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` 对预测CMT焊和脉冲MAG焊焊缝的软件开发在最近几年在焊接之前预测焊缝形状一直是研究人员的挑战研究。
保证焊缝形状有足够的渗透和稀释，好润湿角和无咬边来减少的修复和返工，从而，降低了成本和时间消耗。
CMT是最近引入市场MIG / MAG焊接工艺的变体。
运用焊接焊缝的预测软件帮助用户熟悉这一现代技术。
在本文件中，两种方法进行了软件发展。
这些被应用到CMT和脉冲-MAG焊接并将结果进行了比较。
图形界面是建于帮助用户选择焊接参数并获得焊道剖面的人工智能表达法。
这两个用于预测焊缝剖面方法是神经网络和插值方法。
该数据条目的两种不同方法是：焊接方法（CMT焊和脉冲MAG焊），钢板厚度，焊接速度和送丝速度。
该输出数据是由焊接剖面的点X和Y坐标定义的完整的焊接剖面。
神经网络是一系列不同数量的神经元，多个层和不同的网络，具有不同结构使用的焊接参数，以找到最好的拟合神经网络。
最终每种方法的结果都与实际轮廓核对，得出的结论是该神经网络方法更加灵活，每个方法的精确度几乎相同.1，介绍冷金属过度焊接技术（CMT）是2005年Fronius公司在市场上推出的近代焊接工艺。
凭借其创新的方式来控制焊接过程导线的向后和向前运动，可以保证短路传输中低电流和能量传到工件[1,2]。
这些性能得到无飞溅的清洁接缝。
CMT 也可以焊接薄板（0.3毫米），同时加入不同的材料（例如，钢与铝）[1]。
这个工艺非常适用于具有良好的耐受性自动化流程，从而，它已被汽车工业实现。
示例是与大众辉腾，宾利欧陆GT车型和欧宝雅特模型，在装配生产线上已经使用在CMT焊接工艺。
CMT也用于在包层的应用，作为Lorenzin公司等研究成果[3]。
他们得出的结论是CMT可用于小型面向中型的细节，而传统的高倍率面方法因为它们的高的热输入不能使用。
在这项工作中所使用的第二个方法是常规脉冲惰性/活性气体（MIG / MAG）保护焊接，这种方法是开发于1960年，如今被广泛用于焊接低碳钢，不锈钢和铝。
CMT焊接铝合金的组织和力学性能相似或不同摘要在汽车工业中完整的铝合金结构需要大量的焊接。
因此，接头的性能对整个结构的整体性能有显著影响。
机器人冷金属过渡焊接是一种相对较新的焊接技术，并已用ER5356和ER4043填充金属焊接6082-T4和5182-O铝合金板这项工作。
组织特征通过光学显微镜和能量分散型X射线光谱仪，和通过拉伸测定和硬度测试测试机械性能表示。
做出焊接变量，机械性能和焊缝的显微组织之间的相关性。
结果表明，机器人冷金属过渡焊接在高焊接速度在得到良好的拉伸强度和延展性的接头。
由于机器人冷金属过渡焊接过程的热量输入低，热影响区组织与基体金属组织非常相似，并且焊接金属组织是控制焊接接头效果的因素。
表现最好的是在5182/5182与ER5356焊接接头和这些有机械性能分别是屈服强度，极限抗拉强度和延伸率的100％，98％，和85％。
1 介绍交通运输是最大的能源消耗部门之一，使用世界能源需求约19％。
如今，世界运输系统的96％依赖关于石油燃料和产品，全球交通系统约占世界石油消耗的40％每天近75万桶石油[1]。
使用铝有助于减少汽车，公共汽车，卡车，飞机，火车和船的重量。
当重量减轻，在运输过程中的能耗是降低。
因此，相比替代材料，与铝产品相关的额外的能量和额外的温室气体排放量通过产品的寿命周期可以收回很多次。
另外，铝合金的强度和耐腐蚀性保证耐用性，可靠性和安全性，再加上成本效益。
最后，它的总可回收允许铝行业满足其可持续发展原则的承诺并保证未来新一代有资源。
所以，这不奇怪的是，使用铝的增加跨越运输的整个应用范围。
目前一般欧洲车平均使用超过130公斤的铝，这也在稳步上升[2，3]。
特别感兴趣的是5000系列合金用于内部的车身板，隔热罩，以及结构和焊接件;和6000系列合金为主体部件，外部和内部车身面板，以及结构和焊接件[4-7]。
对设计师确定异种材料牌号制成的变化组件需要连接工艺是一个永久的挑战。
铝及铝合金的焊接cmt焊接参数英文回答：CMT Welding Parameters for Aluminum and Aluminum Alloys.Introduction.CMT (Cold Metal Transfer) welding is a specialized arc welding process designed for welding aluminum and its alloys. It is a variation of the GMAW (Gas Metal Arc Welding) process but utilizes a unique wire feeding mechanism and shielding gas mixture to achieve high-quality welds with minimal spatter and porosity.Parameters.The optimal CMT welding parameters vary depending onthe specific aluminum alloy, thickness, and joint design being welded. However, some general guidelines can be provided:Welding Current: 60-200 amps.Arc Voltage: 15-25 volts.Wire Feed Speed: 2-6 meters per minute.Shielding Gas: Argon-based mix (82% Argon, 18% Helium)。
Electrode Extension: 10-20 mm.Travel Speed: 0.5-2.5 meters per minute.Advantages of CMT Welding.Reduced Spatter: The unique wire feeding mechanism controls the molten metal droplet transfer, minimizing spatter and improving weld quality.Low Porosity: CMT welding produces welds with minimal porosity due to the shielding gas mixture and controlled metal transfer.High Strength: The combination of low spatter and porosity results in welds with excellent mechanical properties and tensile strength.Automation Compatibility: CMT welding is well-suited for automated welding applications due to its stable arc and consistent results.Applications.CMT welding is commonly used in industries where high-quality aluminum welds are required, such as:Automotive.Aerospace.Electronics.Construction.Medical.Conclusion.CMT welding is a versatile and reliable process for welding aluminum and its alloys. By carefully selecting and adjusting the welding parameters, it is possible to achieve high-quality welds with minimal defects.中文回答：CMT焊接铝及铝合金工艺参数。
CMT焊接CMT焊接目前国内外低热输入焊接新工艺CMT(cold metal transfer)一冷金属过渡焊是低热输入焊接工艺中的佼佼者，CMT技术是福尼斯公司开发的一种低热输入焊接工艺。
该技术在熔滴短路时电源输出电流几乎为零，同时焊丝回抽帮助熔滴脱落，实现熔滴“冷”过渡，大大降低了焊接过程的热输入。
1.CMT焊接研究现状图1 CMT焊与P-MIG焊熔滴过渡形式分布CMT技术的发展过程经历了几个阶段:90年代初，奥地利福尼斯公司是为研究钢铝的异种焊接而开始;到90年代末，开发了无飞溅引弧技术(SFI，此技术为CMT的研究奠定了基础;在接下来的几年到1999年，使得CMT技术得以问世;到2010年，Fronius公司对CMT焊接系统进行开发，发展到了CMT Advanced和CMT Advanced +P焊接技术。
发展到现在，CMT焊与P-MIG焊熔滴过渡形式电流电压的分布如图1所示，CMT技术的热输入量达到的范围明显的小于P-MIG。
CMT技术创新的将熔滴过渡过程与送丝运动相结合，该创新处大大降低了焊接过程的热输入量，真正实现了无飞溅焊接。
此焊接工艺不仅提高焊后工件表面质量，还减小金属的损失，降低焊接过程中的烟尘、有害气体，对环境的污染进一步减小是一种绿色环保的焊接技术。
目前CMT焊接的研究主要涉及到薄板焊接、异种焊接、钎焊等，利用的均是其热输入低的特点。
CMT焊可以焊接薄板低至0.3mm的超薄板，CMT焊接工艺己研究应用的有3 mm及以下的铝合金焊接、镁铝异种焊接、铝钢异种焊接、钦铜异种焊接等。
CMT技术问世后专家学者不断的进行研究，目前关于CMT技术复合热源也出现了。
国外学者利用CMT-GMAW焊接镍基超耐热不锈钢，河北科技大学也正在研究利用CMT与高频复合焊接铝锂合金。
2. CMT焊接原理与特点CMT(冷金属过渡技术)的熔滴过渡形式是在短路过渡基础上开发的，普通的短路过渡过程如下:焊丝端部熔化形成熔滴，熔滴与熔池接触形成小桥，焊丝在小桥处爆断，短路时伴有大的电流和飞溅。
1.Pattern MaterialWood is the most common material for patterns. It is easy to work and readily available. Properly selected and kiln-dried mahogany, walnut, white pine, and sugar pine are often used. Sugar pine is most often used because it is easily worked and is generally free from warping and cracking. Moisture in the wood should be about 5 to 6% to avoid warping, shrinking, or expanding of the finished pattern.Metal patterns may be loose or mounted. If usage warrants a metal pattern, then the pattern probably should be mounted on a plate and include the gating system. Metal is used when a large number of castings are desired from a pattern or when conditions are too severe for wooden patterns. Metal patterns wear well. Another advantage of a metal pattern is freedom from warping in storage. Commonly a metal pattern is itself cast from a master pattern and can be replaced readily if damaged or worn.Patterns are made of plaster and plastics. Plaster patterns are easy to make; they can be cast where original molds are available. However, plaster is brittle and not suitable for molding large numbers of sand castings.Plastics serve in several ways for pattern making. Some conventional patterns are made of abrasion-resistant plastics with cost and durability between wood and metal. Another use of certain plastics is to make emergency patterns quickly or to salvage worn or broken patterns.1.模型材料木板是常用的材料，很容易去做并且很通用，普通的选择是桃花芯木，胡桃木，白松和糖松。
Microstructure and Mechanical Properties of Cold Metal Transfer Welding Similar and Dissimilar Aluminum AlloysAhmed Elrefaey •Nigel G.RossReceived:28March 2014/Revised:29January 2015/Published online:10April 2015ÓThe Chinese Society for Metals and Springer-Verlag Berlin Heidelberg 2015Abstract Integrating structures made from aluminum alloys in automotive industry requires a large amount of joining.As a consequence,the properties of the joints have a significant influence on the overall performance of the whole structure.Robot cold metal transfer welding is a relatively new joining technique and has been used in this work to join 6082-T4and 5182-O aluminum alloy sheets by using ER5356and ER4043filler metals.Microstructure characterization was performed by optical microscopy and energy dispersive X-ray spectroscopy,and the mechanical properties were measured by tensile and hardness tests.A correlation is made between welding variables,mechanical properties and the microstructure of welded joints.Results indicate that robot cold metal transfer welding provides good joint efficiency with high welding speed,good tensile strength,and ductility.Owing to the low heat input of robot cold metal transfer welding process,the heat affected zone microstructure was quite similar to base metals,and weld metal microstructure was the controlling factor of joint efficiency.The best performing were the 5182/5182joints welded with ER5356and these had mechanical property coefficients of 100%,98%,and 85%for yield strength,ultimate tensile strength,and elongation,respectively.KEY WORDS:Aluminum;CMT;Welding;Microstructure;Tensile property1IntroductionTransportation is one of the largest energy-consuming sectors,using about 19%of the world’s energy demand.Nowadays,96%of the world’s transportation systems de-pend on petroleum-based fuels and products,with global transportation systems accounting for about 40%of the world’s oil consumption of nearly 75million barrels of oil per day [1].Use of aluminum helps to reduce the weight of cars,buses,trucks,planes,trains and boats.When the weight is reduced,energy consumption during transport is reduced.Thus,the extra energy and extra greenhouse gasemissions related to the production of aluminum compared to alternative materials may be paid back many times through the life cycle of the product.Additionally,the strength and corrosion-resistance of aluminum alloys guarantee durability,reliability and security,coupled with cost-effectiveness.Finally,its total recyclability allows the aluminum industry to fulfill its commitment to the princi-ples of sustainable development and its pledge to future generations.So,it is not surprising that the use of aluminum is increasing across the whole range of transport applica-tions.At present over 130kg of aluminum goes into the average European car,and this is rising steadily [2,3].Of particular interest are the 5000-series alloys used for inner body panels,heat shields,and structural and weldable parts;and the 6000-series alloys for body components,outer and inner body panels,and structural and weldable parts [4–7].A perpetual challenge to the designer is determining the joining process needed to create an assembly made from a variation of grades of dissimilar materials.Additionally,Available online at /journal/40195A.Elrefaey (&)ÁN.G.RossLKR Leichtmetallkompetenzzentrum Ranshofen GmbH,Austrian Institute of Technology,5282Ranshofen,Austria e-mail:ahmed.elrefaei@ait.ac.atActa Metall.Sin.(Engl.Lett.),2015,28(6),715–724DOI 10.1007/s40195-015-0252-6automakers are constantly searching for innovative means of reducing vehicle weight and manufacturing costs in order to meet ever-restricting fuel economy standards while remaining economically competitive.Thus,the au-tomotive industry is interested in more than just the type of metal being used.A promising opportunity to meet these seemingly conflicting requirements is through the use of tailor welded blanks(TWBs).These are blanks made from multiple sheets of material welded together prior to the forming process.The differences in the materials within a TWB can be in the thicknesses and/or grades of the ma-terials[8].Consequently,the mechanical properties of the welded joints are important because the TWBs must stand up to the subsequent forming processes.Cold metal transfer(CMT)technology is a likely inno-vative welding process for joining aluminum parts of similar and dissimilar alloys,and offers versatility,environmental friendliness,and energy efficiency[9,10].CMT utilizes an innovative wire feed system integrated with high-speed digital control in order to control not only the arc length during welding,but also the method of material transfer and amount of thermal input transferred to the work piece.The process is based upon dip transfer welding insofar as mate-rial transfer is initiated at the point of short circuit of the wire electrode into the weld pool and operates in a similar power range.However,whereas material transfer in dip transfer welding is controlled electrically in the CMT process control of the material transfer,short circuit initiation and short circuit duration is by mechanically extending and retracting the wire electrode into and out of the weld pool for defined durations.By incorporating this mechanical process into the electrical process control,the point of short circuit can be detected and the current cut,thus greatly reducing the ther-mal input to the work piece[11].Owing to the lower thermal heat input,gap-bridging ability,low dilution,fast operation and low spatter offered by CMT welding compared to other welding techniques,it is considered very attractive and promising for joining similar and dissimilar aluminum alloys.Reported studies on CMT welding are rather scarce since the process is relatively new.Most of the studies focus on welding aluminum to steel alloys since the heat input can be controlled,and consequently,this controls the volume fraction of intermetallic compounds;thereby en-abling optimization of the joint strength[12–14].For the same reason,aluminum to magnesium CMT welding has attracted the attention of some researchers,but inter-metallic compounds have been difficult to avoid even with the low heat input process[15,16].Some reports have presented a successful application of the CMT process—that is,low spatter and good gap bridging ability—in welding parts made from similar aluminum alloys[11,17]. However,little work has been done in the area of CMT welding of Al sheets of dissimilar alloys.As far as we know,only one study has been reported on CMT welding of6-mm-thick5083-H111and6082-T651aluminum al-loys.Good joint efficiency with high welding speed,good tensile,and fatigue performance has been achieved[18].The choice of CMT welding wire is of fundamental importance.While no aluminumfiller metalfits all needs, ER5356and ER4043are the two most common and make up the majority of aluminumfiller metal applications.They can be used with the most widely used aluminum alloy base metals.Therefore,these twofiller metals were evaluated in this study.A detailed investigation of the characteristic mi-crostructural features and mechanical properties is impor-tant to many industrialfields.The current study explores the relationship between the aluminum alloy with respect to the original microstructure and mechanical properties, and their comparable characteristics after welding.How-ever,to achieve this and produce a weld metal with specific characteristics,proper control is required of the factors that interact during welding.The relationship between the welding parameters,microstructure features,and the me-chanical properties of the joints will then be characterized and evaluated.2Experimental Procedure2.1Material and Welding ProcedureThe base metals for this study were2-mm-thick5182-O and6082-T4aluminum alloy sheets.Coupons were cut with dimension of250mm9100mm and placed in a butt joint configuration without a gap between the plates. Chemical compositions for the base andfiller metals[19] are shown in Table1.Each coupon was cleaned with acetone and steel brush to remove the oxide layer and other contaminants from the surface of the samples.After cleaning,the coupons were clamped and argon shielding gas,at aflow rate of15L/min,was switched on3s before the arc was struck.A torch angle of10o from the perpen-dicular axes was used,and welding was carried out using the robotic CMT technique.A matrix of welding parameters providing different heat inputs to the work piece was selected and is given in Table2.Figure1shows the CMT robot head and welding assembly.Heat input is a relative measure of the energy transferred per unit length of weld.It is an important characteristic because it influences the cooling rate,and this in turn has an effect on the microstructure of the weld metal and the heat affected zone(HAZ)and consequently on the mechanical properties.Heat input(J)is calculated using the equation:J¼g60ÂVIðÞ=v:ð1Þwhere V is the mean welding voltage,I is the mean welding current,v is the welding speed,and g is the efficiency factor for the welding process.For the CMT process,g is 0.9[20,21].2.2MetallographyCross-sectioning of the welds was performed in plane and perpendicular to the welding direction for metallographic analysis to identify the different weld zones and check for the presence of welding defects such as porosity and lack of fusion.The samples were prepared according to standard metallographic practice ASTM E3-11,and etched with Barker’s reagent(5mL HBF4(48%)in200mL water). The microstructures of different zones of interest,such as base metal(BM),HAZ and weld metal(WM)under dif-ferent welding speed combinations,were analyzed on an Olympus BX60M microscopefitted with an Olympus UC30camera using polarized and unpolarized light.Ima-ges were analyzed using the Olympus Stream Motion1.8 image analyzing software.Porosity percentages and im-perfections were studied and evaluated from the cross section of the bead.Additionally,a JEOL JSM7001F scanning electron microscope(SEM)equipped with anTable1Chemical composition(wt%)of the aluminum alloys andfiller metalsMaterial Si Fe Cu Mn Mg Cr Zn Ti Others Al6082-T40.7–1.30.50.10.4–1.00.6–1.20.250.20.10.15Bal. 5182-O0.20.350.150.2–0.5 4.0-5.00.10.250.10.15Bal. ER53560.250.40.10.05 4.50.050.10.060.06Bal. ER4043 5.60.80.30.050.050.050.10.020.08Bal.Table2Welding parametersFiller Base metal combinations No.Current(A)Voltage(V)Welding speed(mm/s)Wire feed rate(m/min)Heat input(kJ/mm)ER53566082/608216914.310 4.30.08929315.115 5.50.084310415.420 6.20.072 6082/518246915.710 4.30.09859314.915 5.50.083610416.220 5.70.076 5182/518276913.910 4.30.08689314.915 5.50.08399615.020 5.70.065ER40436082/6082105515.110 2.90.075118215.515 3.40.076129016.620 3.70.067 6082/5182135514.910 2.90.074148216.115 3.40.079159016.720 3.70.068 5182/5182165515.810 2.90.078177816.715 3.20.078188616.420 3.50.065Fig.1Robot head and sample clamping before weldingenergy dispersive X-ray (EDX)spectrometer was em-ployed to investigate the intermetallic phases.2.3Tensile and Hardness TestingThe weld bead was ground flat,and tensile samples were prepared in accordance with DIN 50125[22].Tensile specimens were machined from four locations from the welded joints,as shown in Fig.2:(1)base material in the rolling or parallel direction (H specimen);(2)base material in the transverse or vertical direction (V specimen);(3)transverse specimens containing the weld in the center of the gauge length (T specimen);and (4)longitudinal spe-cimens machined along the weld metal (L specimen).Six samples were used to calculate the average yield strength,tensile strength,and ductility for each combination of the welding parameters.It is worth noting that transverse weld specimens provide a measure of joint efficiency in terms of strength,but do not provide an accurate ductility mea-surement of the weld.However,the longitudinal weld specimens are more representative of the weld metal properties and joint formability [23].The tests were carried out by Zwick Z100tensile machine at room temperature with a displacement speed was 0.5mm/s.Hardness measurement was taken with the help of a Vickers hardness testing machine type (Leco LM700)ac-cording to ASTM E384-11e1with a 1.0N load,0.35mm between indentations in the weld and adjacent areas,and0.5mm between indentations in the base metals.Mea-surements were conducted through the center of welded joints across the BM,HAZ,and WM.3Results and Discussion 3.1MicrostructureCharacteristic microstructures of the base metals are shown in Fig.3.The presence of silicon in the 6082alloy results in the formation of Mg 2Si in addition to other phases,see Fig.3a.Most 6082commercial alloys also contain iron as an impurity.As a result,the a -Al ?Al 6(Fe,Mn)are formed in addition to Al–Fe,Al–Mg,and Al–Mg-Mn in-termetallic phases [24–26].In contrast,the 5182alloy has coarse insoluble intermetallic phases.These were identified by EDX as Mg 2Al 3and other precipitates containing quite high contents of Fe,Mn,Al,and Si,see Fig.3b.This result is in agreement with other reported studies [27,28].The visual appearance of the joints with regard to surface roughness and bead shape appeared to be very uniform for all joints.Figure 4shows the bead profiles and cross-sectional macrostructures of representative joints.Consistent,tight,and uniform ripples indicate a strong weld,free from con-tamination or cracks.This observation is typical for auto-mated welding processes.The distance between weld ripples is larger in joints No.3and No.12compared to joints No.1and No.10owing to the higher welding speed.Owing to the high content of silicon in ER4043(Table 1),this alloy has a lower melting point and more fluidity than ER5356.There-fore,joints welded with ER5356filler metal have much less spatter than the joints welded with ER4043filler metal,and weld spatter from the latter is very low.This phenomenon has been reported by other researchers [29,30].This con-firms the ability of the CMT process to produce joints with virtually no weld stly,a visual observation of the joints found an absence of obvious surface flaws such as lack of fusion,undercuts,overlaps,and cracks.Influence of heat input on welded metal area is clearly shown in Fig.4.Weld size and mechanical properties are both influenced by the heat input.Heat input is usually chosen to be high enough to achieve full penetration and sufficient fusion to base metals,but also as low as possible to decrease weld distortion and phase transformation in the fusion and heat affected zones [31,32].The welded areas in the high heat input joints (joints No.1and No.10)are almost 25%larger than the welded area in joints welded with low heat input (joints No.3and No.12).Furthermore,full and continuous penetration of the welded beads is present in the high heat input joints (Fig.4b and h)in contrast to the interrupted penetration in low heat input joints (Fig.4e andk).Fig.2Schematic view of the welded joint showing the locations of test samples (unit:mm)Representative macrostructures of the joints welded with ER5356and ER4043are shown in Fig.5a and e, respectively.During the weld thermal cycle,the BM in the fusion zone partially melts.The weld microstructures ap-pear dendritic and owing to the temperature gradient the edge of the weld has more columnar dendrites in contrast to the equiaxed dendrites found at the center of the weld.For bothfiller metals,the weld metal is characterized by an Al-rich matrix and secondary eutectic phases.In the case of the ER5356filler metal,the secondary eutectic phases are present as spherical precipitates(Fig.5b),and as an in-terdendritic network in the case of ER4043(Fig.5f).Comparing the WM grain sizes of the twofiller metals, the grains are larger when ER5356is used.The average WM grain size in the cases of joints welded with ER5356 and ER4043are217and164l m,respectively.In general, the higher heat input into the welded area needed for this filler metal promotes grain growth and results in larger grain sizes.This phenomenon is limited only to WM since the HAZ close to the weld metal showed almost no change in grain size of HAZ compared to the BM(Fig.5e,d,g and h).Surprisingly,while no observed change in precipitates types and shape in6082HAZ,the5182HAZ close to WM containsfine precipitates in the aluminum matrix and next to this area,to the direction of the BM,coarsening of the Mg2Al3precipitates can be seen in Fig.5e and g.This coarsening has been reported in other studies[6,33].Figure6shows SEM micrographs of the WM,6082 HAZ,and5182HAZ from sample No.6.The excess of Mg in thefiller metal promotes the formation of spherical precipitates identified by EDX as Mg2Si.Additionally,the light intermetallic Mg–Al phases scattered in the alu-minum matrix also contain Si,Mn and Fe.These ele-ments constitute phases such as Al3Mg2,Al3Fe and AlMg2Mn,see Fig.6a.The Mg2Si precipitates are clearly visible in the6082HAZ,see Fig.6b.Meanwhile in the 5182HAZ,there is a transition zone containing some coarse particles of Al3Mg2and veryfine precipitates which could not be identified accurately by EDX analysis, as shown in Fig.6c.It is proposed that the Al3Mg2pre-cipitates close to the WM dissolved during the heating cycle and re-precipitated again during cooling depending on the cooling rate.Maps of elements present in the weld metal,sample No. 6,are shown in Fig.7.The distribution of elements shows concentrations of Si and Mn at the grain boundaries and a uniform distribution of Mg in all areas.Also,a depletion of aluminum is obviously seen at the grain boundaries where the precipitates formed.The EDX analyses revealed lower concentrations of Si and Mg in the joints welded with ER5356filler metal than in the joints welded with ER4043. As shown in Fig.7b,the WM produced from ER5356filler contains low contents of Si and Mg.There is an increase in the Si content with respect to thefiller metal composition (0.2–0.98wt%),and this will be due to dilution from the 6082sheet.Mg content is reduced(4.5–1.22wt%)because it is prone to evaporation during welding[34,35].In contrast,the ER4043filler produced WM with high Si content(9.6wt%)and the Mg concentration(0.92wt%) have increased by dilution from the base metals,especially when at least one of the base metals is the5182alloy(the spectrum of the weld metal is not shown).Changes in the alloying elements in the weld metal area will certainly affect the mechanical properties of the joints.3.2Mechanical PropertiesMicro-hardness profiles across the welded joints produced by the different welding parameters are shown in Fig.8.It is observed that the lowest hardness values are measured in the WM area of the joints welded with ER5356filler,especially when at least one of the base metals is6082alloy.In contrast to ER5356filler,joints welded with ER4043filler show higher hardness in the weld metal when at least one ofbase Fig.3Polarized(bottom)and unpolarized(top)light images showing the microstructure of the base metals:a6082alloy;b5182alloymetals is 5182alloy.In fact,several factors are controlling the hardness of the weld area.The main factors are the al-loying elements in solid solution;precipitate volume frac-tion,morphology and size and grain size.During welding,the elements in the WM are a combination of the filler metal and the dilution from the base metal.When at least one of the base metals is 6082alloy,which contains low Mg and Si,the amount of alloying elements and precipitates is lower in the weld metal area,see Fig.5b and f.This results in the decrease of WM hardness,as shown in Fig.8a,c and e.Due to the higher heat input required for the ER5356filler metal,it can also be concluded that these joints ex-perience a slower cooling rate than the ER4043filler metal joints,and hence have a coarser-grained WM microstruc-ture.Furthermore,after experiencing a melting process,the solidified alloy was expected to be in the form of super-saturated solution,and few particles precipitate from the supersaturated solid solution.Thus,the lower micro-hard-ness in the WM of the joints welded with ER5356is due to the action of grain coarsening and weak precipitation strengthening.On the other hand,when using ER4043filler metal,the concentrations of alloying elements and pre-cipitates in the WM increases when at least one of base metals is 5182alloy.The filler metal is already high in Si and becomes richer in Mg due to the dilution of the base metal.The increase in alloying elements andprecipitatesFig.4Bead profiles and cross-sectional macrostructures of representative joints:a ,b ,c sample No.1;d ,e ,f sample No.3;g ,h ,i sample No.10;j ,k ,l sample No.12causes the WM hardness to proportionally increase,as shown in Fig.8b,d and f.It is worth noting that the hardness of the 6082HAZ is slightly affected by weld thermal cycle.Despite the unclear change in the HAZ microstructure,a small drop in hardness in HAZ is observed compared with the BM.It is reported that the strength losses in the 6000alloys are less in the naturally aged metal than in the artificially aged alloys [6].On the other hand,the 5182HAZ showed similar hardness values to the base metal.The yield strength,tensile strength,and elongation for all joints are shown in Fig.9.Table 3shows the average mechanical properties of the base metals for comparison.In the case of joints welded with ER5356filler,the 5182/5182joints have the highest ultimate tensile strength (UTS),and in most cases also the highest elongation.Since there is homogeneity between the base and weld area,this com-bination achieved good mechanical properties.The me-chanical property coefficients of the 5182/5182joints (known as the ratio between the mechanical properties of a welded joint and that of base metal)are 100%,98%,and 85%for YS,UTS,and elongation,respectively.The corresponding ratios with respect to the 6082/6082joints are 83%,79%,and 66%.It is interesting to note that there is little variation in YS for all joints and L samples were higher in UTS and elongation than T samples in all joints which have low hardness in weld metal area.In T welded joints,since the weld is weaker than the surrounding transverse areas,it possesses all plastic deformation and the fracture took place at a low tensile load.On the other hand,L welded joints display entirely different tensile behavior.The different zones have different resistance to deforma-tion due to difference in microstructure,grain size,pre-cipitate size and distribution.The increase in strength properties compared to T joints is due to the presence of the stronger HAZ and BM longitudinal to WM and resisting the deformation [36,37].With a much higher elongation than all other joints,the 6082/6082joints welded with both filler metals show a good combination of properties.The high elongation of these joints is owing to the low hardness level in the weld metal area,as clearly shown in Fig.8a and b.High hard-ness in the case of 5182/5182joints and 5182/6082joints resulted in a lower elongation of these joints,but withnoFig.5Typical microstructures of the dissimilar joints No.6a ,No.15e ,polarized (bottom )and unpolarized (top )optical light images showing the microstructures of the WM of joints No.6b ,No.15f ,5182HAZ/WM interface of joints No.6c ,No.15g 6082HAZ/WM interface of joints No.6d ,No.15hFig.6SEM images showing the microstructure of sample No.6:a WM;b WM/6082HAZ;c WM/5182HAZapparent effect on the UTS or YS.The YS and UTS of the ER4043welded joints are comparable to joints welded with ER5356filler metal.Visual examination of the tensile samples after fracture indicated that the fracture always occurred within the weld area for the6082/6082joints and in the majority of 5182/6082and5182/5182joints welded with ER5356fil-ler.Meanwhile,the fracture path shifted mainly to the5182 base metal in case of5182/5182and5182/6082joints welded with ER4043filler.This result indicates that the HAZ is not the weakest location for these latter joints. These observations are in agreement with the hardness results.Failure occurs in the WM when the hardness of the WM is less than the BM hardness and vice versa.Failure of the joint in the5182alloy BM when the WM has the highest hardness is to be expected because the5182alloy has the lowest YS of the two BMs(Table3).This clearly shows the welded joint properties and fracture paths are controlled to a large extent by the WM microstructure and composition.For the6082/6082joints,the best combination of me-chanical properties was achieved using ER5356filler since the composition of WM contained less amount of alloying elements,compared to joint welded with ER4043,which improve the ductility of the joints in addition to keeping the strength at an optimum level.Since the softest part of the joints is in WM up to the hardness measurements,the fracture originated and propagated in this area.ER4043 showed less compatibility with5182alloy due to the high alloying elements and consequently the high hardness of the produced WM.In spite of achieving good YS and UTS by using ER4043filler metal,the joints ductilitysharply Fig.7SEM image a,EDS spectrum of the weld metal area b,EDX maps of elements in the weld metal of sample No.6c–fdegrade.The fracture path in joints welded with ER4043,except 6082/6082joints,is mainly in 5182BM since it is the softest area in the joints.4Conclusions5182and 6082aluminum sheets were successfully joined using different CMT welding parameters.The following results were obtained:1.The 6082HAZ showed almost no change in shape and size of precipitates compared to the BM.However,in the 5182sheets,the HAZ microstructure showed fine precipitates of second phases and coarsening of the Mg 2Al 3precipitates in the aluminum matrix.2.Mechanical property coefficients for 5182/5182joints welded with ER5356are 100%,98%,and 85%for YS,UTS,and elongation,respectively.The corre-sponding ratios for 6082/6082joints are 83%,79%,and 66%.In general,joining 5182to 6082alloy did not show worse mechanical properties compared to the other joints.3.The welded joints mechanical properties are controlled by the weld metal microstructure and composition.4.Use of ER5356filler metal in welding 5182alloy sheet is better than ER4043since it produces a weld metal with low Mg and Si content.The corresponding joints are characterized by low hardness,high ductility,and high strength.However,for the 6082alloy,both filler metals arecompatible.Fig.8Micro-hardness profiles across the welded joints produced under different welding parameters,the 0mm position is taken as the left-most edge of the weld metal:a samples No.1and No.3;b samples No.10and No.12;c samples No.4and No.6;d samples No.13and No.15;e samples No.7and No.9;f samples No.16and No.18Acknowledgments The authors would like to thank the Austrian Institute of Technology (AIT)for the financial support in this project.References[1]J.W.Mcauley,Environ.Sci.Technol.37,5414(2003)[2]Factsheet,Aluminium in transport.(European Aluminium Asso-ciation),http://www.alueurope.eu/pdf/Fact%20Sheet_Transport.pdf[3]Factsheet 1,Aluminium in taransport.(UK Aluminium Industry),/downloads/documents/XN6LU4R3TW_1_aluminium_in_transport.pdf[4]G.E.Totten,S.Mackenzie (eds.),Handbook of Aluminum,Vol.1:Physical Metallurgy and Processes (Marcel Dekker Inc,New York,2003)[5]J.G.Kaufman,Introduction to Aluminum Alloys and Tempers(ASM International,Materials Park,2003),pp.90–97[6]G.Mathers,The Welding of Aluminium and its Alloys (Wood-head Publishing Limited,Cambridge,2002),pp.1–6[7]M.M.Mossman,J.C.Lippold,Weld.J.9,188s (2002)[8]B.L.Kinsey,J.Cao,J.Manuf.Sci.Eng.125,344(2003)[9]G.Antonsson (ed.),Springer Handbook of Mechanical Engi-neering (Springer,New York,2008)[10]T.Rosado,P.Almeida,I.Pires,R.Miranda,L.Quintino,Paperpresented at the 58Congresso de Engenharia de Moc ¸ambique,Maputo,2–4Sept 2008[11]C.G.Pickin,K.Young,Sci.Technol.Weld.Join.11,583(2006)[12]R.Cao,G.Yu,J.H.Chen,P.Wangc,J.Mater.Process.Technol.213,1753(2013)[13]J.Lin,N.Ma,Y.Lei,H.Murakawa,J.Mater.Process.Technol.213,1303(2013)[14]Y.Zhou,Q.Lin,J.Alloys Compd.589,307(2014)[15]J.Shang,K.Wang,Q.Zhou,D.Zhang,Mater.Des.34,559(2012)[16]R.Cao,B.F.Wen,J.H.Chen,P.Wang,Mater.Sci.Eng.A 560,256(2013)[17]J.Feng,H.Zhang,P.He,Mater.Des.30,1850(2009)[18]B.Gungor,E.Kaluc,E.Taban,A.SIK,Mater.Des.54,207(2014)[19]J.R.Davis (ed.),Properties and Selection:Nonferrous Alloysand Special-Purpose Materials (ASM International,Materials Park,1990)[20]P.M.S.Almeida,S.Williams,Innovative process model of Ti-6Al-4V additive layer manufacturing using cold metal transfer (CMT),in proceeding of the 21st Annual International Solid Freeform Fabrication Symposium,University of Texas,Austin,TX,USA,9–11Aug 2010[21]N.C.Pepe,Dissertation,Cranfield University,2010[22]Deutsche Norm,Pru ¨fung Metallischer Werkstoffe –Zugproben ,(Deutsches Institut fu¨r Normung,DIN 50125,2009)[23]M.C.Stasik,R.H.Wagoner,Int.J.Form.Process.1,9(1998)[24]D.G.Eskin,Physical Metallurgy of Direct Chill Casting ofAluminium Alloys (CRC Press,Taylor and Francis Group,Boca Raton,2008),pp.45–50[25]L.F.Mondolfo,Aluminum Alloys,Structure and Properties(Butterworths,London,1976)[26]N.Dolic,J.Medved,P.Mrvar,F.Unkic,Mater.Technol.46,563(2012)[27]S.Katoh,Weld.Int.4,944(1990)[28]R.C.Calcraft,M.A.Wahab,D.M.Viano,G.O.Schumann,R.H.Phillips,N.U.Ahmed,J.Mater.Process.Technol.92–93,60(1999)[29]C.L.N.Azida,A.Jalar,N.K.Othman,N.M.Rashdi,M.Z.Ibel,Adv.Mater.Res.146–147,1402(2010)[30]L.Hwang,C.Gung,T.Shih,J.Mater.Process.Technol.116,101(2001)[31]P.M.G.P.Moreira,L.F.M.da Silva,P.M.S.T.de Castro,Struc-tural Connections for Lightweight Metallic Structures (Springer,New York,2012)[32]J.A.Vargas,J.E.Torres,J.A.Pacheco,R.J.Hernandez,Mater.Des.52,556(2013)[33]J.R.Davis (ed.),Aluminum and Aluminum Alloys (ASM Inter-national,Materials Park,1993)[34]X.Wang,W.Huang,Q.Wei,X.Shen,Trans.China Weld.Inst.27,61(2006)[35]K.Subbaiah,M.Geetha,B.Shanmugarajan,S.R.K.Rao,Sad-hana 37,587(2012)[36]B.Kinsey,X.Wu (eds.),Tailor Welded Blanks for AdvancedManufacturing (Woodhead Publishing Limited,Cambridge,2011)[37]U.S.Dixit,R.G.Narayana,Metal Forming:Technology andProcess Modelling (Tata McGraw Hill Education Private Limited,New Delhi,2013),pp.134–135Fig.9Mechanical properties of different weld joints made with ER5356a ,ER4043b filler metalsTable 3Properties of the aluminum substrates at room temperature Material Tensile strength (MPa)Yield strength (MPa)Elongation (%)6082-T429618028.35182-O29714324.1。
CMT焊接工艺及其应用一、冷金属过渡（CMT）焊概述：1、意义：冷金属过渡技术 (CMT)是近年来焊接工艺的一次突破，其创造性地将焊丝运动与熔滴过渡过程相结合，实现了低能耗、高品质的焊接。
2、特点：（1）、良好的电弧稳定性：CMT焊接系统送丝过程受控并且和电弧过程相结合，可以机械检测弧长并快速调节，这使得CMT的电弧非常的稳定。
（2）、精确的能量输入控制：CMT技术实现了无电流状态下的熔滴过渡。
当短路电流产生，焊丝即停止前进并自动地回抽。
在这种方式中，电弧自身输入热量的过程很短，短路发生，电弧即熄灭，热输入量迅速地减少，可以获得最低能量的输入。
（3）、优异的搭桥能量输入：CMT技术具有优异的电弧稳定性和精确的低能量输入，具有优异的搭桥能力，对装配间隙和错边的要求低，根焊焊道也可以获得很好的的背面成型（4）、更快的焊接速度：CMT过渡的频率高达60—70 Hz，焊丝主动回抽促进熔滴的脱落，焊接速度可达450—600 mm／min，能够明显地提高焊接效率。
3、应用：（1）、材料应用领域：CMT技术拥有广泛的应用领域。
几乎可以应用与所有已知的材料。
（2）、行业应用：机车制造行业、航天领域、桥梁和钢结构。
二、CMT工艺原理及设备：2.1、CMT工艺原理：（1）、数字式焊接控制系统感知电弧生成的开始时间，自动降低焊接电流，直到电弧熄灭，并调节脉冲式的焊丝输送，这种脉冲式焊丝输送有效改善了焊丝熔滴的过渡。
（2）、在熔滴从焊丝上滴落之后，数字控制系统再次提高焊接电流，并进一步将焊丝向前送出。
之后重新生成焊接电弧，开始新一轮的焊接过程。
（3）、或者说系统监测到一个短路信号，就会反馈给送丝机，送丝机作出回应回抽焊丝，从而使得焊丝与熔滴分离，使熔滴在无电流状态下过渡（70HZ）。
2.2、CMT与传统短路焊接工艺比较：CMT焊与普通 GMAW 有三个最大的不同：（1）、将焊丝运动与焊接过程相结合：在焊丝前行过程中，一旦数字过程控制器检测到短路电流，便控制送丝机构回焊丝，以促成焊丝与熔滴的分离。
激光-CMT复合焊接铝合金AA6061的组织和工艺特性实施光纤激光-冰冷金属过渡（CMT）复合焊接AA6061铝合金薄板。
微观结构通过光学分析显微镜，扫描电子显微镜，和能量色散光谱法进行分析。
测试交叉焊缝拉伸强度和硬度，以评估焊接接头的机械性能。
获得可接受细显微结构和无缺陷的焊缝。
拉伸强度达到223兆帕，比激光脉冲金属惰性气体（PMIG）混合焊接接头强10％。
与激-PMIG混合焊接比较，由于CMT电弧电流波形的特征，在激光CMT复合焊接的熔池产生更强成分过冷和更多的异质形核。
相比激光PMIG联合焊接焊缝，导致了该激光CMT接头具有较细显微并与窄柱状枝晶区。
由于CMT在激光诱导锁孔稳定的电弧，激光CMT混合焊接飞溅少和焊缝中有少量氢气孔。
1.介绍铝合金的广泛应用需要先进的焊接技术。
激光焊接有高焊接速度和低热量输入的特点，是一个有吸引力的技术。
由于在高温下流动性差，铝对激光光束高的反射率（Al）引起不稳定锁孔，孔隙度很容易地发生在激光焊接铝合金[1，2]。
它减少激光焊接铝组件的可靠性，然后限制该激光焊接在铝工业的应用。
通过激光和电弧的协同效应，激光脉冲金属惰性气体（PMIG）混合焊接可以提高激光锁孔的稳定性和增加在熔池中的气泡的逸出能力，从而减少焊缝气孔[3-7]。
然而，这些研究表现出较高的电弧电流（通常超过180 A）必须用于去除焊接气孔[5]。
两大挑战出现了。
第一，增加热输入增加焊接变形。
其次，增加电弧压力增加焊接过程中的烧穿敏感性。
这些挑战阻碍激光PMIG复合焊接Al合金薄板的应用。
焊接技术的最新发展是自动化冷金属过渡（CMT）的工艺[8-11]。
这个工艺是一个短路金属过渡，由线材的向后运动协助熔滴脱离。
当金属丝端头上的液滴接触的熔池，所述导线给料机给丝向后拉力;同时，短路电流降低到一个非常低的水平。
在结束时，液滴过渡到熔池无液体桥梁断裂而产生传统的电弧焊。
它表明CMT不仅减少了热输入，也保持自由飞溅，因为它并不需要一个高电流压裂液桥[11]。
ORIGINAL ARTICLEInvestigation of droplet transfer behaviours in cold metal transfer (CMT)process on welding Ti-6Al-4V alloyZhe Sun1&Yaohui Lv1&Binshi Xu1&Yuxin Liu1&Jianjun Lin2&Kaibo Wang1Received:6December2014/Accepted:19April2015#Springer-Verlag London2015Abstract Ti-6Al-4V alloy is widely used in the aeroengine industry due to its excellent comprehensive mechanical prop-erties.In this paper,the droplet transfer behaviours of cold metal transfer(CMT)welding Ti-6Al-4V alloy have been studied by analysing captured electrical waveforms and high-speed images of droplet transfer process.The results in-dicated that the current and voltage waveforms of CMT welding Ti-6Al-4V alloy are different from that of typical CMT cycle.A current pulse appears in short-circuit phase to adjust energy distributions of different stages,and it results in smoother droplet transfer process.Frequency of droplet trans-fer increases with augment of wire feed rate or decrease of inductance correction value.Droplet size is greater with high wire feed rate or small inductance correction value. Keywords Cold metal transfer.Droplet transfer.Transfer frequency.Droplet size1IntroductionTi-6Al-4V alloy is widely used in manufacturing compressor discs and blades of aeroengine by right of its excellent com-prehensive mechanical properties and mass reduction[1–3]. Various weld procedures are applicated to welding Ti-6Al-4V alloy such as laser beam welding(LBW)[4,5],electron beam welding(EBW)[6–8],linear friction welding(LFW)[9–11],gas tungsten arc welding(GTAW)[12]and gas metal arc welding(GMAW)[13].GMAW has advantages of high pro-ductivity and convenience for automatic applications[14]. However,energy density of GMAW is relatively lower,and thus heat affect zone(HAZ)is wide.It goes against the welding quality especially in thin-wall parts.Cold metal transfer(CMT),as a modified process of GMAW,completes droplet detachment by the motion of welding wire in short-circuit phase,hence,it nearly re-quires no electromagnetic force when droplet detaching from the welding wire.Therefore,CMT process not only has the GMAW characteristics of low cost and high ef-ficiency,but also reduces heat input effectively.Basic operating principles of CMT process were previously re-ported by Pickin and Young[15]through comparing the process stability between conventional arc welding and CMT process for welding thin aluminum alloy sheets. And this work has been expanded by Feng et al.[16] for welding aluminum sheets.Additional studies by Zhang et al.[17]of joining aluminum to steel also have similar findings about the typical characteristics of CMT process.The conventional arc characteristics and droplet transfer behaviours of CMT process have been summa-rized based on the studies mentioned above.While the current and voltage in short-circuit phase almost ap-proach zero,the input heat is greatly reduced.Further studies have been expanded to nickel alloy welding and dissimilar metals joining.Benoit et al.[18]have researched on Inconel718weldability using CMT pro-cess,and the microstructure of Inconel625CMT overlay on steel has been described by Rozmus-Górnikowska et al.[19].Weldability of CMT joining of AA6061-T6 to boron steels has been studied by Cao et al.[20],and Zhang et al.[21]have studied laser-CMT hybrid welding of AA6061aluminum alloy.Shang et al.[22]has *Zhe Sunbit20081277@1Academy of Armored Forces Engineering,Beijing,China2Academy of Armored Forces Engineering,Shanghai Jiao TongUniversity,Beijing,ChinaInt J Adv Manuf TechnolDOI10.1007/s00170-015-7197-9researched the microstructure and mechanical properties of CMT joining Mg/Al dissimilar metals.However,it is notable that typical characteristics of CMT process are revelated though experiments with filler metal of alumi-num or aluminum alloy.No exhaustive works on tran-sient phenomena of droplet transfer behaviours for welding with filler metal which has different dynamic viscosity and surface tension coefficient from aluminum alloy have been investigated,such as titanium or titani-um alloy.In this paper,we have investigated droplet transfer behaviours of CMT welding Ti-6Ai-4V alloy.Energy distribution of different phases in droplet transfer pro-cess is studied by analysis of current and voltage wave-forms,and the dynamic procedure of droplet transfer is observed by high-speed images.Furthermore,droplet transfer behaviours with different parameters have also been researched.2ExperimentalBase metal and welding wire are both Ti-6Al-4V alloy and specified composition ranges are detailed in Table 1.Welding wire diameter was maintained at 1.2mm,and base metal was machined to dimension of 150×100×8mm for all experiments.A simple vision sensing system shown in Fig.1was developed for capturing high-speed images of molten droplet and capturing electrical signal.The high-speed charge coupled device (CCD)camera used was HiSpec5.Frame capture rate was set at 1150frames/s with a resolution of 1280×1024.The current and voltage tran-sient capturing module was made based upon Hall current and voltage sensors.The oscilloscope used to capture the waveform was Aligent MSO6034A and the sampling widths were set at 5ms/grid or 20ms/grid.Droplet size shown in Fig.2was calculated through the measured proportion relation between welding wire and droplet.Instantaneous values were acquired from captured current and voltage waveforms.The arc instantaneous power value P i was calculated using Eq.(1),this being derived from the product of measured instantaneous cur-rent I i and instantaneous voltage V i .Studies by Joseph et al.[23]and Koiotynskii et al.[24]have shown that the approach would have a greater accuracy than that which is on the basis of average values when investi-gating pulsed welding.A CMT cycle consists of two phases with huge differences in process values;hence,greater accuracy will be realized by using this approach rather than adopting the average value.Therefore,the output energy in time interval (t 1,t 2)was calculated by the Eq.(2),which is the integration of P i in that time interval.p i ¼V i I ið1ÞE Δt ¼Z t 2t 1V i I i dtð2ÞAll trials were conducted by a CMTwelding system which is made up of Fronius Advanced 4000welding power source and KUKA KR6robotic.The motions of welding wire were controlled by CMT torch with digital controlled wire feeder.Trials were investigated with parameters detailed in Table 2.Values of applied mean current and voltage were set throughTable 1Specified composition ranges of Ti-6Al-4V alloy TiAlVFeOCNHBalanced 5.5∼6.75 3.5∼4.5≤0.25≤0.18≤0.05≤0.05≤0.012Fig.1Configuration of vision sensingsystemFig.2Schematic diagram of droplet sizeInt J Adv Manuf Technolsetting wire feed rate due to unified adjusting mode.The welding wire melting rates with various welding current and voltage are shown in Fig.3,and the data are from Fronius Expert System.3Results3.1Transient phenomena in CMT process on welding Ti-6Al-4V alloyIn order to demonstrate the stability of CMT process on welding T-6Al-4V alloy,the captured transient values of cur-rent and voltage are connected according time order in the current-voltage coordinate plane,as is shown in Fig.4.The line cluster is concentrated,and there is almost no deviant line.It indicates that the process of CMT welding of Ti-6Al-4V is quite stable.CMT current and voltage waveforms of Ti-6Al-4V alloy are shown in Fig.5,the wire feed rate is 5.2m/min and the inductance correction value is maintained at 0.The typical CMT cycle to be compared with is ac-quired while welding 308L stainless steel with same av-erage wire feed rate of Ti-6Al-4V alloy.According to the comparison between the two waveforms,it is obvious that the current waveform of Ti-6Al-4V is quite different from the typical.As is shown in Fig.5b ,current value in whole short circuit phase of typical CMT cycle remains low and nearly constant.However,a current pulse shown in Fig.5a appears at the arrival of short-circuit phase when welding Ti-6Al-4V alloy.Obviously,the current pulse results in increase of output energy distribution in short-circuit phase.In this trial,input energy is calculat-ed using Eq.(2)during short circuit phase,and result shows that in CMT welding Ti-6Al-4V alloy,it accounts for about 7%of total energy in one CMT cycle,while the ratio in typical CMT process is only 5%.In order to study the necessity of that current pulse,an additional experiment has been conducted.The peak value of pulsed current in welding trial is reduced to half level through weld power source setting.The acquired current and voltage waveform is shown in Fig.5c .Process is obviously unstable with arc interruption phenomenon ac-cording to Fig.5c .High-speed photographs of droplet transfer in one Ti-6Al-4V alloy CMT cycle are shown in Fig.6.During the later stage of arcing phase shown in Fig.7a ,cath-ode spot transfers from baseplate to formed weld seam.Hence,the droplet axis is deviated from welding wire axis towards the direction away from weld seam.An additional trial has been carried out with welding speed of 600mm/min,and arc blow phenomenon is not obvi-ously observed shown in Fig.7b .The result shows that arc blow phenomenon appears in a relatively low inter-val of welding speed and disappears when the welding speed ishigh.Fig.3Welding wire meltingratesFig.4Transient I -V diagramTable 2Welding parametersParametersValues Wire feed rate (m/min)2∼9Inductance correction (%)−5∼+5Welding current (A)40∼150Welding voltage (V)13∼16Welding speed (mm/min)400Arc correction (%)0Shielding gasPure ArgonShielding gas flow rate (L/min)15Contact tip-to-work distance (mm)18Int J Adv Manuf Technol3.2Droplet transfer behaviours with different wire feed ratesCurrent and voltage waveforms with wire feed rates of 3,5.2and 7.4m/min are shown in Fig.8,and inductance correction values of trials are maintained at −2.5%.With wire feed rate increases,droplet transfer frequency also increases,and theshort-circuit phase arrives more early.While the wire feed rate is low,in order to offer enough energy,the average short-circuit current value is even higher than average arc-ing current value,but the current pulse width becomes narrower.As is intuitively shown in Fig.9,the proportion of arc output energy in short-circuit phase decreases with the increase of wire feedrate.Fig.5CMT current and voltage waveforms:a Ti-6Al-4V alloy,b typical,c current pulse of half peakvalueFig.6High-speed images of droplet transfer in one Ti-6Al-4V alloy CMT cycleInt J Adv Manuf TechnolTable 3shows the droplet sizes of different wire feed rates.Diameter (D )and length (L )of droplet both increase as the increase of wire feed rate,and the length increasing rate is slightly higher than that of diameter.The proportion between diameter and length is maintained at about 0.89.3.3Droplet transfer behaviours with different inductance correction valuesCurrent and voltage waveforms with different inductance cor-rection values are shown in Fig.10,and wire feed rate is main-tained at 5.2m/min.The welding process is unstable when inductance correction is set above 0due to the low response speed of welding power source,as is shown in Fig.10d .Fig-ure 11shows the comparison of current and voltage wave curves in one CMT cycle,while the welding processes were stable.Droplet frequency increases as inductance correction value de-creases due to the more early arrival in short-circuit phase.Droplet sizes are shown in Table 4,and as inductance cor-rection value increases,diameters of droplets remain essen-tially constant,and lengths of droplets decline slightly.4Discussion4.1Analysis of transient phenomenaDuring CMT short-circuit phase,various forces acting on a droplet [25–27]such as gravity (G ),surface tension (F t),Fig.7Droplet behaviours in later stage of arcing phase:a WS=400mm/min,b WS=600mm/minFig.8Current and voltage waveforms with different wire feed rates:a 3m/min,b 5.2m/min,c 7.4m/minFig.9Droplet transfer frequency with different wire feed ratesInt J Adv Manuf TechnolTable 3Droplet sizes of different wire feed ratesWire feed rate (m/min)Diameter (mm)Length (mm)D /L 3 1.48 1.670.8865.2 1.66 1.850.8977.41.721.960.896Fig.10Current and voltage waveforms with differentinductance correction values:a −5%,b −2.5%,c 0,d>0Fig.11Comparisons of current and voltage waveforms in one CMT cycleTable 4Droplet sizes of different inductance correction valuesInductance correction value (%)Diameter (mm)Length (mm)D /L 0 1.65 1.830.901−2.5 1.66 1.850.897−51.681.880.895Int J Adv Manuf Technolelectromagnetic force (F e )and pulling force (F p )generated by welding wire motion.Due to low heat input of CMT process and physical properties of Ti-6Al-4V alloy,the surface tension coefficient of droplet is relatively greater.As is shown in Fig.12,the horizontal component electro-magnetic force (F ej )has shearing effect on liquid bridge to promote it to be abrupted.When welding Ti-6Al-4V alloy,the surface tension hindering the formation and breaking of liquid metal bridge are obviously bigger,the liquid metal bridge breaking process is more violent.The current pulse appearing in short circuit phase could exert larger electro-magnetic force to promote the forming and breaking of liquid metal bridge.Besides,larger heat is inputted to droplet and liquid metal bridge to reduce the surface ten-sion of liquid metal bridge so that the hindering effect of surface tension on liquid bridge decreases effectively.Therefore,the current pulse in short-circuit phase is neces-sary for CMT process stability when welding Ti-6Al-4V alloy.At a relative low welding speed,arc blow phenomenon is easily observed,as is shown in Fig.7a .In later stage of arcing phase,while the distance from droplet to weld seam is short,the cathode spot moves to weld seam to find the shortest path [28]with the principle of minimum voltage.Thus,as shown inFig.13,electromagnetic force on droplet changes and gener-ates a component force vertical with welding wire axis.Under the action of the component force,droplet axis will be not coaxial with welding wire axis.With welding speed increases,distance from formed droplet to weld seam is lengthened be-fore droplet entering into molten pool.Thus,arc blow phe-nomenon turns weaker gradually.4.2Effects of wire feed rate on droplet transfer behaviourIn CMT process,while droplet has reached molten pool,a signal is returned to digital controlled wire feeder,then the welding wire is being pulled back and the CMTcycle arrives short-circuit phase,control circuit of CMT process is shown in Fig.14.Obviously,the mass of molten welding wire is larger with increase of wire feed rate.Thus,droplet size is enlarged,as is shown in Table 3.While droplet is being formed in arcing phase,the surface tension acting on droplet towards the opposite direction of gravity and helps droplet to be kept on the welding wire.As the heat inputted to droplet increases,the droplet surface tension coefficient de-creases.And the droplet is pulled longer with impact of gravity.Therefore,the length increasing rate is slightly higher than that of the diameter.4.3Effects of inductance correction value on droplet transfer behaviourInductance correction value has great effect on dynamic qual-ity of welding power source.The implication of welding pow-er source dynamic quality includes two parts:instantaneouscurrent increasing rate d id tand instantaneous voltage recovering rate d u d t .As is shown in Fig.11,while inductance correction value is small,instantaneous values of current and voltage change rapidly in arcing phase,and it results in shorter period of arcing phase.Therefore,the frequency increases.Besides,length of droplets shown in Table 4increases as the induc-tance correction value decreases,thus the droplet reaches mol-ten pool slightly more early.The effect of inductance correc-tion value on energy distribution adjustment in different stages is little,and it mainly affects the stability in weldingprocess.Fig.12Electromagnetic force on liquid metalbridgeFig.14Control circuit of CMTprocessFig.13Electromagnetic force on droplet in later stage of arcing phase Int J Adv Manuf Technol5ConclusionsDroplet transfer behaviours of CMT welding Ti-6Al-4V alloy have been investigated by analysing captured electrical wave-forms and high-speed images of droplet transfer process. 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