Measuring the process efficiency of controlled gas metal arc welding processes

Abstract

The thermal or process efficiency in gas metal arc welding (GMAW) is a crucial input to numerical models of the process and requires the use of an accurate welding calorimeter. In this paper, the authors compare a liquid nitrogen calorimeter with an insulated box calorimeter for measuring the process efficiency of Fronius cold metal transfer, Lincoln surface tension transfer and RapidArc, Kemppi FastRoot and standard pulsed GMAW. All of the controlled dip transfer processes had a process efficiency of ～85% when measured with the liquid nitrogen calorimeter. This value was slightly higher when welding in a groove and slightly lower for the RapidArc and pulsed GMAW. The efficiency measured with the insulated box calorimeter was slightly lower, but it had the advantage of a much smaller random error.     

Effect of trailing heat sink on residual stresses and welding distortion in friction stir welding Al sheets

Abstract

This paper investigates a trailing heat sink, which was designed and applied to friction stir welding (FSW) in order to control the residual stresses and welding distortion. Residual stresses, residual plastic strains and welding distortion of 2024-T3 and 5083-H321 Al sheets welded by FSW with and without the trailing heat sink were compared. The optimal placement of the heat sink was discussed. The results revealed that the reductions in peak tensile stresses were 66% for 2024-T3 and 58% for 5083-H321 by application of the trailing heat sink in FSW. In addition, the welding distortion could be reduced drastically by this method. The 5083-H321 sheet with a size of 1000×100×3·5 mm welded by this method was very flat and had almost no distortion. This method achieved in-process control of stresses and welding distortion, without additional complicated work before or after welding operation.     

Residual stress in thermal spray coatings measured by curvature based on 3D digital image correlation technique

Residual stress is an important factor in thermally sprayed deposits which affects both processing and performance. High stress can influence the structural integrity of sprayed parts and impair their function. Therefore, it is important to know the stress state, understand its origin and be able to control it. In this paper, three-dimensional digital image correlation as a non-destructive full-field optical measurement method is used to measure the strip curvature on thermal spray coatings given a NiCrCoAlY bonding layer and an yttria-stabilized zirconia (YSZ) layer on 304 stainless steel substrate as an example. The stress profile through thickness can be determined in each step of deposition by finite element method based on an inverse analysis and the stress interpretation of curvature. The individual contribution of quenching and thermal stresses to residual stress is analyzed from temperature evolution during post-deposition cooling. In addition, the multilayer progressing deposition and elastic-plastic model are used for the accurate predictions in the simulations.     

Strain and distortion monitoring during arc welding by 3D digital image correlation

A novel three-dimensional (3D) digital image correlation (DIC) approach using a stereovision system was developed to measure the evolution of strain and distortion near the fusion zone during the gas tungsten arc welding process. Unlike the previously reported two-dimensional (2D) DIC approach using a single camera, the 3D DIC method was immune to the out-of-plane displacement and was capable of measuring 3D deformation. Both 2D and 3D DIC approaches in welding applications were based on the utilisation of the novel high-temperature speckle and the pulsed laser illumination plus bandpass filters. However, the speckle pattern was partially specular reflective causing issues in subset matching in the 3D approach. A new algorithm and experimental procedure was incorporated to solve this problem.     

Finite element simulation and experimental investigation of thermal tensioning during welding of DP600 steel

The locally introduced heating and cooling cycle of welding generates residual stresses and distortion. Costly post-weld heat treatments are required in order to reduce welding residual stresses and distortion. Thermal tensioning is one of the more promising in-process techniques to control welding distortion and can be classified into transient thermal tensioning (TTT) and side heating (SH). In thermal tensioning additional heat sources are applied during welding. If the additional heaters are close to the weld centre line and influence the thermal field of the weld, the process is called TTT. If there is no interference to the thermal field of the weld, the process is called SH. In this work, the SH during welding of 2?mm thick DP600 steels has been extensively investigated using numerical and experimental approaches. The thermal and mechanical fields during conventional welding and SH were examined by means of finite element models, and validated by comparison with experimental observation of temperature, distortion and residual stresses. The microstructure was investigated experimentally.     

松下KRII-500型弧焊电源焊接火电厂铝母线工艺研究

通过分析火力发电厂铝母线的使用特点和性能要求,结合铝的焊接性和KRII-500型晶闸管控制MIG/MAG焊弧焊电源特点,研究了本焊机MIG焊焊接铝母线工艺。制定合理的焊接工艺规范参数,并在600MW超临界机组铝母线安装中顺利实施,效果良好,对以后在铝母线的检修和安装中取代昂贵的铝焊接专机有一定的参考价值。     

Measurement of residual stress in thermal spray coatings by the incremental hole drilling method

The experimental measurement of residual stresses originating within thick coatings deposited by thermal spray processes onto solid substrates plays a fundamental role in the preliminary stages of coating design and process parameters optimization. The main objective of the present investigation was to determine the residual stresses by means of the incremental hole drilling method in order to perform the measurement of the stress field through the thickness of two different HVOF Nickel-based coatings. The holes through the coatings were carried out by means of a high velocity drilling machine (Restan). A finite element calculation procedure was used to identify the calibration coefficients necessary to evaluate the stress field. The Integral method was used for the analysis of non-uniform through-thickness stresses. The results for both coatings indicate that the nature of the residual stresses is tensile and their values are between 150-300 MPa.     

Laser welding of complex aluminium structures

Abstract

Automotive manufacturing is making significant strides towards producing lighter but stronger vehicles. Developments in high strength steels and lightweight materials, especially aluminium, have contributed to these advances. The body shell contributes nearly 25% of a car's weight. Current predictions estimate the contribution of aluminium in a car to be of the order of 10% of the total vehicle weight, whereas the use of aluminium alloys in car body manufacture could lead to potential weight savings of up to 43%. The use of aluminium is expected to grow in body in white and tailored welded blank applications within the automotive sector. Complex joint configurations are encountered in spaceframe assembly. One such joint structure, which involves the welding of a rectangular section to a flat plate, has been considered in the work reported. Traditionally, a MIG welding solution would be adopted to manufacture such a joint. Work was performed to establish the feasibility of using laser welding to manufacture the component and to compare the laser and MIG processes. A fibre optic delivered Nd-YAG laser beam, manipulated with an articulated arm robot, was successfully used to weld the tube to the aluminium plate. Spatter free welds with no externally visible porosity were achieved and no cracking was observed in any weld cross-section examined. Mechanical tests indicated that the static strength of the laser welds was lower than that achieved by MIG welding, which is attributed to the lower load bearing area in the laser welded joints.     

Correlation between corrosion resistance properties and thermal cycles experienced by gas tungsten arc welding and laser beam welding Alloy 690 butt weldments

This study investigates the correlation between the thermal cycles experienced by Alloy 690 weldments fabricated using gas tungsten arc welding (GTAW) and laser beam welding (LBW) processes, and their corresponding corrosion resistance properties. The corrosion resistance of the weldments is evaluated using a U-bend stress corrosion test in which the specimens are immersed in a boiling, acid solution for 240 h. The experimental results reveal that the LBW inputs significantly less heat to the weldment than the GTAW, and therefore yields a far faster cooling rate. Moreover, the corrosion tests show that in the GTAW specimen, intergranular corrosion (IGC) occurs in both the fusion zone (FZ) and the heat affected zone (HAZ). By contrast, the LBW specimen shows no obvious signs of IGC.     

Numerical modeling of inconel 738LC deposition welding: Prediction of residual stress induced cracking

A predictive tool has been implemented to determine the suitable conditions for deposition welding of inconel 738LC which is particularly susceptible to hot cracking. Cracking criteria related to liquation phenomenon and reheating rely on comparisons between residual stresses derived from numerical modeling and cracking resistance identified from the confrontation between numerical and experimental results of deposition welding performed on fir tree turbine blades test specimens. A contribution to the validation of the proposed simultaneous thermal, metallurgical and mechanical numerical modeling was based on their application to the repair of upstream guide vanes. In addition, the study has shown the importance of the heating temperature prior to welding.     

Internal residual stress measurement on inertia friction welding of nickel-based superalloy

The internal residual stress in the narrow inertia friction welding (IFW) welds of FGH96 nickel-based superalloy was measured with the contour method (CM). The as-welded internal hoop and axial residual stresses were obtained after two cuts and the detailed steps of the CM measurement were described. In addition, the hole-drilling method was used to obtain the surface stress. Furthermore, the internal hoop residual stress of a FGH96 superalloy IFW specimen after post-weld heat treatment (PWHT) was also measured with the CM after a single cut. The measured results show that the peak hoop residual stress is not symmetrical about the weld centreline. Axial residual stress on the outer surface at the weld centreline is compressive stress, while tensile stress appears on the inner surface, and it varies linearly along the thickness. The peak values of hoop tensile and compressive stresses decrease dramatically after PWHT.     

Residual stress evolution during the thermal cycling of plasma-sprayed zirconia coatings

X-ray diffraction has been used for the residual stress evaluation in two plasma-sprayed zirconia coatings: yttria-partially-stabilized zirconia and dysprosia-partially-stabilized zirconia. Following the evolution of these stresses with thermal cycling, an attempt at correlation between this evolution and the characteristics of the two coatings is undertaken.     

Effects of heat input on welding buckling distortion by experimental measurement method

An experimental method based on digital image correlation technology is proposed to measure welding buckling distortion for bead-on-plane with tungsten inert gas welding. This study focuses on the effect of welding heat input for dynamic buckling and full-field distribution. The experimental results demonstrate that heat input has a great influence on welding buckling distortion. The longitudinal shrinkage of the weld bead is in direct proportion to the heat input. In the weld bead, the longitudinal plastic strain is small, while the transverse strain is large. The key points on the line, that is perpendicular to the bisector of the weld bead, experience a distorted trend of positive increase, constant displacement, negative increase and stable displacement, although the heat inputs are different.     

Numerical and experimental study of thermally induced residual stress in the hybrid laser-GMA welding process

A model based on a double-ellipsoidal volume heat source to simulate the gas metal arc welding (GMAW) heat input and a cylindrical volume heat source to simulate the laser beam heat input was developed to predict the temperature field and thermally induced residual stress in the hybrid laser-gas metal arc (GMA) welding process. Numerical simulation shows that higher residual stress is distributed in the weld bead and surrounding heat-affected zone (HAZ). Effects of the welding speed on the isotherms and residual stress of the welded joint are also studied. It is found that an increase in welding speed can reduce the residual stress concentration in the as-weld specimen. A series of experiments has been performed to verify the developed thermo-mechanical finite element model (FEM), and a qualitative agreement of residual stress distribution and weld geometrical size is shown to exist.     

Non-destructive determination of residual stress state in steel weldments by Magnetic Barkhausen Noise technique

The purpose of this study is non-destructive determination of residual stresses in the welded steel plates by Magnetic Barkhausen Noise (MBN) technique. A MBN-stress calibration set-up and a residual stress measurement system with scanning ability were developed. To control the accuracy and the effectiveness of the developed system and procedure, various MBN measurements were carried out. The MBN results were verified by the hole-drilling method. Microstructural investigation and hardness measurements were also conducted. It was concluded that if the calibration procedure including the effect of microstructure is appropriate then MBN is a very promising method for non-destructive, fast and accurate prediction of residual stresses in the welded plates.     

基于DIC的镍基焊材焊接接头局部力学性能研究

本文通过显微组织分析、数字图像相关技术（DIC）试验以及硬度测试,系统研究了15CrMoR母材与镍基焊材焊接接头各区域的局部力学性能。基于DIC方法获取了镍基焊材焊接接头拉伸过程的全场应变云纹发现：焊缝区域应变较小且不均匀,母材区域应变量大,热影响区均匀过渡。在此基础上构建了焊接接头不同区域的局部应力应变曲线,并揭示了焊接接头局部力学性能参数的分布规律。由焊接接头显微组织的分析可知,局部力学性能参数的分布与显微组织的分布存在密切联系。最后,结合显微硬度与强度参量在焊接接头的分布规律,构建了两者的关联方程,便于镍基焊材焊接接头局部力学参数的估算。     

棒料热应力预制V型槽尖端理想裂纹的可行性研究

针对研究新型低应力精密下料系统时,需要获得热应力预制V型槽尖端理想裂纹方法的有效性问题。利用ANSYS软件,建立了棒料的有限元分析模型,并对其V型槽尖端区的温度场和应力场进行了数值模拟研究。研究结果表明,对于45#钢、20#钢、60Si2Mn棒料来说,在现有给定的参数下,其V型槽尖端均发生了塑性变形,但只有60Si2Mn棒料在其V型槽尖端点产生了热裂纹。     

Numerical analysis of residual stress distribution generated by welding after surface machining based on hardness variation in surface machined layer due to welding thermal cycle

Recently, stress corrosion cracking (SCC) has been observed near the welded zone of the primary loop recirculation pipes made of low-carbon austenitic stainless steel type 316L in boiling water reactors. SCC is initiated by superposition effect of three factors. They are material, environmental and mechanical factors. For non-sensitized material such as type 316L, residual stress as a mechanical factor of SCC is comparatively important. In the joining processes of pipes, butt welding is conducted after surface machining. Surface machining is performed in order to match the inside diameter and smooth surface finish of pipes. Residual stress is generated by both processes. Moreover, residual stress distribution generated by surface machining is varied by subsequent welding processes, and it has the maximum residual stress around 900 MPa near the weld metal. The variation of metallographic structure, such as recovery and recrystallization, in the surface machined layer due to the welding thermal cycle is an important factor for this residual stress distribution. In this study, thermal ageing tests were performed in order to evaluate hardness variation due to the thermal cycle in the surface machined layer. Results of thermal ageing tests were applied to the finite-element method as the additivity rule of the hardness variation. Varied hardness was converted into equivalent plastic strain. Then, thermo-elastic-plastic analysis was performed under residual stress fields generated by surface machining. As a result, analytical results of surface residual stress distribution generated by bead-on-plate welding after surface machining show good agreement with measured results by the X-ray diffraction method. The maximum residual stress near the weld metal is generated by the same mechanism as in the both-ends-fixed bar model in the surface machined layer that has high yield stress.     

Joining of titanium to 304L stainless steel by friction welding

The dissimilar metal joint of titanium (Ti) to 304L stainless steel (SS) is essential in the nuclear industry for the dissolution of spent fuel that is carried out in boiling nitric acid in the dissolver vessel (made of Ti) and the dissolved solution is transported through the 304L SS pipes to the other plant components made of 304L SS. Because of the radioactive environment, leak tightness and corrosion resistance of this dissimilar joint are important. In this work, friction welding process was attempted to join Ti to 304L SS. Direct friction welding of Ti to 304L SS results in a stronger weld in which failure occurs in the Ti base metal during tensile testing. However, the joints have almost zero bend ductility that has been attributed to the formation of intermetallics due to mechanical alloying, strain hardening of Ti near the joint interface and residual stresses. Post-weld heat treatment marginally increases the bend ductility to 5° because of relieving of the effects of strain hardening and of residual stresses at the joint interface. Corrosion test in boiling nitric acid as per ASTM A-262 Practice C shows that the average corrosion rate is 10 mpy with the joints remaining intact after the corrosion test. The details of mechanical tests, microstructure analysis using optical and scanning electron microscopy, are discussed.     

Submicron-scale depth profiling of residual stress in amorphous materials by incremental focused ion beam slotting

This paper reports a new technique, namely the incremental micro-slotting cutting method, for the investigation of residual stress profiles as a function of depth on a micron scale. The residual-stresses in a peened bulk-metallic glass (BMG) (Zr₅₀Cu₄₀Al₁₀, in atomic per cent) are estimated using finite-element analysis of the surface relaxations, as measured by digital image correlation analysis from field-emission gun scanning electron microscopy images, which occur when a micro-slot is stepwise micro-machined by focused ion beam. The calculation algorithm, which solves this inverse problem of residual-stress estimation, is based on the unit pulses method and is stabilized by a Tikhonov regularization scheme. It is demonstrated on a peened BMG that the new technique allows residual-stress profiles in amorphous materials to be inferred with high spatial definition (∼400 nm). Observations point to the scalability of this method to study residual-stress profiles in volumes as small as 1 × 1 × 0.2 μm³ or less, and is particularly well suited to glasses, but can also be applied to crystalline materials.