超声交流TIG焊声场及电弧特性分析

目的研究复合焊接过程中,发射端高度、喷嘴高度及保护气流量等工艺参数对空气中声场强度的影响规律及作用机理。方法采用分组实验的方法,分析了各焊接工艺参数对声场强度的影响。结果研究结果表明:发射端高度及喷嘴高度对声场强度影响最大,保护气流量几乎无影响;不同发射端高度下,声场强度峰值所对应的喷嘴高度不同。结论通过对实验结果进行分析,得到了优化的焊接工艺参数:发射端高度为30 mm,喷嘴高度为11 mm,弧长为4 mm,电流为120A,保护气流量为25 mL/min;得出了交流复合超声TIG堆焊中,超声可细化组织并使组织均匀化。     

Development of an advanced A-TIG (AA-TIG) welding method by control of Marangoni convection

A new type of tungsten inert gas (TIG) welding has been developed, in which an ultra-deep penetration is obtained. In order to control the Marangoni convection induced by the surface tension gradient on the molten pool, He gas containing a small amount of oxidizing gas was used. The effect of the concentration of O₂ and CO₂ in the shielding gas on the weld shape was studied for the bead-on-plate TIG welding of SUS304 stainless under He–O₂ and He–CO₂ mixed shielding gases. Because oxygen is a surface active element for stainless steel, the addition of oxygen to the molten pool can control the Marangoni convection from the outward to inward direction on the liquid pool surface. When the oxygen content in the liquid pool is over a critical value, around 70 ppm, the weld shape suddenly changes from a wide shallow shape to a deep narrow shape due to the change in the direction of the Marangoni convection. Also, for He-based shielding gas, a high welding current will strengthen both the inward Marangoni convection on the pool surface and the inward electromagnetic convection in the liquid pool. Accordingly, at a welding speed of 0.75 mm/s, the welding current of 160 A and the electrode gap of 1 mm under the He–0.4%O₂ shielding, the depth/width ratio reaches 1.8, which is much larger for Ar–O₂ shielding gas (0.7). The effects of the welding parameters, such as welding speed and welding current were also systematically investigated. In addition, a double shielding gas method has been developed to prevent any consumption of the tungsten electrode.     

Laserstrahlschweißen von Titanwerkstoffen unter Berücksichtigung des Einflusses des Sauerstoffes

Influence of the oxygen content in the shielding gas on microstructure and mechanical properties of laser welds of titanium and titanium alloys In the present work, a new tool concept for laser welding of titanium in high volume production has been presented and evaluated. Through the innovative application of a six‐layer metal web it is possible to calm the argon gas flow and avoid pernicious turbulences during welding. The integration of the mentioned metal web at the base of an open welding chamber allows the automated welding of highly reactive materials, such as titanium, under atmospheric pressure and inert shielding conditions. The higher density of argon relative to air offers the unique possibility to leave the chamber open on the top, so that a higher degree of flexibility than gas shielding devices for TIG welding, especially for industrial robots, is attained and can be successfully used for industrial mass production. Furthermore this device is important for welding three‐dimensional contours or to shield the regions of overlap (in overlapped joints) where shielding gas trailers are unsuccessful. By means of the presented gas shielding procedure and a modern laser welding process such as Nd:YAG laser welding, systematic investigations on the effect of oxygen on the microstructure as well as on the mechanical properties of reference bead‐on‐plate weldments could be performed for the first time. As a result of these welding trials it can be concluded that in order to avoid discolorations and hardness increase, lower restrictions to the purity of the shielding gas, in comparison to TIG welding condition, can be allowed. The maximum tolerable value of oxygen in the welding atmosphere was found to be approximately 1000 ppm for laser welding. On the contrary the maximum value for TIG welding is about 30 ppm. Further investigations on the microstructural and mechanical properties of the joints confirm that the optical quality assurance criteria for TIG welding due to the standards of aircraft construction transferable to Nd:YAG welding are.     

Effectiveness of amorphous silica encapsulation technology on welding fume particles and its impact on mechanical properties of welds

Stainless steel welding generates nano-sized fume particles containing toxic metals which may cause serious health effects upon inhalation. The objective of this study was to investigate the effectiveness of an amorphous silica encapsulation (ASE) technology by evaluating its silica coating efficiency (SCE), particle morphology, and its impact on the weld’s mechanical properties. Tetramethylsilane (TMS) added to the welding shielding gas decomposed at the high-temperature arc zone to enable the silica coating. Collected welding fume particles were digested by two acid mixtures with different degrees of silica solubility, and the measured mass differences in the digests were used to determine the SCE. The SCEs were around 48–64% at the low and medium primary shielding gas flow rates. The highest SCE of 76% occurred at the high shielding gas flow rate (30 Lpm) with a TMS carrier gas flow of 0.64 Lpm. Transmission electron microscopy (TEM) images confirmed the amorphous silica layer on the welding fume particles at most gas flow rates, as well as abundant stand-alone silica particles formed at the high gas flow rate. Metallography showed that welds from the baseline and from the ASE technology were similar except for a tiny crack found in one particular weld made with the ASE technology. Tensile tests showed no statistical difference between the baseline and the ASE welds. All the above test results confirm that welding equipment retrofitted with the ASE technology has the potential to effectively address the toxicity problem of welding fume particles without affecting the mechanical properties of the welds.     

Dissimilar Joining Between Austenitic and Duplex Stainless Steel in Double-Shielded GMAW: A Comparative Study

In the present work, the effect of double-layer shielding and five other process parameters, namely welding voltage, current, primary shielding gas type, its flow rate, and filler material, is studied during dissimilar gas metal arc welding (GMAW) between austenitic and duplex stainless steels (SSs). A simple modification over the GMAW setup is made for additional supply of secondary shielding gas at different flow rates. Two different sets of welding are performed between austenitic and duplex SSs, i.e., AISI 304 with Duplex 2205 and AISI 316 with Duplex 2205, and the contributions of process parameters, their interactions on joint distortion, tensile strength, toughness, and fusion zone microhardness are evaluated. Improvements in joint quality due to the double-shielding environment are also highlighted. Double-layer shielding with secondary shielding by CO₂ supply significantly improves tensile strength and toughness and reduces distortion. Fusion and interface zone microstructures are observed by scanning electron microscopy to study the metallurgical behavior of joints fabricated under single- and double-layer shielding environment.     

Spatter and blowhole formation phenomena in pulsed gas shielded metal arc welding

Abstract

This paper investigates systematically the effect of pulse conditions, type of shielding gas, and wire composition on blowhole and spatter formation and on wire melting rate to obtain guidelines for selection of welding conditions. It was found that spatter formation can be reduced significantly by using a shielding gas of composition Ar + 2·4%O₂ +20% CO₂ for pulsed gas shielded metal arc (MAG) welding. It was also found that it is possible to reduce blowhole formation using pulsed MAG welding. However, blowhole formation is affected greatly by variations of pulse condition and by shielding gas composition.

MST/1423     

氦−氩混合保护气体角接头旋转激光+电弧复合焊电弧特性数值分析

目的 研究激光+电弧复合焊中氦−氩混合保护气体成分对电弧等离子体物理特性的影响,从而改善焊接性能。方法 综合考虑角接头几何特性和氦−氩混合保护气体的物理特性,建立氦−氩混合保护气体角接头旋转激光+电弧复合焊电弧行为的数值分析模型。使用FLUENT软件对不同体积比氦−氩保护气体下电弧等离子体的温度场、流场、压力场和电势场进行模拟计算,对比分析保护气体成分改变对电弧等离子体的影响规律,考虑其对焊接性能的影响,并将计算结果与高速摄影试验进行对比,验证数值分析模型的准确性与合理性。结果 保护气体分别为纯Ar、95%Ar+5%He、90%Ar+10%He时,电弧向激光侧偏移收缩,电弧整体形貌被压缩,位置A纵截面处电弧等离子体的峰值温度分别为25 603、25 080、23 904 K,最大流速分别为336.34、334.34、317.58 m/s,压力最大值分别为899.08、943.40、957.67 Pa,电势梯度分别为11.56、12.17、13.18 V。结论 在氦−氩混合保护气体激光+电弧复合焊中,当保护气体中氦气体积分数增加到5%和10%时,随着氦气所占比例的增大,电弧处于动态变化过程,电弧被压缩,等离子体的峰值温度逐渐降低,最大流速下降,电弧压力和电势梯度增大,有利于焊缝熔深的增大。     

Gap-free fibre laser welding of Zn-coated steel on Al alloy for light-weight automotive applications

As a result of new policies related to global warming announced by the European Union, avoiding unnecessary energy waste and reducing environmental pollution levels are becoming a major issue in the automotive industry. Accordingly, the lap welding of Zn-coated steels process, which is commonly used for producing car doors, has been gradually developed to lap welding of Zn-coated steel to light materials, such as Al alloy, Mg alloy and composite materials, in order to effectively reduce the vehicle weight. In certain part of car manufacture, organic glues are used to temporally join the Zn-coated steels and Al alloys before permanent welding takes place. The stability of such temporary joining by glues needs improving. Laser “stitching” or low strength welding could be considered as an alternative. However, challenges exist in joining Zn-coated steel on Al alloy by laser welding, due to significant differences of material properties between the two welding materials. Porosity, spatter and intermetallic brittle phases are readily produced in the weld. In this study, the effects of welding speed, laser power, number of the welding passes and type of shielding gas in gap-free welding of Zn-coated steel on Al alloy were investigated using a 1 kW single mode continuous wave fibre laser. Results show that a weld with higher shear strengths in the laser stitching application and less intermetallic phases could be obtained when nitrogen gas was used as the shielding gas. The corrosion resistance and the surface finish of the weld could be improved in double pass welding, especially when argon gas was used as the shielding gas.     

Weld Shape Variation and Electrode Oxidation Behavior under Ar-(Ar-CO_2) Double Shielded GTA Welding

Double shielded gas tungsten arc welding (GTAW, also known as tungsten inert gas (TIG) welding) of an SUS304 stainless steel with pure inert argon as the inner layer shielding and the Ar-CO₂ or CO₂ active gas as the out layer shielding was proposed in this study to investigate its effect on the tungsten electrode protection and the weld shape variation. The experimental results showed that the inner inert argon gas can successfully prevent the outer layer active gas from contacting and oxidizing the tungsten electrode during the welding process. Active gas, carbon dioxide, in the outer layer shielding is decomposed in the arc and dissolves in the liquid pool, which effectively adjusts the active element, oxygen, content in the weld metal. When the weld metal oxygen content is over 70×10^-6, the surface-tension induced Marangoni convection changes from outward into inward, and the weld shape varies from a wide shallow one to a narrow deep one. The effect of the inner layer gas flow rate on the weld bead morphology and the weld shape was investigated systematically. The results show that when the flow rate of the inner argon shielding gas is too low, the weld bead is easily oxidized and the weld shape is wide and shallow. A heavy continuous oxide layer on the liquid pool is a barrier to the liquid pool movement.     

Time Dependant Weld Shape in Ar-O2 Shielded Stationary GTA Welding

The stationary gas tungsten arc welding （GTA） is carried out on SUS304 stainless steel under Ar-0.1%O2 and Ar-0.3%O2 mixed shielding to observe the evolution of the molten pool and investigate the role of Marangoni convection on the weld shape. After welding, the oxygen content in the weld metal was measured by using an oxygen/nitrogen analyzer. Small addition of oxygen to the argon based shielding gas can effectively adjust the weld pool oxygen content. Oxygen plays an important role as an surface active element in determining the pattern of Marangoni convection in the stainless steel weld pool. When the weld metal oxygen content is over the critical value, 0.01 wt pct, corresponding to the Ar-0.3%O2 mixed shielding gas, the Marangoni convection changes from outward to inward direction and the weld shape dramatically changes from wide shallow shape to narrow deep shape.     

Optimization of deep penetration laser welding of thick stainless steel with a 10 kW fiber laser

Deep penetration laser welding of 12 mm thick stainless steel plates was conducted using a 10 kW high-power fiber laser. The effect of the processing parameters on the weld bead geometry was examined, and the microstructure and mechanical properties of the optimal joint were investigated. The results show that the focal position is a key parameter in high-power fiber laser welding of thick plates. There is a critical range of welding speed for achieving good full penetration joint. The type of top shielding gas influences the weld depth. The application of a bottom shielding gas improves the stability of the entire welding process and yields good weld appearances at both the top and bottom surfaces. The maximum tensile stress of the joint is 809 MPa. The joint fails at the base metal far from the weld seam with a typical cup–cone-shaped fracture surface. The excellent welding appearance and mechanical properties indicate that high-power fiber laser welding of a 304 stainless steel thick plate is feasible.     

Low Heat Welding of Titanium Materials with a Pulsed Nd:YAG Laser

Titanium materials exhibit a property profile that is just as versatile as that of steel materials. Titanium materials therefore have outstanding properties, such as excellent resistance to corrosion and high strength values at low densities, which makes them ideal for use in the chemical industry and as structural materials in lightweight construction. Due to the high affinity of titanium to atmospheric gases at increased temperatures above 500 °C, titanium components have to be welded in a sophisticated process under inert shielding gas by TIG welding or by an electron beam in a vacuum. A novel innovative laser beam welding process using a pulsed laser with free pulse shaping will be presented here with which oxidation‐free titanium weld seams with excellent mechanical and technological properties can be produced. For this low heat welding process, the otherwise commonly used inert gas covering can be substituted with a shielding gas nozzle. The process‐specific low heat input and the resulting low energy input per unit length both have a positive effect on the microstructure and thus on the mechanical properties. This welding process offers both technological and economical advantages over the processes used up until now, particularly for the machining of complex components and for series production.     

Pore formation during hybrid laser-tungsten inert gas arc welding of magnesium alloy AZ31B—mechanism and remedy

One of the major concerns during high speed welding of magnesium alloys is the presence of porosity in the weld metal that can deteriorate mechanical properties. This study seeks to analyze the presence method and quantity of pore during hybrid laser-tungsten inert gas arc (TIG) welding of magnesium alloy AZ31B by radiography, optical microscopy and electron probe microanalysis (EMPA). At the same time, it identifies both the mechanism of pore formation and a remedy for this problem. The experimental results indicate that lacking of shielding gas for laser beam is the dominant cause of macroporosity formation during the hybrid of laser-TIG welding of magnesium Alloys AZ31B plate, and hydrogen is not main cause to form large pores. A favorable weld without porosity can be obtained by appending lateral shielding gas for laser beam.     

Effect of Welding Parameters on GTA Weld Shape for Pure Iron Plate under Ar-O2 Mixed Shielding

Weld shape variation for different welding parameters is investigated on pure iron plate under gas tungsten arc (GTA) welding with argon-oxygen mixed shielding. Results showed that small addition of oxygen to the argon base shielding gas can effectively adjust the oxygen adsorption to the molten pool. An inward Marangoni convection occurs on the pool surface when the oxygen content in the weld pool is over the critical value, 80×10-6, for pure iron plate under Ar-0.3%O2 mixed shielding. Low oxygen content in the weld pool changes the inward Marangoni to an outward direction under the Ar-0.1%O2 shielding. The GTA weld shape depends to a large extent on the pattern and strength of the Marangoni convection on the pool surface, which is determined by the content of surface active element, oxygen, in the weld pool and the welding parameters. The strength of the Marangoni convection on the liquid pool is a product of the temperature coefficient of the surface tension (dσ/dT) and the temperature gradient (dT/dr) on the pool surface. Different welding parameters will change the temperature distribution and gradient on the pool surface, and therefore, affect the strength of Marangoni convection and the weld shape.     

Development of simplified active flux tungsten inert gas welding for deep penetration

A new advanced active flux tungsten inert gas (AA-TIG) welding technique, named cap active flux tungsten inert gas (CA-TIG) welding using atmospheric oxygen, was proposed to increase the penetration depth of a weld. Only a simple nozzle cap with an air inlet was used for the welding. Flowing inert gas used as a shielding gas through a nozzle center led to the aspiration of oxygen from the atmosphere to the molten pool. The penetration depth was increased by the reversal of the Marangoni convection due to the entrained oxygen, and it reached three times deeper than that of the conventional TIG welding. Additionally, no degradation of the tungsten electrode was observed because it was protected by the inert gas. The penetration depth was changed by the oxygen content in the molten pool and it could be easily controlled by the nozzle cap design and the welding parameters.     

Mechanical and Metallurgical Studies in Double Shielded GMAW of Dissimilar Stainless Steels

In the present work, a simple arrangement is made to provide double layer shielding gas supply in addition to primary shielding during gas metal arc welding (GMAW) of two dissimilar stainless steels, i.e., AISI 316 and duplex 2205. Influences of double layer shielding in addition to five more process parameters like welding current, voltage, material of the electrode wire, the type of primary shielding gas, and flow rate on joint tensile strength and fusion zone microhardness are studied. An experimental design technique is used to design the experimental conditions and the results are analyzed to observe the influences of each process parameter and their interactions. The tensile strength is more influenced by the electrode material and the type of shielding, whereas current, interaction between current × voltage and current × flow rate significantly influence microhardness. Welding voltage influences both tensile strength and microhardness. Double layer shielding with CO₂ as an outer shielding layer helps in controlling the cooling rate which improves the tensile strength and microhardness. Microstructural observations by scanning electron microscopy reveal that moderate to low heat input with a single layer of shielding results in poor joint strength and severe damage or lack of fusion, and the duplex 2205 filler gives the maximum joint strength due to the presence of a ferrite structure.     

Near-resonance enhanced O2 detection for dual-broadband pure rotational coherent anti-Stokes Raman scattering with an ultraviolet-visible setup at 266 nm

Broadband and dual-broadband coherent anti-Stokes Raman scattering (CARS) are widely established tools for nonintrusive gas diagnostics. Up to now the investigations have been mainly performed for electronic nonresonant conditions of the gas species of interest. We report on the enhancement of the O2-N2 detection limit of dual-broadband pure rotational CARS by shifting the wavelength of the narrowband pump laser from the commonly used 532-266 nm. This enhancement is caused when the Schumann-Runge absorption band is approached near 176 nm. The principal concept of this experiment, i.e., covering the Raman resonance with a single- or dual-broadband combination of lasers in the visible range and moving only the narrowband probe laser near or directly into electronic resonant conditions in the UV range, should also be applicable to broadband CARS experiments to directly exploit electronic resonance effects for the purpose of single-shot concentration measurements of minority species. To quantify the enhancement in O2 sensitivity, comparative measurements at both a 266 and a 532 nm narrowband pump laser wavelength are presented, employing a 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyram (DCM) dye laser as a broadband laser source at 635 nm. An increase of approximately equal to 13% in the ratio of the rotational CARS cross sections of O2 and N2 was obtained. The broad spectral width of the CARS excitation profile was approximately equal for both setups. Further enhancement should be achievable by shifting the narrowband pump laser closer toward 176 nm, for example, with a frequency-doubled optical parametric oscillator or an excimer laser. The principal concept of this experiment should also be applicable to broadband CARS experiments to directly exploit electronic resonance effects of the narrowband pump laser with electronic transitions of minority species for the purpose of single-shot concentration measurements of those species.     

Determination of stability of MIG/MAG welding processes

The paper treats several methods of evaluating the stability of MIG/MAG welding processes, which are based on measurement of time‐varying welding‐current intensity and welding voltage. The stability of the welding process is affected by numerous parameters. The most unfavourable result of poor stability are spatters, which are problematic in terms of material losses and extension of production times due to cleaning, as well as appearance. The experimental part of the paper is based on stability analyses carried out with three different gas‐shielding atmospheres. Two different welding domains were compared. The first was short‐circuit material transfer, and the second spraying material transfer. The results obtained in the analysis of the signals measured indicate a more stable short‐circuit material transfer in the case of welding with the pure CO gas, and a more stable spray material transfer in the case of welding with the transferred ionized molten energy mixture. Copyright © 2001 John Wiley & Sons, Ltd.     

小电流高频TIG焊电弧稳定性的研究

本文以大量的试验结果表明了电流频率、钨极尺寸和保护气体成分等对小电流 TIG 焊电弧稳定性的影响规律。并得出6～8A 小电流时的电弧稳定性条件是采用20KHz 以上的高频电流。(Ar+2%H_2)混合气体保护和直径为φ0.5mm 的细铈钨极。     

Effects of type of shielding gas upon local mechanical properties of laser welded joint in heat‐treatable steel

Small additions of oxygen or carbon dioxide to argon shielding at laser beam welding can increase welding speed and productivity and decrease the mechanical properties of welded joints. The effect of the type of active shielding gas mixtures based on argon with additions of oxygen and/or carbon dioxide upon the local mechanical properties of laser welded joints of heat‐treatable steel 25 CrMo 4 was studied. Microshear test method has been used to investigate the local mechanical properties of welded joints, including microshear strength, microshear plasticity and microshear thoughness. The obtained data were statistically processed, and a mathematical modeling of mechanical properties applying the method of response surfaces was carried out. The analysis revealed that the impact of the used shielding gas mixtures upon the local mechanical properties of the joint is not very significant. The results indicate that the microshear test can be used successfully for estimation of the local mechanical properties distribution of laser welded joints.