短路過程發(fā)生之前一直保持著低電流。（3）短路階段。在這個(gè)階段，焊絲接觸熔池電弧電壓變?yōu)榱?。在傳統(tǒng)的MIG短路金屬轉(zhuǎn)移焊的過程中這會(huì)導(dǎo)致通過較大的電磁力推動(dòng)液體的電流的快速增長。但是在CMT過程中，短路電流被降低在一個(gè)較低的水平。同時(shí)，返回信號(hào)被送到送絲裝備。然后送絲裝備給焊絲一個(gè)回抽力。這種回抽力會(huì)幫助液體橋接斷裂并轉(zhuǎn)移到焊接熔池。然后電弧重燃重復(fù)循環(huán)。圖2 CMT過程中電流和電壓的波形圖3為金屬在一個(gè)波形周期的過渡過程。附著在焊絲頂端的液滴尺寸和電弧燃燒期間的焊絲直徑很接近。斷裂的液體橋是非常穩(wěn)定的。因?yàn)闆]有大的短路電流，所以在金屬轉(zhuǎn)移過程中沒有電爆炸現(xiàn)象。當(dāng)熔滴接觸熔池時(shí)，反饋信號(hào)進(jìn)入焊絲支流改變焊絲移動(dòng)方向，所以后面的力能幫助液體橋斷裂使金屬流進(jìn)熔池里。圖4是典型的MIG短路金屬過渡的過程。由圖可以看出，電爆炸不能因?yàn)榇蟮亩搪冯娏鞫苊?。這種電爆炸通常會(huì)導(dǎo)致影響成形質(zhì)量的微小的焊接飛濺的產(chǎn)生。圖3 CMT過程中短路金屬過渡過程（幀/1ms）圖4 傳統(tǒng)的MIG過程中短路金屬的過渡過程（幀/1ms） CMT焊接過程的特性CMT焊接過程的特性主要是通過調(diào)查熔焊焊接厚度為1mm的純鋁板得到的。雖然金屬過渡是一個(gè)短路的過程，但是固定的方向和回抽力的大小保證進(jìn)程的穩(wěn)定性而且當(dāng)采用CMT焊接電源焊接時(shí)可以實(shí)現(xiàn)無飛濺焊接。焊縫表面質(zhì)量的評(píng)估分為三個(gè)方面：保持焊道和焊接方向的一致性，焊縫表面的平滑度和嚴(yán)重的飛濺。圖5展示的是CMT進(jìn)程中產(chǎn)生的焊縫?？梢郧宄目闯鲞@一過程顯著的提高了焊縫表面的質(zhì)量，焊縫均勻有規(guī)律。根據(jù)以上的分析，CMT與使用相同材料焊接的傳統(tǒng)MIG焊相比較需要較小的焊接電流。回抽力取消金屬過渡時(shí)所需的電流，所以熱輸入急劇降低。研究厚度為1mm的純鋁熔焊的撓曲變形，兩個(gè)夾具固定焊絲盤附近電弧的啟動(dòng)和結(jié)束點(diǎn)。焊接允許自由變形的板之后立刻移走夾具，板大約150mm長原始的板長200mm。焊接試驗(yàn)過后板的撓曲變形大約10mm與傳統(tǒng)的MIG焊相比較是非常小的。一個(gè)低的熱輸入也會(huì)導(dǎo)致大的間隙產(chǎn)生如圖6所示。因?yàn)榇罱拥纳蠈影宓娜刍瘻p少間隙和容易被橋接。圖5 焊縫的宏觀外貌圖6 兩板搭接間隙2mm的圖片4 結(jié)論電流和電壓波形傳感系統(tǒng)和金屬轉(zhuǎn)移視覺傳感系統(tǒng)被用來檢驗(yàn)CMT的工藝特點(diǎn)。結(jié)果表明，金屬轉(zhuǎn)移過程基于特殊的波形特征控制是非常穩(wěn)定的并且有助于回抽力。較薄的純鋁熔焊試驗(yàn)時(shí)，可以由無飛濺的焊接過程獲得外觀良好的焊縫。同時(shí)，CMT中較低的焊接熱輸入不僅在焊接純鋁的過程中減少撓曲變形，而且可以提升焊縫的橋接能力。參考文獻(xiàn)[1] Haraga K. Strength properties of aluminum/aluminum and aluminum/steeljoints for light weighing of automotive body. Weld World 2000。44(4):23–7.[2] Larsson JK. Overview of joining t