激光切割金屬塗層薄鋼板
G.V.S. Prasad, E. Siores, W.C.K. Wong *
School of Mechanical and Manufacturing
Engineering, Queensland Uni6ersity of
Technology, G.P.O. Box 2434, Brisbane,
Qld. 4001, Australia
Received 1 June 1996
提要
本文討論的激光束加工的金屬塗層薄鋼板如
ZINCALUME，ZINCANNEAL
GALVABOND 的 1 毫米的厚度。這些材料基本
上是鋅和鋁塗層的皮膚厚度變化對鋼。
試點工作，探索了一些方法減少熱損傷的塗料面
對面父：基本金屬
實行嚴格的加工憑藉其高反射率和熱導率的限
制。 A 500 W 的連續波，10.6
毫米 CO2 激光數控中心，以提高切割質量，良
好的表面光潔度，減少切縫寬度和糟粕。一個
切削過程特性的有限元分析模型的開發是為了
建立和已經澄清
一種有效的工藝參數的選擇是最小的熱損傷的
塗層的一個先決條件。的地形
特性的未切割的貫通的切縫和表面粗糙度進行
了討論。實驗也有些 visualisational
進行的微觀和宏觀力學切削過程的進一步理
解。事實證明，切削速度
是一個函數的輸入功率，並且這些材料的激光加
工是一種商業上可行的選項。 1998 年發布
由 Elsevier Science S.A.
關鍵詞：激光中心;塗金屬薄板鋼;工藝參數
1。介紹
激光器被用來在許多工業加工操作，
特別是用於處理“難以機'
材料。激光加工相比有幾個優點
常規方法進行。首先，它是一種非接觸式的過程
消除這種影響刀具磨損，振動機
和機械誘導熱損傷。其次，
激光加工是一種熱的過程和材料
具有良好的熱性能可以成功
不管處理它們的機械性能。
第三，激光加工是一個靈活的過程。
塗金屬薄板鋼已加工成功
使用常規的設備如印刷機
斷頭台相當長的一段時間，但現在這些
方法帶來了他們的問題，如低
生產力和快速的工具，穿，雖然他們也
無屑加工方法。本文探討了
作為一個潛在的高能量的激光束的應用
工具處理這些材料。
基本切割機構是依賴於
上升到熔融形成的輻射陷阱
池在局部的位置，然後通過彈出
使用一個合適的輔助氣體射流的工件的根目錄
中。
以往的研究激光材料的相互作用
對金屬的切削區表明，屬性，例如
作為反射率和熱導率決定的效率
切削過程中，大多數的金屬是高度
反射的激光波長。由於這個原因，在
光束和工件的耦合往往是低效
和非常低的。
然而，該材料的吸收係數是
溫度的函數的，在改變
過渡階段的過程。最初的弱吸收
在工件的表面上開始增加
工件溫度的直接下的光學
束，從而降低了反射率相當迅速。
直到熔化溫度和吸收增加
蒸發溫度達到允許一個
鎖孔或輻射陷阱的形成在局部點。
激光束作為一個充滿活力的線熱源
因此，顯而易見的是，對於該鎖孔切削過程
以發起，至關重要的是，功率密度
高到足以克服反射障礙。一次
做到這一點，該過程可以使用控制
熔化和蒸發的關係。
任何激光加工過程中的目標是最大化
材料去除率，同時減少
熱影響區（HAZ）
。這個實驗的目標
研究發現：（i）確定的參數
有不利的影響的結果的切削
過程;（二）建立之間的關係
進給速度，輸入功率;（三）檢查
通過分析表面質量方面的 HAZ
過程中形成的激光材料相互作用憑藉
氧化的塗料直接下
光束;及（iv）之間的權衡分析
材料的去除速率和 HAZ。
2。激光與金屬的相互作用
雖然 CO2 激光切割的金屬已經成為
以及建立的製造過程中，該處理
金屬塗層薄鋼板被視為
“困難”。這些材料是在較低的速度切割和
在稀釋劑最大的部分比大多數其他
金屬。背後的原因，這種減少在切割
效率比，例如鋼，可以
佔通過檢查的物理性質
這些材料：（一）其反射率的 10.6 毫米 CO2
激光輻射是非常高的，高達 99％，在室溫下;
及（ii）它們的熱導率約為
三次的其它金屬，如低碳鋼。
原則的激光加工的金屬中表明
切削過程取決於建立
的局部區域的熔融和：或者蒸發
整個深度的工件。熔融從而
除去所產生的聚焦光束的從切
由入射的氣體射流，這也是化學區
與熔體反應。的化學反應最
通常採用的是放熱的氧化
激光輻射的熱量下的金屬表面。
熔體化學降解和反應形成
二次熱輸入到切割區。
的高反射率和熱導率，使
很難建立一個本地化的熔融區。在
環境溫度下，所有的金屬具有高的反射率
（\99％）的入射激光束。所佔比例很小，
所吸收的光的有加熱的效果
下面積的光束和隨後的溫度上升
是伴隨著降低的反射率。這
減少的反射率的結果，在進一步加熱直到
建立高度吸收的熔池。
的很小的百分比]是所吸收的熱
材料轉換成熱能，但很快就消散
憑藉其高跨在片材的表面
熱傳導性。作為一個結果，低的熱
輸入和快速的熱耗散，一個高度
吸收，局部熱點較難成立
，在其他金屬的情況下。持續的比較
與鋼，鋁的放熱反應：
遠不如有效的切割區域中比作為熱源
切削鋼時，採用類似的反應
的鋁的氧化反應能夠產生
更多的能量比鐵反應，但
產生的氧化物的形成不透水的密封
底層的鋁表面，從而抑制
任何進一步的與氧的反應。在激光
切割，這種密封由於不斷骨折
滿分切割區的熔體流動湍流性質。
鑑於此湍流，氧化反應
可以作為大量的熱輸入到切割
雖然其貢獻的過程是不相同的
量級的鐵氧化
在切割過程中的鋼。
上述理論方面的考慮
在試樣上進行的實驗調查。
使用辛辛那提進行的工作
CL-5 數控激光中心，結合高
功率調製和良好的模式，實現非常
高的能量密度，從而使問題的高
反射率和熱導率，可以被克服。
3。實驗方法
這些一系列的實驗進行了使用
在辛辛那提 CL-5 數控激光在不同的中心
電源輸入的視圖，以優化的切割質量。
這台機器在一個波長的光束
的範圍內的 310？7-3？10？3 毫米。梁
集中使用 127 毫米焦距鏡頭和一個簡單的
圓錐形噴嘴出口處，有一個直徑 1.7
毫米與噴嘴工件偏離距離
1 毫米。
的機器設計的切削頭組件
例如，把材料追隨者改變
從透鏡到工件的距離。調整
材料跟隨移動的光束焦點
材料的表面的上方或下方。該材料
從動旋轉（相對於透鏡組件的）上
線程，它移動，垂直 0.05 英寸（1.27 毫米）每
革命，超過總範圍為 0.4 英寸或八分飽
轉動。
氧氣作為輔助氣體，這也是
主輔助氣體為計算機指定，而
多達八個不同的氣體可以被納入
實驗設計。的流率和操作
輔助氣體的壓力，通常依賴於每個
特定的應用程序。三個工藝參數
確定為重要的在切割的標本
正在研究的，即，輸入功率，切割速度和
輔助氣體壓力。
3.1。輸入功率
在熔化和蒸發的劇烈波動
溫度的塗層和父：基
金屬呈現的輸入功率的關鍵因素之一
在實現最佳的加工質量。的初始集
實驗進行不同的電源輸入
450-700 W 範圍內的生產質量惡化
削減所有的標本。這是決定調節
在 500 瓦的功率，改變切割速度和
輔助氣體壓力。
3.2。切割 6elocity
大多數激光系統是基於低壓力
這意味著，最大切割的切割頭
壓力是有限的切割中的光學系統
頭。鏡頭通常製成的 GaAs 或硒化鋅的
指定要承受的最大壓力為約 5
吧。通常已知的切割速度
隨著氣體壓力的增加。有一個特定的
領域，高品質的削減。最大的
速度約 5 巴壓力。
調查表明，有之間的間隙
理論上計算和實驗得到
切割速度，這表明更多的餘地
改善。
3.3。輔助氣體壓力
範圍在 5-20 的氧氣壓力增加
吧。在 20 巴的水平，材料開始像
反射鏡和材料之間不存在交互
和入射光束。這意味著，對於一個
特定的激光功率，有一個特定的壓力範圍內
內，該材料可以被處理。的氣體
壓力變化，因此限於最多 14
吧。的切削速率的輸入功率的函數是
調查。
此次裁員精細，良好的評價，
可以接受的，質量差。的品質進行了優化
通過優化的焦點位置
聚焦光學系統的激光系統。
4。結果與討論
討論背後的基本理念是：
的能源浪費，這並不有助於
切削過程中，不應該被視為作為其一個方面
激光材料相互作用。忽略的反射
能源，這是不，根據定義，輸入到
通過傳導，對流切割過程中的損失
和輻射可以僅作為一個功能的處理
融化在其周圍。
一個簡單的分析模型可以解釋上述
與下面的能量平衡的幫助下
方程：
輸入能量（能量用於切割）？
（傳導，對流和輻射損失）
作為一個初始近似值，假設特定
降低能源用於去除單位體積的切
材料是獨立的材料的厚度。 “
切割中使用的能量，因此，這種功能
特定的切割能量乘以的音量
在切割過程中材料的去除。的損失
傳導，對流和輻射的函數的
切割前的溫度和其表面積
在與周圍環境的接觸。在這些條件下，
能量平衡方程可以寫為
follows.If 激光功率 P 在時間 t，切直線 L
然後：
（二）T（X：100）Ecutldk？tBdk：2（A？B？
C）（3）
其中，b 是發送到切割區的激光功率;
x 是吸光係數的切斷區; Ecut 是具體
需要的能量，熔化和除去一個單位體積的
材料從切割區; d 為材料的厚度
和 A，B 和 C 是導電的，輻射和
對流損失函數。
理論表明，在切割過程中，它通常是
切割前的後緣的情況下，不
延伸到的完整的入射光束的直徑。一
因此，比例的光直接穿過
不與切割前交互的切縫。因此，
吸收率將部分要高得多
比在室溫的溫度下的理論值。
這是因為切區具有其吸收率
增加的高溫度的結果，存在
氧化物，淺角度的入射
的粗糙度和吸收層的激光束
蒸汽。
特定的切割能量可以假設不變
對於任何給定的材料，所有的切口出現類似
因此可以被認為是被產生類似
機制。基於這種假設，平均
切削溫度也可以被假定為保持
常數為一個給定的材料。考慮到損失。
在上面的方程中，表示導電性損失
每單位面積的切削前可以再次被假定為
對於一個給定的材料是恆定的，所以，雖然
一般由導電熱損失
散熱器和溫度的熱源，這
因子不會干擾的中心思想
的討論。
上述還能夠方便地暗示
對流和輻射熱損失可以是每單位面積的
近似成比例的表面積
前面。據如下方程中，能量
切割中使用的是獨立的切割時間。 “
損失然後將切削時間成正比，所以
的比例的有用的，浪費的能量
會改變，如果切割速度被改變，以
切割不同厚度的材料。
4.1。材料厚度的切割速度的影響
在公式中，假設在同一時間，d 被減半
激光功率輸入：
（P B）
（T：2）
（100）？？tBdk Ecutlk（D：2）：
4（A？B？C）
（4）
為了比較，讓一切一倍
式中。 （4）
：
（P B）T（X：100）Ecutldk（T）BDK（A？B？
C）（5）
很顯然，方程中的不平衡
尊重式。 （3），損失已經減少了一半。
因此，平衡方程可以通過簡單地操縱
噸。
上述清楚地意味著有一個具體的
超過該限制的材料的厚度的切割
機制打破了，不能重新建立
在任何切割速度。這樣做的原因是相對
從切割區作為熱損失增加
切割速度下降。在機械加工的情況下
所研究的材料，其熱傳導
塗層決定了切割速度雖然
材料的厚度並不是主要的問題。
的快速氧化反應的影響，必須
被認為在確定的最佳選擇
的輸入功率和按比例增加的
切割速度。在這種情況下，如下，隨
材料厚度的增加，有一個比例
增加的能源浪費，但是這是更
漸進的，這主要是由於能量的耗散
整個材料表面憑藉具有高導熱
導電性。換句話說，有更少的濃度
跨越任何特定區域中吸收的能量
材料的表面。
4.2。的入射光束的影響的表面上的
材料
圖。 1 所示的圖形繪製與輸入功率
降息。由此可以推斷，GALVABOND
標本被切斷速度比 ZINCALUME
ZINCANNEAL 標本。這是由於鋁
塗層高度吸收，在激光的
波長為 10.6 毫米。由此形成的氧化物
牢固地結合到基板和不汽化
的入射光束，由於其熔點高，並
沸點。這種高度吸收和耐火材料
表面取代了原來的鋁表面等
使用反射相關聯的問題被最小化
4.3。切割輔助氣體的影響
在圖的照片。 4 顯示的效果的
各種標本的表面上的切割氣。
有較大的明顯的表面解體
箱子 GALVABOND 標本相比
與其他人。這表示由不同的氧化物
形成沿著切割長度。切緣
表面代表極端邊緣的熔融切
區，然後切割過程中留下的。在
該區域中，熔體是與父接觸：鹼
金屬，在溫度為不大大超過
基體金屬的熔點。
這樣低的溫度下熔體具有較高的表面張力
比熱得多的頂表面的塗層在
的激光材料相互作用區的中心。這
表面張力梯度的作用是繪製的熔融材料
向的兩側的切口，而它是在同一
時間推進垂直向下的衝擊
氣體射流。在這種方式中，熔融材料可以
上積聚的切割邊緣的底部和
此後凝固為糟粕。另外，熔融區是
與氧化物塗層覆蓋，這往往會增加
的整體的熔體的表面張力。此外，經驗
釬焊已表明，較熱的表面往往會吸引
更有效地因此較熱的，較低的熔體
的表面張力。
高表面張力的力傾向於限制表面
流體的大半徑的幾何形狀。這減少了
熔體表面張力可能已經預料到
加快在切割過程。移動的氣體通過
切割區主要充當一個機械的推進劑
的液體金屬，滿分切割區。化學，
也能作為能量的來源，如果氧氣
輸入到切割過程中，但它必須緊
記住，氣也用於冷藏切割
通過強制對流冷卻區。採用這種解釋，
可以預見，較高的比熱
和熱導率的一些其他的輔助氣體，例如
如氮氣或氦氣，使其更有效
的裝置的冷卻熔融區的激光。
4.4。切縫寬度分析
所有的切口的切口寬度進行該
實驗方案只是稍微的變化
的 250 毫米與 ZINCANNEAL 中的平均值和
ZINCALUME 標本佔用範圍 220-250
mm 和 GALVABOND 的試樣在 250-270
mm 範圍內（參見圖 2）。這是緊密一致
類似的其他研究人員進行的研究
金屬和大多數結果報告的迄今表現出
切縫寬度的均勻性這傾向。
這幾乎是獨立的切縫寬度與尊重
切割速度快，材料厚度或協助的類型
機械切割方法，用氣，讓人想起
並且它可以推測，對於一個特定的組合
激光透鏡金屬的，聚焦的激光假定
因而未必改變的有效寬度
改變工藝參數。金屬本身的決定
這個寬度作為一個結果，其高的熱導率，
有效地冷卻了所有的材料不
由光束直接照射，並由此防止
橫向擴展的切縫寬度。
4.5。上切割的輔助氣體壓力的影響
在約 5 巴壓力，質量好，削減
獲得 ZINCALUME 河畔 ZINCANNEAL（見
圖。微不足道的毛刺和熱影響區的 3）與
（HAZ）
，而 GALVABOND 表現出更寬一點
HAZ。的切割速度的增加而增加氣體
由約 60％的壓力，作為氣體壓力從
5 至 20 巴。重複實驗 GALVABOND
顯示，在參數區中哪個質量好
削減縮小相比低氣壓
參數。在低剪切速度，高 O2
這是一個強大的毛刺效果的壓力導致
不可控的，產生了廣泛的不規則的切縫。
由此可以推斷，這是由於高壓力
純氧，用鋅和鋼反應，形成
鋅和鉻氧化物沿邊緣的削減。
最終的結果表明，在較高的氧壓，
切割速度增加在 40-70％的範圍內，
證明這些標本的加工有利可圖的使用
高能量的激光。
4.6。金相 in6estigation
在圖的顯微照片。圖 5 顯示了正面意見
的標本。雖然熱影響區很窄，ZINCALUME
和 ZINCANNEAL 標本，它更
宣判案件 GALVABOND。這也是
如此氧化物沉積跨的長度的情況下
切。圖覆蓋的區域朝下方
切割邊緣，並清楚地顯示了粘合劑的糟粕，
再凝固的熔化區的多孔性。 “
大部分的毛孔的棱角意味著，它們是
結果，卡夾的氣體從頂部到
切口的底部。此外，種折疊機構是
揭示的切割邊緣的線平行。在
從中央熱點側的流體流
切區，它可能是可能的，兩個相鄰的，
氧化的表面可以接觸到，並成為
困為所示的類型中的一個線性列入
的身影。
4.7。控制糟粕
雖然沉積的下邊緣上的鋅渣
切造成不良影響，這是相當容易
除去機械的情況下，這些標本
無論是刮或磨損。為了盡量減少其糟粕，
有許多技術可用。其中一個選項
了可以使用脈衝激光束，而不是一個
CW 模式。這是因為，一個脈衝的峰值能量
激光束是遠遠高於的 CW 輸出，但
平均產量一般較低。
每個脈衝的峰值功率高應採取行動
迅速融化，蒸發，這些金屬塗層。一個切
可以以這種方式進行的與最低的盈餘
融化的傳導效應。這減少了
盈餘熔化能進一步抑制代
，浮渣雖然切割速度往往為低於
更高的功率 CW 模式。
5。結論
所研究的金屬塗層薄鋼板，即
ZINCALUME，ZINCANNEAL 和 GALVABOND
可以在商業上可接受的價格削減
觀察厚度範圍為 0.5-1.0mm，高的激光
權力。雖然切割的速度是相同的情況下，
ZINCALUME 和 ZINCANNEAL，它是稍微
更高（約 20％）在箱子 GALVABOND。 “
輸入功率，切割速度，輔助氣體壓力
規定獲得加工的切割質量
這些材料。
是相當有效的氧氣作為輔助氣體的
切割過程中盡可能切割速度而言。
然而，困難與本地化
過熱，特別是在箱子 GALVABOND
標本中，可能會遇到，如果詳細的工作
需要。在激光切割 GALVABOND 標本，
氧化的邊緣，可以完全消除
使用一些其它的輔助氣體（如氮或氦），
這應該呈現完全氧化切割邊緣。
如果兩個不同的激光功率進行比較，它是
可能有較高的功率將有一個劣質
模式質量不會將重點放在為小點作為
更低的功耗。這間較大的焦點會產生
更廣泛的削減，從而使這個過程更高效，
將有更多的材料被除去，以生成
切。
的剪裁區的流體動力學扮演著非常
在確定材料的去除的重要作用
率。輸入功率增加的傾斜變化
和切割前的幾何形狀，這將在反過來，
發生變化的材料切除率。因此，
以上的限制的切割速度，熔體粘度
可能成為速率確定因子。
作為一個結果，減少熱損失的
工件在切割時，更高的速度，熱
切割區周圍的梯度變得更加嚴重
沿切割線的材料還沒有預熱
移動切割前面，因此需要更多的能量
成為熔化，噴出。
無渣削減可在切割 ZINCALUME
和 ZINCANNEAL 但在箱子
GALVABOND，形成的氧化物都集中在
爐渣。由於高的熱導率和
熔點，爐渣固化前，它離開
切縫。
它是觀察到部分壓入到所說的熔渣
熔融區域在切口的切口，這可能會導致問題的
當這些樣品進行進一步的處理。
遺民浮渣一般的下邊緣處形成
作為結果，高表面張力的切口和
內的熔體的表面張力梯度。雖然糟粕
可以容易地除去，但也可以被最小化由
脈衝激光的切割，這是當然的，在犧牲
的切割速度，或可能由使用的浮渣
噴氣指示所有的渣滓上的浪費材料
切口側。
5.1。參數的啟示
這是顯而易見的，從上述的結果和討論
分析模型，從而發展呈現出
有限元的字符，當施加到材料
在考慮中。這是由於存在
材料的各種不同的厚度和其
夾在中間的影響力。簡化模型排除
激光材料的相互作用的可能性的研究
在的各種接口，在實時
互動程度繼續保持激烈和複雜的，
的金屬，即根據不同的膨脹率。
鋅，鋁，鋼，作為行進的光束功率電
上下夾層，從而產生不同的
的界面處的溫度梯度。
雖然它被用於放大模型
研究激光切割的金屬塗層板
鋼在今後的工作中，這種情況是相當複雜的，
考慮到組合物，這些標本。 “
模型發展至今從而提供範圍進一步
修改在上述複雜的光
並且可以被修改，以適應的相互作用
的界面處的標本的夾心
中的溫度梯度和不同的
的各種金屬的線性膨脹係數。
致謝
作者希望表達自己的感謝，並
感謝必和必拓鋼有限公司，澳大利亞，
提供標本和昆士蘭
製造研究所，布里斯班，澳大利亞，
辛辛那提 CL-5 數控激光中心開展
實驗。
原文
Laser cutting of metallic coated sheet steels
G.V.S. Prasad, E. Siores, W.C.K. Wong *
School of Mechanical and Manufacturing Engineering, Queensland Uni6ersity of Technology, G.P.O. Box 2434, Brisbane,
Qld. 4001, Australia
Received 1 June 1996
Abstract
This paper discusses the laser-beam machining of metallic coated sheet steels such as ZINCALUME,
ZINCANNEAL and
GALVABOND of 1 mm thickness. These materials are essentially zinc and aluminium coatings of varying skin
thickness on steel.
The experimental work explores some methods for reducing thermal damage of the coatings vis-a-vis the
parent:base metal which
impose severe machining restrictions by virtue of their high reflectivity and thermal conductivity. A 500 W
continuous-wave, 10.6
mm CO2 CNC laser centre was used to improve the cut quality in terms of good surface finish, reduced kerf width
and dross. An
analytical model was developed to establish the finite-element characteristic of the cutting process and it has been
clarified that
an efficient choice of the process parameters is a pre-requisite for minimum thermal damage of the coatings. The
topographical
characteristics of the uncut-through kerf and surface roughness are discussed. Some visualisational experiments
were also
performed for further understanding of the micro- and macro-mechanics of the cutting process. It is proven that the
cutting speed
is a function of the input power and that the laser processing of these materials is a commercially viable option. ©
1998 Published
by Elsevier Science S.A.
Keywords: Laser centre; Metallic coated sheet steels; Process parameters
1. Introduction
Lasers are used in many industrial machining operations,
especially for processing the ‘difficult-to-machine’
materials. Laser machining has several advantages over
conventional methods. First, it is a non-contact process
that eliminates such effects as tool wear, machine vibration
and mechanically induced thermal damage. Second,
laser machining is a thermal process and materials
with favourable thermal properties can be successfully
processed regardless of their mechanical properties.
Third, laser machining is a flexible process.
Metallic coated sheet steels have been machined successfully
using conventional equipment such as presses
and guillotines for quite some time now but these
methods have brought with them problems like low
productivity and rapid tool wear although they too are
chipless machining methods. This paper examines the
application of a high energy laser beam as a potential
tool for processing these materials.
The basic cutting mechanism is dependent upon the
formation of the radiation trap giving rise to a molten
pool at a localised spot that is then ejected through the
root of the workpiece using a suitable assist gas jet.
Previous studies on the laser–material interactions at
the cutting zone for metals indicate that properties such
as reflectivity and thermal conductivity dictate the efficiency
of the cutting process, as most metals are highly
reflective at the laser wave lengths. Due to this, the
coupling of the beam and the workpiece is often inefficient
and very low.
However, the absorption coefficient of the material is
a function of temperature, which changes during the
transient phase of the process. The initial weak absorption
at the surface of the workpiece begins to increase
the workpiece temperature directly under the optical
beam and thus decreases the reflectivity quite rapidly.
Temperature and absorption increase until melting and
evaporation temperatures are reached that permit a
keyhole or radiation trap to form at the localised spot.
The laser beam acts as an energetic line heat source
* Corresponding author. Fax: _61 7 38641529. within
the material and initialises the cutting process.
0924-0136:99:$19.00 © 1998 Pubished by Elsevier Science S.A. All rights reserved.
PII S0924-0136(97)00276-8
G.V.S. Prasad et al. : Journal of Materials Processing Technology 74 (1998) 234–242 235
Thus, it is evident that for this keyhole-cutting process
to be initiated, it is essential that the power density be
high enough to overcome the reflection barrier. Once
this is achieved, the process can be controlled using the
melting and evaporation relationships.
The goal in any laser machining process is to maximise
the material removal rate whilst minimising the
heat affected zone (HAZ). The objectives of this experimental
study are: (i) to identify the parameters that
have detrimental influence on the outcome of the cutting
process; (ii) to establish a relationship between
traverse speed and input power; (iii) to examine the
surface quality aspect through an analysis of the HAZ
formed during the laser–material interaction by virtue
of the oxidation of the coatings directly under the
optical beam; and (iv) to analyse the trade-off between
the material removal rate and the HAZ.
2. Laser–metal interactions
Although the CO2 laser cutting of metals has become
a well established manufacturing process, the processing
of metallic coated sheet steels is considered as
‘difficult’. These materials are cut at lower speeds and
at thinner maximum sections than for most other
metals. The reasons behind this reduction in the cutting
efficiency as compared with, for example steels, can be
accounted for by examining the physical properties of
these materials: (i) their reflectivity to the 10.6 mm CO2
laser radiation is very high, up to 99% at room temperature;
and (ii) their thermal conductivity is approximately
thrice that of other metals such as mild steels.
The principles of laser processing of metals suggest
that the cutting process depends upon the establishment
of a localised area of melting and:or evaporation
throughout the depth of the workpiece. The melt thus
generated by the focused beam is removed from the cut
zone by the incident gas jet, which is also chemically
reactive with the melt. The chemical reaction most
often employed is the exothermic oxidation of the
metallic surface under the heat of the laser radiation.
The melt is chemically degraded and the reaction forms
a secondary heat input to the cut zone.
The high reflectivity and thermal conductivity makes
it very difficult to establish a localised molten zone. At
ambient temperatures, all metals have high reflectivity
(\99%) to the incident laser beam. The small proportion
of the absorbed light has the effect of heating the
area under the beam and the subsequent rise in temperature
is accompanied by a reduction in reflectivity. This
reduction in reflectivity results in further heating until a
highly absorptive molten pool is established.
The small percentage of heat that is absorbed by the
material is converted into heat but is quickly dissipated
across the surface of the sheet by virtue of its high
thermal conductivity. As a result of the low thermal
input and the rapid dissipation of the heat, a highly
absorptive, localised hot spot is established less readily
that in the case of other metals. Continuing the comparison
with steels, the aluminium exothermic reaction:
(4)Al_(3)O2_(2)AL2O3_1670 kJ:mol (1)
is far less effective as a heat source in the cut zone than
1
the similar reaction employed when cutting steels:
(4)Fe_(3)O2_(2)Fe2O3_822 kJ:mol (2)
The aluminium oxidation reaction is capable of generating
more energy than the iron reaction but the
oxide generated forms an impermeable seal on the
surface of the underlying aluminium and thereby suppresses
any further reaction with oxygen. During laser
cutting, this seal is continuously fractured due to the
turbulent nature of the melt flow out of the cut zone.
Consequent to this turbulence, the oxidation reaction
can act as a substantial thermal input to the cutting
process although its contribution is not of the same
order of magnitude as that of the oxidation of iron
during the cutting of steels.
The foregoing theoretical aspects were considered in
the experimental investigations conducted on the specimens.
The work was carried out using a Cincinnati
CL-5 CNC Laser Centre with the combination of high
power modulation and good mode, attaining extremely
high energy densities, so that the problems of high
reflectivity and thermal conductivity could be overcome.
3. Experimental methodology
These series of experiments were carried out using
the Cincinnati CL-5 CNC Laser Centre at different
power inputs with a view to optimise the cut quality.
The machine produces a beam with a wave length in
the range of 3_10_7–3_10_3 mm. The beam was
focused using a 127 mm focal length lens and a simple
conical cutting nozzle that had a exit diameter of 1.7
mm with the nozzle–workpiece standoff distance being
1 mm.
The cutting head assembly of the machine is designed
such that turning the materials follower changes the
distance from the lens to the workpiece. Adjusting the
material follower thus moves the beam focal point
above or below the material surface. The material
follower rotates (relative to the lens assembly) on
threads that move it vertically 0.05 in. (1.27 mm) per
revolution, over a total range of 0.4 in. or eight full
turns.
1 Enthalpies
of formation at 293 K (for comparison only).
G.236 V.S. Prasad et al. : Journal of Materials Processing Technology 74 (1998) 234–242
Oxygen was employed as the assist gas which is also
the primary assist gas specified for the machine, whilst
up to eight different gases can be incorporated into the
experimental design. The flow rates and the operating
pressures of the assist gas generally depend on each
specific application. Three process parameters were
identified as important in the cutting of the specimens
under study viz., input power, cutting velocity and the
assist gas pressure.
3.1. Input power
The drastic fluctuation in the melting and evaporation
temperatures of the coatings and the parent:base
metal renders the input power one of the crucial factors
in achieving optimum machining quality. The initial set
of experiments conducted for varying power input in
the range 450–700 W produced deteriorating quality of
cuts in all of the specimens. It was decided to modulate
the power at 500 W and vary the cutting velocity and
the assist gas pressure.
3.2. Cutting 6elocity
Most laser systems are based on a low pressure
cutting head that means that the maximum cutting
pressure is limited by the optical system in the cutting
head. Lenses normally made of GaAs or ZnSe are
specified to withstand a maximum pressure of about 5
bar. It is generally known that the cutting velocity
increases with increasing gas pressure. There is a particular
area in which high quality cuts appear. The maximum
velocity is found at pressures of around 5 bar.
Investigations indicate that there is a gap between the
theoretically calculated and the experimentally obtainable
cutting velocities, which indicate more scope for
improvements.
3.3. Assist gas pressure
The oxygen pressure was increased in the range 5–20
bar. At levels of 20 bar, the material begins to act like
a mirror and there is no interaction between the material
and the incident beam. This implies that for a
specific laser power, there is a particular pressure range
within which the material can be processed. The gas
pressure variation was thus limited to a maximum of 14
bar. The cutting rate as a function of input power was
investigated.
The cuts were evaluated in terms of fine, good,
acceptable and poor quality. The qualities were optimised
by optimising the focal point position of the
focusing optics of the laser system.
4. Results and discussion
The basic philosophy behind the discussion is that
the wasted energy, which does not contribute to the
cutting process, should not be viewed as an aspect of
the laser–material interaction. Ignoring the reflected
energy which is not, by definition, an input to the
cutting process, the losses by conduction, convection
and radiation can be treated solely as a function of the
melt on its surroundings.
A simple analytical model of the above can be interpreted
with the help of the following energy balance
equation:
Input energy_(energy used in cutting)_
(losses by conduction, convection and radiation)
As an initial approximation, assume that the specific
cutting energy used to remove a unit volume of cut
material is independent of the material thickness. The
energy used in cutting is, therefore, a function of this
specific cutting energy multiplied by the volume of the
material removed during the cutting. The losses by
conduction, convection and radiation are a function of
the temperature of the cutting front and its surface area
in contact with its surroundings. Under these conditions,
the energy balance equation can be written as
follows.If a laser power P can cut a line L in time t,
then:
(P_b)t(x:100)_Ecutldk_tBdk:2(A_B_C) (3)
where b is the laser power transmitted to the cut zone;
x is the absorptivity of the cut zone; Ecut is the specific
energy needed to melt and remove a unit volume of
material from the cut zone; d is the material thickness
and A, B and C are the conductive, radiative and
convective loss functions.
Theory suggests that during cutting, it is often the
case that the trailing edge of the cut front does not
extend to the full diameter of the incident beam. A
proportion of the light therefore passes straight through
the kerf without interacting with the cut front. Consequently,
the absorptivity will partially be much higher
than the theoretical values at ambient room temperature.
This is because the cut zone has its absorptivity
increased as a result of the high temperature, the presence
of oxides, the shallow angle of incidence of the
laser beam, the roughness and the absorptive layer of
vapour.
The specific cutting energy can be assumed constant
for any given material as all cuts appear similar and
thus can be thought of as being produced by similar
mechanisms. Based on this assumption, the average
cutting temperature can also be assumed to remain
constant for a given material. Considering the losses
G.V.S. Prasad et al. : Journal of Materials Processing Technology 74 (1998) 234–242 237
indicated in the above equation, the conductive losses
per unit area of cutting front can again be assumed to
be constant for a given material, so that while the
conductive heat loss is generally determined by the
temperature of the heat sink and the heat source, this
factor will not interfere with the central idea of this
discussion.
The foregoing will also conveniently imply that the
convective and radiative heat losses per unit area can be
approximated to be proportional to the surface area of
the front. It follows that in the equation, the energy
used in cutting is independent of the cutting time. The
losses will then be proportional to the cutting time, so
that the proportion of the useful and the wasted energy
will change if the cutting speed is changed in order to
cut materials of different thickness.
4.1. Effect of material thickness on cutting speeds
In the equation, suppose that d is halved at the same
laser power input:
(P_b)(t:2)(x:100)_Ecutlk(d:2)_tBdk:4(A_B_C)
(4)
For the sake of comparison, let everything be doubled
in Eq. (4):
(P_b)t(x:100)_Ecutldk_(t:2)Bdk(A_B_C) (5)
It is clear that the imbalance in the equation with
respect to Eq. (3) is that the losses have been halved.
Thus, the equation can be balanced by simply manipulating
t.
The foregoing clearly implies that there is a specific
limit to the material thickness beyond which the cutting
mechanism breaks down and cannot be re-established
at any cutting speed. The reason for this is the relative
increase in thermal losses from the cut zone as the
cutting speed is decreased. In the case of the machining
of the materials under study, the thermal conductivity
of the coatings dictates the cutting speed although the
material thickness is not the primary concern.
The effects of the rapid oxidation reactions have to
be considered in determining the optimum selection of
the input power and the proportional increase in the
cutting speeds. In this case, it follows that with an
increase in the material thickness, there is a proportional
increase in the energy wasted, but this is more
gradual, due mainly to the dissipation of the energy
across the material surface by virtue of high thermal
conductivity. In other words, there is less concentration
of energy absorbed in any particular region across the
material surface.
4.2. Impact of the incident beam on the surface of the
material
Fig. 1 shows graphs plotted for input power versus
cutting rates. It can be inferred that GALVABOND
specimens are cut faster than the ZINCALUME and
ZINCANNEAL specimens. This is due to the aluminium
coating being highly absorptive at the laser
wavelength of 10.6 mm. The oxides thus formed are
firmly bonded to the substrate and are not vaporised by
the incident beam, owing to their high melting and
boiling points. This highly absorptive and refractory
surface replaces the original aluminium surface and so
the problems associated with reflection are minimised.
Fig. 1. Graphs of input power (W) vs. cutting rate (mm min1): (a)
GALVABOND; (b) ZINCANNEAL; (c) ZINCALUME.
G.238 V.S. Prasad et al. : Journal of Materials Processing Technology 74 (1998) 234–242
Fig. 2. Comparison of kerf widths: (a) GALVABOND; (b) ZINCALUME; (c) ZINCANNEAL.
4.3. Influence of the assist gas on cutting
The photographs in Fig. 4 show the effect of the
cutting gas on the surface of the various specimens.
There is greater:pronounced surface disintegration in
the case of GALVABOND specimens as compared
with the others. This is indicated by the distinct oxide
formation along the length of the cut. The cut edge
surfaces represent the extreme edge of the molten cut
zone which is then left behind by the cutting process. In
this region, the melt is in contact with the parent:base
metal, where the temperature is not greatly in excess of
the melting point of the base metal.
This low temperature melt has a higher surface tension
than the much hotter top surface of the coating at
the centre of the laser–material interaction zone. This
surface tension gradient acts to draw the molten material
towards the sides of the cut whilst it is at the same
time being propelled vertically downwards by the impinging
gas jet. In this way, the molten materials can
accumulate on the bottom of the cut edge and will
thereafter solidify as dross. Further, the melt zone is
covered with the oxide coating, which tends to increase
the overall melt surface tension. Also, experience from
brazing has shown that a hotter surface tends to attract
the melt more efficiently hence the hotter, the lower the
surface tension.
High surface tension forces tend to restrict the surface
geometry of fluid to large radii. This reduction in
the melt surface tension might have been expected to
accelerate the cutting process. The gas moving through
the cut zone acts primarily as a mechanical propellant
of the liquid metal out of the cut zone. Chemically, it
can possibly also act as a source of energy if oxygen is
input to the cutting process but it must be borne in
mind that the gas also serves to refrigerate the cutting
zone by forced convective cooling. Using this interpretation,
it can be predicted that the higher specific heat
and thermal conductivity of some other assist gas such
as nitrogen or helium should render it more effective as
a means of cooling the laser melt zone.
4.4. Kerf width analysis
The kerf width of all the cuts carried out for this
experimental programme varied only slightly about an
G.V.S. Prasad et al. : Journal of Materials Processing Technology 74 (1998) 234–242 239
Fig. 3. Microphotographs of the transverse edge: (a) GALVABOND; (b) ZINCANNEAL; (c) ZINCALUME.
average value of 250 mm with the ZINCANNEAL and
ZINCALUME specimens occupying the range 220–250
mm and the GALVABOND specimens in the 250–270
mm range (see Fig. 2). This is in close conformity with
the studies undertaken by other researchers for similar
metals and most of the results reported thus far exhibit
this tendency towards uniformity of the kerf width.
This almost independence of the kerf width with respect
to cutting speed, material thickness or the type of assist
gas used, is reminiscent of mechanical cutting methods
and it can be postulated that for a particular combination
of laser–lens–metal, the focused laser assumed an
effective width which is not necessarily changed by
altering the process parameters. The metal itself determines
this width as a result of its high thermal conductivity,
which effectively cools all the material not
directly irradiated by the beam and thereby prevents
lateral expansion of the kerf width.
4.5. Influence of assist gas pressure on the cutting
At pressure of around 5 bar, good quality cuts were
obtained for ZINCALUME nad ZINCANNEAL, (see
Fig. 3) with insignificant burr and heat-affected zone
(HAZ) while GALVABOND exhibited a little wider
HAZ. The cutting velocity increased with increasing gas
pressure by about 60% as the gas pressure moved from
5 to 20 bar. Repeat experiments for GALVABOND
revealed that the parameter area in which good quality
cuts are obtained narrowed compared to low gas pressure
parameters. At low cutting velocities, the high O2
pressure resulted in a strong burring effect that was
uncontrollable and produced a wide irregular kerf.
It can be inferred that this is due to the high pressure
of pure oxygen that reacts with zinc and steel, forming
zinc and chromium oxides along the edges of the cuts.
End results indicated that at higher O2 pressures, the
cutting velocities increased in the range of 40–70%,
proving machining profitable for these specimens using
the high energy laser.
4.6. Metallographic in6estigation
The micrographs in Fig. 5 show the front views of
the specimens. While the HAZ is very narrow in ZINCALUME
and ZINCANNEAL specimens, it is more
pronounced in the case of GALVABOND. This is also
true for the case of oxide deposition across the length
G.240 V.S. Prasad et al. : Journal of Materials Processing Technology 74 (1998) 234–242
of cut. The figure covers the area towards the bottom of
the cut edge, and clearly shows the adhesive dross and
the porous nature of the resolidified molten zone. The
angularity of most of the pores implies that they are the
result of the entrapment of the gas from the top to the
bottom of the cut. Also, a kind of folding mechanism is
revealed in the lines parallel to the cut edge. During the
fluid flow from the central hot spot to the side of the
cut zone, it could be possible that the two adjacent,
oxidised surfaces can come into contact and become
trapped as a linear inclusion of the type shown in the
figure.
4.7. Control of dross
While the deposition of dross on the lower edge of
the cut is an undesirable effect, it is rather easy to
remove mechanically in the case of these specimens by
either scraping or abrasion. To minimise the dross,
there are numerous techniques available. One option
could be the use of a pulsed laser beam rather than a
CW mode. This is because the peak energy of a pulsed
laser beam is much higher than the CW output but the
average output is generally lower.
The high peak powers of each pulse should act to
rapidly melt and vaporise these metallic coatings. A cut
can be carried out in this way with minimum surplus
melting by conduction effects. This reduction in the
surplus melting could further inhibit the generation of
dross although the cutting speeds tend to be lower than
for the higher power CW mode.
5. Conclusions
The metallic coated sheet steels under study, i.e.
ZINCALUME, ZINCANNEAL and GALVABOND,
can be cut at commercially acceptable rates in the
observed thickness range of 0.5–1.0 mm at high laser
powers. While the cutting speed is same in the case of
ZINCALUME and ZINCANNEAL, it is slightly
higher (about 20%) in the case of GALVABOND. The
input power, cutting velocity and the assist gas pressure
dictate the quality of cuts obtainable in the machining
of these materials.
Oxygen is quite effective as an assist gas for the
cutting process as far as the cutting speeds are concerned.
However, difficulties associated with localised
overheating, particularly in the case of GALVABOND
specimens, may be encountered if detailed work is
required. In the laser cutting of GALVABOND specimens,
the oxidised edges can be totally eliminated by
using some other assist gas such as nitrogen or helium,
which should render totally oxidised-free cut edges.
If two different laser powers are compared, it is
probable that the higher power will have an inferior
mode quality which will not focus to as small spot as
the lower power. This larger focal spot will produce a
wider cut, thus rendering the process less efficient, as
more material will have to be removed to generate the
cut.
The fluid dynamics of the cut zone play a very
important role in determining the material removal
rate. Increase in power input changes the inclination
and the geometry of the cut front, which will in turn,
induce changes in the material removal rates. Thus,
above a limiting cutting speed, the viscosity of the melt
may become the rate determining factor.
Fig. 4. Microphotographs of the top surface: (a) ZINCALUME; (b)
ZINCANNEAL; (c) GALVABOND.
G.V.S. Prasad et al. : Journal of Materials Processing Technology 74 (1998) 234–242 241
Fig. 5. Microphotographs of the front view: (a) GALVABOND; (b) ZINCALUME; (c) ZINCANNEAL.
As a result of the reduction in thermal losses to the
workpiece when cutting at higher speeds, the thermal
gradients around the cutting zone become more severe
as the material along the cut line is not preheated by
the moving cut front and therefore requires more energy
to become melted and ejected.
Slag-free cuts are obtainable in the cutting of ZINCALUME
and ZINCANNEAL but in the case of
GALVABOND, the oxides formed are concentrated in
the slag. Owing to the high thermal conductivity and
melting point, the slag solidifies before it leaves the
kerf.
It is observed that the slag is partly pressed into the
melt zone in the cut kerf, which could cause problems
when these specimens are subjected to further processing.
Adherent dross is generally formed at the lower edge
of the cut as a result of high surface tension forces and
surface tension gradients within the melt. While dross
can be easily removed, it can also be minimised by
pulsed laser cutting, which is of course, at the expense
of the cutting speed or probably by the use of a dross
jet which directs all the dross onto the waste-material
side of the cut.
5.1. Implication of the arguments
It is evident from the foregoing results and discussion
that the analytical model thus developed exhibits a
finite-element character, when applied to the materials
under consideration. This is due to the presence of
different materials of various thicknesses and their
sandwiching influence. The simplified model precludes
the possibility of studying the laser–material interactions
at the various interfaces, where the real time
interactions continue to remain drastic and intricate, by
virtue of different expansion rates of the metals, viz.
zinc, aluminium and steel, as the beam power travels
down the sandwich, thereby giving rise to different
temperature gradients at the interfaces.
Although it is intended to amplify the model to
investigate the laser cutting of metallic coated sheet
G.242 V.S. Prasad et al. : Journal of Materials Processing Technology 74 (1998) 234–242
steels in future work, the situation is rather complex,
considering the composition of these specimens. The
model developed so far thus provide scope for further
modifications in the light of the above-mentioned intricacies
and can be modified to accommodate the interactions
at the interfaces of the sandwich of the specimens
in terms of the temperature gradients and the different
coefficients of linear expansions of the various metals.
Acknowledgements
The authors wish to express their thanks and
appreciation to BHP Steels Ltd., Australia, for
providing the specimens and the Queensland
Manufacturing Institute, Brisbane, Australia, for the
Cincinatti CL-5 CNC laser centre to carry out the
experiments.
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