Flashing phenomena in square wave alternating current. Flash welding control by use of PWM inverter power supply

Flash welding, through conferring a number of important advantages, such as high productivity, good-quality welds, and suitability for welding of components with large cross-sections, is extensively used for shop welding of rails, plating of hot coils, welding of wheel rims, etc.^1-3 More compact welding facilities are also being sought through further exploitation of mechanical control technology for practical applications in job-site welding of rails, H-steel sections, etc.⁴     

Mechanical and metallurgical properties of welding zone in rail welded via thermite process

Abstract

The quality of railway construction is an important factor in providing safe and comfortable rail transportation. Thermite welding, which is a long established technique, prevents rail failure and derailing of trains. This technique assists in providing a smooth ride and long service life for rails and railway carriages. In the present study, type S49 SAE/AISI 1050 steel rails have been welded via a short preheating thermite process.The hardness distributions of the upper surface and inside sections of the welding zone have been obtained and its microstructure has been examined. In addition, specimens taken from the upper side (head), web, and base of the welding zone were subjected to tensile and Charpy V notch impact tests. In the hardness profiles for these specimens maximum hardness values were observed at the edges of the weld metal. The hardness values were 109 HB at the interface between the melting zone and the heat affected zone, 108 HB in the melting zone, and 104 HB in the unwelded rail region. Examination of the microstructure revealed proeutectoid ferrite and pearlite, but not martensite.     

Optimisation of friction stir welding process parameters to weld cast aluminium alloy A413 – an experimental approach

Abstract

A413 is a high strength eutectic aluminium silicon cast alloy used in the food, chemical, marine, electrical and automotive industries. Fusion welding of these cast alloys can lead to problems such as porosity, microfissuring and hot cracking, etc. However, friction stir welding can be used to weld these cast alloys effectively, without defects. In this investigation, an attempt was made to optimise the friction stir welding process parameters for joining the cast aluminium alloy A413. Joints were made using four levels each of tool rotation speed, welding speed and axial force. The quality of the weld zone was analysed using macrostructure and microstructure analysis. Tensile strength of the joints were evaluated and correlated with the weld zone hardness and microstructure. The joint fabricated using a tool rotation speed of 900 rev min^?1, a welding speed of 75 mm min^?1 and an axial force of 3 kN showed the best tensile strength.     

Modifying residual stress levels in rail flash-butt welds using localised rapid post-weld heat treatment and accelerated cooling

Abstract

Flash-butt welding is used in the manufacture of continuously-welded rails. Finished welds typically exhibit high tensile residual stresses in the rail web and at the upper surface of the rail foot, which may increase the risk of fatigue failure in service. An understanding of the influence of the welding process, including post-weld cooling, on the residual stress distribution is necessary to improve the performance of flash-butt welds by post-weld heat treatment (PWHT), since incorrect treatment may have adverse effects on both residual stress and weld material characteristics. A finite element model has been developed to simulate post-weld cooling in flash-butt welded AS60 kg m^–1 rail. Computed thermal histories for normal (air) cooling, rapid PWHT, and accelerated cooling (water spray) were used as inputs to calculate sequentially coupled stress–time histories, including phase transformations. In addition, the localised influence of the initiation time for rapid PWHT, after final upset, on the reduction of tensile residual stresses was investigated. Heating the rail foot immediately after final upset reduced tensile residual stresses in the web region of the weld. Preliminary numerical predictions showed that water quenching the entire weld region too soon after the austenite–pearlite transformation is completed can induce further tensile residual stresses without affecting the microstructure. The results of the numerical analysis can be used to modify the flash-butt welding procedure to lower residual stress levels, and hence improve weld performance.     

Suppression of porosity in beam weaving laser welding

Abstract

The present paper describes a beam weaving laser welding technique to suppress argon or nitrogen porosity, which may appear during laser welding of low carbon steel. Bead on plate welding was performed using a 3 kW CO₂ laser. The weaving frequency was varied within 0–30 Hz and the weaving amplitude within 0–2 mm during welding. The experimental results show that under 2.4 kW laser power and 1.0 m min^-1 welding speed, the nitrogen porosity decreases remarkably with increasing frequency, and it can be eliminated for a weaving frequency of 22 Hz with 0.5 mm weaving amplitude. Under 2.4 kW laser power and 1.5 m min^-1 welding speed, beam weaving laser welding can also effectively reduce argon porosity at a weaving frequency of 22 Hz and amplitude of 1.0–1.5 mm.     

Methods of eliminating pollutant emissions from the work environment in welding and related processes

Abstract

The welding and joining of dissimilar metals which have very different properties, such as aluminium and carbon steel, is considered to be a subject for research and development in the welding/joining sector continuing into the 21st century. There are also huge requirements and expectations for this sector.¹ Due to the aforementioned, the research and development of welding and joining of dissimilar materials have been carried out over many years; for instance, eutectic bonding of copper pipe and aluminium pipe was developed 30 years ago and this process is still applied for the heat pipes of refrigerators. Recently it has even progressed for applications in joining of wide plate materials of aluminium alloy and stainless steel by means of the vacuum rolling process² and also for weldments of aluminium alloy and carbon steel joined by means of friction welding and employed as automobile components.³ However, there are problems from aspects of cost and restrictions concerning the configurations for which joining is feasible using conventional welding and joining processes and these techniques have not yet reached the stage where they can be applied in a number of industrial sectors. Accordingly, an extensive programme of research and development has been deployed in recent years using fusion welding processes, such as electron beam and laser welding and brazing, diffusion bonding and also friction stir welding (FSW).⁴     

Pulsed gas metal arc welding of Al–Cu–Li alloy

Abstract

An experimental Al–Cu–Li–Mg–Ag–Zr type alloy in the form of 13.7 mm thick plates was studied for its fusion characteristics using gas metal arc welding (GMAW) and pulsed gas metal arc welding (P-GMAW). High copper 2319 filler of 1.6 mm diameter was used. The burn-off characteristics of 2319 filler wire in GMAW and P-GMAW were experimentally determined, including the relation between pulse current and pulse duration for the desired one-drop detachment per pulse (ODPP) condition and feasible range of pulse parameters. The effect of welding parameters on bead geometry and shape relationships was investigated through beadon-plate experiments in the welding current range above the spray transition current. Reasonably good weld beads were obtained in P-GMAW at currents as low as 194 A and welding speeds of 45 cm min^–1. P-GMAW yielded significantly higher weld penetration compared to GMAW.     

An apparatus for the determination of the shear and peel strengths in pad welds

Abstract

Friction stir welding (FSW)^1,2, developed in 1991 by The Welding Institute, UK is a joining process which overthrew welding concepts which existed at the time; its application has been progressed not only for soft alloys including aluminium, but also, in recent years, for steel.³ Sato and others carried out friction stir welding on 1080-O and 5083-O materials and investigated variations in the hardness.⁴ Ten years or so have passed since the development of friction stir welding, the process has been globally investigated and widely employed in various sectors such as vehicles, ships and the aerospace industry. Furthermore, investigations have also been carried out into the manufacture of dissimilar metal joints. For example, Enomoto studied 2024/AC4C cast alloy and reportedly obtained satisfactory joints.⁵ Li and others have investigated 2024/6061 and reported that both alloys are distributed at the weld zone in a stratified and complex manner.⁶     

Correlation between weld formation and processing parameters in CO2 laser welding of steels

Abstract

Laser welding, which has undergone rapid development in the past few decades, is one of the most important applications in laser materials processing. Although some general data are available, precise welding parameters are equipment specific. In the present study, a series of autogenous laser welds on mild and stainless steels has been investigated, using a Trumpf 3·0 kW CO² laser system, to establish welding parameter windows. The correlation between laser power, welding speed, and weld bead profile for bead on plate welding has been obtained. For a constant laser power, penetration depth reaches a stable value as welding speed exceeds 11 000–13 000 mm min^-1. This value is defined as the penetration threshold. Lower welding speed produces deeper penetration. However, under such conditions, the unstable keyhole and weld pool could result in undercut and porosity. The maximum penetration achievable for sound welds on both mild steel and stainless steel was investigated. The correlation between penetration threshold and power level was also established. The parameter windows established for autogenous welds can be adopted effectively on butt jointsif welding speed is reduced by 25%.     

The technology of extrusion‐fusion welding of polythene and polypropylene butt and T joints

Abstract

Laser welding, regarded as one of the welding techniques for use with steel materials and employed in the fabrication of automobiles, has increasing applications in the manufacture of transmission systems and car bodies; however, there are not many examples of application to aluminium alloy components.¹ The reasons for this are thought to be as follows: aluminium alloys have a higher laser reflectivity compared with that of steel materials; consequently an even higher power laser is required to input energy to the material for welding and also, due to the low viscosity of molten aluminium alloy, stable welding is difficult.^2,3     

Laser stabilisation of arc cathode spots in titanium welding

Abstract

Cathode spot formation is very pronounced during arc welding of titanium and titanium alloys. The dynamic behaviour of these spots was observed to interfere with metal transfer during welding, this interference being a fundamental cause of poor weld quality in these alloys. In the present work, stabilisation of the arc cathode spot with a focused Nd–YAG laser beam during pulsed gas metal arc welding of titanium was investigated. The laser beam was focused near the leading edge of the weld pool and the laser power and focus spot size were varied to determine the values required to confine the cathode spot to the laser focus position. The results showed that, for fixed welding conditions, the laser power required to prevent cathode spot motion varied as a function of focus spot size. The required laser power was minimised at 200 W for a spot size of 0.6 mm. The laser stabilised arcs had lower voltage but approximately the same current density as stabilised arcs. Increased welding speeds required marginally higher laser powers to stabilise the spot, but the minimum power was still attained with a 0.6 mm focus spot diameter. The laser power density required for stabilisation decreased as spot size was increased, varying from almost 10⁶ W cm^?2 at the smallest spot size to approximately 10⁴ W cm^?2 at the largest. Cathode spot stabilisation improved weld quality by reducing spatter generation and weld bead irregularity.     

Vision based neurofuzzy logic control of weld pool geometry

Abstract

The geometry of the weld pool contains accurate, instantaneous information about welding quality. Thus, weld pool sensing and control plays a significant role in automated arc welding. Previous studies have focused on inferring penetration through models and controlling penetration by various methods, such as adaptive control, model based fuzzy logic, etc. In the present work, a weld pool imaging system employing a LaserStrobe (tradename) high shutter speed camera is used to obtain contrasting images and eliminate arcing interference. Two image processing tools based on edge detection and connectivity analysis extract online information about the weld pool length and width. A neurofuzzy control system elicited from both human experience and experimental results has been developed to control the welding current and welding speed in real time based on changes in weld pool dimensions. Closed loop control of welding speed is used to achieve desirable weld pool geometry.     

Joining of 1050/5052 by resistance seam welding and materials evaluation of joints

Abstract

Spot welding is extensively used for joining aluminium alloys, although it is necessary to have available a large-capacity power source with a maximum capacity of around 300 000 A for joining by resistance seam welding¹, low-current applications not being very often encountered. Recently developed welders, however, allow welding to proceed with ease at low currents of 20 or so kA. Joining of 1050/5052 by resistance seam welding was therefore tried together with a materials evaluation of joints.     

Development of friction stir welding of high strength steel sheet

Abstract

For friction stir welding (FSW) of advanced high strength steel (AHSS) sheets with tensile strength grades between 590 and 1180?N?mm^?2, the appropriate welding condition range and the influence of welding conditions on microstructures and mechanical properties of the welds were investigated. The appropriate welding conditions to avoid defects such as the incomplete consolidation at the bottom of the weld were obtained for the steel sheets up to 1180?N?mm^?2 grade. The higher tool rotation speed evidently resulted in the larger volume fraction of martensite and higher hardness in the stir zone (SZ), attributed to an increase in the peak temperature of its thermal cycle. The tensile strength of the weld joint was as high as that of the base metal for the steels up to 980?N?mm^?2 grade, but slightly lower than that of the base metal for the steel of 1180?N?mm^?2 grade due to the heat affected zone (HAZ) softening.     

焊轨列车专用焊机及成套对轨装置研制

随着高速铁路服役时间的增长,线路换轨施工越来越多。为了实现既有线钢轨的"焊铺换"一体化施工,提出将钢轨闪光焊接整合到长轨运输车上的构想。由于车载设备运行情况复杂且长钢轨质量太大,要在车上完成钢轨焊接面临很多难题,包括焊机位置和轨头配合的问题以及钢轨之间的对轨问题等。分析车载钢轨焊接的技术难点并提出具体的设计方案,形成了以焊机为核心的成套对轨系统。通过研制焊轨列车,解决了车载长钢轨的焊接及对轨难题,为今后的换轨一体化施工提供技术支持。     

Penetration in spot GTA welds during centrifugation

Convective flow during arc welding processes mainly depends on electromagnetic force, Marangoni force, and buoyancy force. The Marangoni flow (caused by surface tension gradient,dγ/dT)and the buoyancy driven flow are the major factors in controlling weld penetration in austenitic stainless steels, such as types 304 and 316. Alloys 304 and 316 were subjected to a 7 s spot gas-tungsten arc (SGTA) welding at 1 g (g = 9.8 m/s ² )and 5 g accelerations. The welds at 5 g were performed on Clarkson University’s multigravity research welding system (MGRWS). The cross sections of the fusion zones were polished/etched, and their depth (D)and width (W)were measured to ± 0.025 mm. It was determined that the depth/width ratio (D/W)of the welds decreased as the acceleration increased from 1 to 5 g. This result indicates that increase in buoyancy driven flow will produce wider but shallower welds during SGTA welding.     

Study of weld strength variability for capacitor discharge welding process automation

Abstract

The capacitor discharge welding (CDW) process is an autogenous, rapid solidification joining process ideal for joining small parts incorporating dissimilar metals. Potential applications include welding of solderless electrical contacts, cutting tool inserts, and automotive valves. Because of high cooling rates, in excess of 10⁶ K s^-1, the production rate for CDW processes is potentially very high. However, potential industrial users are hesitant to use the CDW process, owing largely to the unavailability of automated process control. The objective of the present study was to investigate the source of weld strength variability in CDW as a foundation for process automation and control. Results show that welding time significantly affects weld strength variability for both high thermal conductivity and high gas content materials, independent of the t_w/2RC ratio.     

The influence of process parameters in linear friction welded 30CrNiMo8 small cross-section: a modelling approach

ABSTRACT

The linear friction welding process is a solid-state welding technology enabling high-quality joints in chains, thus competing with the currently in use flash butt welding process. In the present study, the effects of welding parameters were investigated, using 30CrNiMo8 small cross-section with 10?mm in diameter. To that end, a 2^k full factorial design was used. The influence of the process inputs was assessed experimentally and numerically using the commercial software DEFORM. A fairly good agreement was obtained between experiments and modelling. Some differences in the statistical effect of the parameters were observed regarding burn-off rate. Confirmation experiments were carried out to evaluate the adequacy of the attained regression models and good predictive ability was achieved.     

Friction stir welding of pure titanium lap joint

Abstract

The effects of welding parameters on friction stir welding of pure titanium lap joint were investigated together with the microstructural characteristics of the sound joint. Three kinds of welding defects were found under the condition of tool load control, namely, the groove-like defect, the inner cavity defect and the overheating rough surface and tool penetration defect with increasing heat input. The tool plunge depth control effectively increased the lap width compared with the tool load control, so the sound joints fractured in the base metal were acquired at 250 rev min^–1–75 mm min^–1 and 200 rev min^–1–50 mm min^–1. The sound joint consisted of the thermomechanically affected zone, the stir zone, the lap zone and the top layer. The microstructure was fined obviously after welding, and finer grains were observed in the lap zone and top layer.     

Control of nitrogen content and porosity in gas tungsten arc welding of high nitrogen steel

Abstract

In welding of high nitrogen steel (HNS), it is essential to control the nitrogen content and porosity in the weld metal. In this paper, the influence of shielding gas composition and heat input on the nitrogen content and porosity in the weld metal of HNS was investigated by gas tungsten arc welding. The experimental results indicate that the weld nitrogen content increases as N₂ in the shielding gas is increased in the same heat input of welding. The weld nitrogen content decreases with increasing the heat input for pure argon used as a shielding gas, whereas it increases with increasing the heat input for the shielding gas including some nitrogen. The nitrogen pore can be avoided when the nitrogen content in the shielding gas is <4% in the heat input range of 528–2340 J mm^–1.