Acceptable Weld Quality Research Articles

Seam tracking and gap bridging during robotic laser beam welding via grayscale imaging and wobbling

The use of laser beam welding with robotic manipulators is expanding towards wider industrial applications as the system availability increases with reduced capital costs. Conventionally, laser welding requires high positioning and coupling accuracy. Due to the variability in the part geometry and positioning, as well as the thermal deformation that may occur during the process, joint position and fit-up are not always acceptable nor predictable a-priori if simple fixtures are used. This makes the passage from virtual CAD/CAM environment to real production site not trivial, limiting applications where short part preparations are a need like small-batch productions. Solutions that render the laser welding operations feasible for production series with non-stringent tolerances are required to serve a wider range of industrial applications. Such solutions should be able to track the seam as well as tolerating variable gaps formed between the parts to be joined. In this work, an online correction for robot trajectory based on a greyscale coaxial vision system with external illumination and an adaptive wobbling strategy are proposed as means to increase the overall flexibility of a manufacturing plant. The underlying vision algorithm and control architectures are presented; the robustness of the system to poor illumination conditions and variable reflection conditions is also discussed. The developed solution employed two control loops: the first is able to change the robot pose to follow varying trajectories; the second, able to vary the amplitude of circular wobbling as a function of the gap formed in butt-joint welds. Demonstrator cases on butt-joint welds with AISI 301 stainless steel with increased complexity were used to test the efficacy of the solution. The system was successfully tested on 2 mm thick, planar stainless-steel sheets at a maximum welding speed of 25 mm/s and yielded a maximum positioning and yaw-orientation errors of respectively 0.325 mm and 4.5°. Continuous welds could be achieved with up to 1 mm gaps and variable seam position with the developed control method. The acceptable weld quality could be maintained up to 0.6 mm gap in the employed autogenous welding configuration.

Optimization of CMT welding process parameters of dissimilar hot rolled E250 and polymer sandwich steel lap joints

Lap joints of 1.5 mm thin sheets of dissimilar hot rolled E250 with polymer sandwich steel (MPM) were produced by cold metal transfer (CMT) process, closer to the real application. The polymer layer polystyrene-butadiene-styrene sandwiched between two DC06 sheets of MPM was found to be intact at significantly lower heat inputs (0.16 − 0.24 kJ/mm), which otherwise had been quite challenging during TIG welding leading to huge rejections. After several iterative trials for acceptable weld quality, experiments were conducted as per L9 orthogonal array, Taguchi technique with welding speed (WS), wire feed rate (WFR) and welding torch orientation (TO) as the process parameters. The joints were investigated through optical macrograph, micrographs, hardness, tensile lap shear tests and fractography on the fractured specimen. Optimum parameters were determined for maximizing shear strength. The sample with optimized parameters exhibited 6% improvement in shear strength achieving 152.09 MPa and polymer layer retention. ANOVA analysis suggested WS to be most significant parameter with 69.85% contribution affecting shear strength. Coefficient of determination (R 2) for the shear strength was 87.26% derived from the linear regression model. Significantly, lower error 0.96% computed from the confirmatory test concluded very effective optimization. Highlights Thin sheets of polymer sandwich steel (MPM) sheets were joined successfully with hot rolled E250 sheet without thermal disengagement of the polymer layer in MPM by CMT welding. Under optimal parameters MPM/E250 joints exhibited excellent mechanical properties. Fractography analysis revealed ductile failure mode. The error between experimental and predicted results by Taguchi was significantly lower and very well within the acceptable range.

The influence mechanism of variable polarity plasma arc pressure on flat keyhole welding stability

The variable polarity plasma arc (VPPA) welding process is well suited for the welding of aluminum alloy, but the stable welding process and good-quality weld bead are difficult to be obtained in the flat position. The purpose of this work is to study the characteristic of variable polarity plasma arc pressure with different polarity time and find the key fact to achieve welding process stability and good quality welding joint in the flat position. The arc shape variation, arc pressure evolution and the molten pool behavior were observed and measured to discover the relationship to the weld joint formation. It was found that the small plasma arc pressure in the center with a similar affected region and enough upward shear force of arc plasma are the necessary conditions to get a stable keyhole process, which could drive the liquid metal of the back side weld pool converges at the bottom of the keyhole and flows upward and form a high-quality weld bead. Comparing the welding stability and quality, adjusting polarity time is the good method for completing the necessary conditions. With the variable polarity time decreasing, the stable keyholes process and acceptable weld quality were obtained in the flat position.

Gravity effects on horizontal variable polarity plasma arc welding

Welding defects easily appear in variable polarity plasma arc (VPPA) horizontal welding of aluminum alloy. The influence of gravity on the molten metal flow plays a critical role. Thus, the effect of the dip angle of the torch and the position of the feed wire were investigated. The stable keyhole of the dip angle torch along the welding direction was easily achieved compared to that of the right-angle torch. The distribution of thermal-force input to the keyhole was changed and the effect of gravity on the molten metal flow decreased. The melting metal bridging between the upper and the lower side became uniform by adjusting the position of the feed wire, the metal liquid state was then adjusted to ensure that gravity is a beneficial force on the molten metal flow. By introducing these optimizing methods, stable keyholes process and acceptable weld quality were obtained in VPPA horizontal welding.

A selectively-coupled shear localization model for friction stir welding process window estimation

This paper presents a novel computational procedure specifically aimed at gaining modeling capabilities for estimating friction stir welding (FSW) process window. The proposed model first combines a three-dimensional (3D) transient heating effects with a one-dimensional (1D) shear localization process leading to shear band development within one pin revolution. The resulting shear band width is then compared with the minimum material flow layer thickness required for satisfying both mass conservation and velocity continuity conditions in a two-dimensional (2D) planar material flow around the tool pin. If the shear band width formed within one pin revolution is equal or larger than the minimum material flow layer thickness, conditions for developing a quality weld prevail. Otherwise, conditions for developing various forms of weld defects can be identified, depending upon shear localization behavior predicted. Specifically, the proposed model is shown capable of elucidating some of the major defect formation mechanisms observed in experiments, such as “lack of fill”, “abnormal stirring”, “surface galling”, and “excessive flash”, etc. As a result, the selectively-coupled shear localization model enables a theoretical estimation of FSW process window typically represented as a regime of welding speed and stir tool rotation speed combination for a given application, within which acceptable weld quality should be expected. Its application in FSW process window estimation is demonstrated by considering three types of aluminum alloys. In all cases, good agreements are achieved between model-estimated and experimentally-determined process windows. In addition, the proposed model also enables a theoretical estimation of optimum welding parameters within an established process window, e.g., for achieving maximum welding speed while maintaining good weld quality.

Effect of welding parameters and the heat input on weld bead profile of laser welded T-joint in structural steel

The high power fiber laser has become one of the most efficient energy sources for deep penetration welding processes used in heavy manufacturing and marine industries. Combinations of cost-efficient, easily automatable process together with fairly mobile and flexible welding equipment have raised high expectations for improved quality and economic feasibility. In this study, the fillet welding of a low alloyed structural steel was studied using a 10 kW fiber laser. Plates of 8 mm thick AH36 were welded as a T-joint configuration in flat (1F) and horizontal (2F) positions using either an autogenous laser welding or a hybrid laser arc welding process. The effect of heat input on the weld bead geometry was investigated using one variable at a time approach. The impact of single process parameter such as laser power of 4.5–6 kW, welding speed of 0.5–2.5 m/min, beam inclination angle of 6°–15°, focal point position of −2 to +2 mm, and welding positions of 1F and 2F were studied. All welds were visually evaluated for weld imperfections described in EN ISO 13919-1 standard. Penetration depth, geometries of the fusion and heat affected zones, and hardness profiles were measured. Produced joints have a high depth to width ratio and a small heat affected zone; full penetration welds with acceptable weld quality on both sides of the joint were produced. The parameter configurations for optimizing the welding processes are proposed.

Laser ultrasound and simulated time reversal on bulk waves for non destructive control

Laser welding of aluminium generally creates embedded welding defects, such as porosities or cracks. Non Destructive Inspection (NDI) after processing may ensure an acceptable weld quality by defect detection. Nowadays, NDI techniques used to control the inside of a weld are mainly limited to X-Rays or ultrasonics. The current paper describes the use of a Laser Ultrasound (LU) technique to inspect porosities in 2 and 4-mm thick sheet lap welds. First experimentations resulted in the detection of 0.5-mm drilled holes in bulk aluminium sheets. The measurement of the depth of these defects is demonstrated too. Further experimentations shows the applicability of the LU technique to detect porosities in aluminium laser welds. However, as the interpretation of raw measures is limiting the detection capacity of this technique, we developed a signal processing using Time-Reversal capabilities to enhance detection capacities. Furthermore, the signal processing output is a geometrical image of the material's inner state, increasing the ease of interpretation. It is based on a mass-spring simulation which enables the back-propagation of the acquired ultrasound signal. The spring-mass simulation allows the natural generation of all the different sound waves and thus enables the back-propagation of a raw signal without any need of filtering or wave identification and extraction. Therefore the signal processing uses the information contained in the compression wave as well as in the shear wave.

Feasibility study of laser welding assisted by filler wire for narrow-gap butt-jointed plates of high-strength steel

Laser welding assisted by a cold filler wire (LWCFW) and hybrid laser–gas metal arc (HLGMA) welding were applied to execute the welding of 6.63-mm-thick high-strength steel (HSS) plates. The corresponding hardness distribution, tensile strength, and micrographs of the welded HSS were experimentally obtained. Results show that LWCFW can achieve a good weld quality without the groove preparation that is usually required in HLGMA welding. A narrower fusion zone and heat-affected zone can be achieved by LWCFW as compared to HLGMA welding. However, the size of the gap in a butt joint in LWCFW is limited. It was found that when a gap of about 1 mm is used in joining 6.63-mm-thick plates in a square butt joint with 4 kW of fiber laser power and a wire diameter of 0.9 mm, an acceptable weld quality could be achieved.

Quality and costs analysis of laser welded all steel sandwich panels

Laser welding is a fast and flexible way to manufacture all steel sandwich panels. Economically the most feasible solution is often to weld as large panels as possible. In corrugated core all steel sandwich panel applications the effect of an air gap in welding is difficult to eliminate when welding large panels. A system for welding large panels with a laser is often considered to be complicated and expensive. The processibility of large panels with specified core types was estimated by welding large all steel sandwich panels with two different core types. Materials used were low-carbon steel DC01 and stainless steel ASTM 304 with sheet thickness from 0.5 to 1.5 mm. The panels were welded with a 6 kW CO2 laser. The manufacturing of large steel panels was possible with acceptable weld quality (EN-ISO 13919-1). The total flatness of the panels was better than 11.3 mm, depending on the manufacturing procedure, core type, and material. Total manufacturing costs of high power CO2, Nd:yttrium–aluminum–garnet and diode lasers were compared in welding of large weight optimized panels. Laser system, panel size, fixturing, and the welding parameters affect the total costs. The size and type of the panel core has together with the batch size a strong effect on the manufacturing costs. The economical evaluation of the cost of the panel depends considerably on the application.

Friction Stir Welding Tools and Developments

The basic principle of Friction Stir welding (FSW) is described. Certain applications (butt and lap welding) are reviewed, with particular emphasis on probe types currently under development (Flared-Triflute™ probe and an A-Skew™ probe). Both give lap welds of 190% of the plate thickness, an improvement in weld integrity, a reduction in upper plate thinning and an increased welding speed over current practice. Butt welds produced with Whorl™ and MX-Triflute™ frustum-shaped probes that gave acceptable weld quality are also described.

Weld growth mechanism of resistance spot welds in zinc coated steel

Whilst resistance welding has been the primary joining technique for automotive body-in-white (B-I-W) assembly since the 1930s, far more welds than necessary are used to fabricate the structure to achieve the required structural performance. This can be attributed due to lack of confidence in weld quality, particularly when joining three or more sheet combinations. To achieve the necessary confidence, there is a need to understand the mechanism of heat development and weld formation, and develop appropriate welding conditions to ensure acceptable weld quality. With the high proportion of zinc coated steels in B-I-W assembly, one of the key issues is to understand the role of the zinc coating in weld nugget development. Consequently, in this study, the mechanisms of weld nugget growth for three thickness joints of both uncoated low carbon and hot dip zinc (GI) coated steels have been investigated using similar welding conditions. The welding conditions selected were based on those recommended for the uncoated low carbon steel. Since these are unlikely to be optimal in the case of the coated steel, further studies were undertaken to establish the effect of electrode force on the mechanism of weld growth. The results obtained indicated that the rate of heat development was reduced in the case of the coated steel compared to the uncoated steel variant. This can be attributed to the lower surface resistance in the presence of the zinc coating. Variations in electrode force indicated flexibility in the selection of welding conditions to produce an acceptable weld quality. Where, an increase in the level of electrode force was used, a significant change in the location of heat development and weld nugget formation was noted compared to that observed for the low force variant. This was attributed to the change in the dominant resistive heating mechanism that took place.

Effect of wire feed position on laser welding with filler wire

The utilization of the laser welding process with filler wire addition is often considered a complicated and difficult process which has too high accuracy demands for a work shop floor. This study shows the effect of some of the most important welding variables to the quality and efficiency of laser welding when welding with filler wire. The material used was a common structural steel St52/37 of 6 mm in thickness. The joint type used was a butt joint. Acceptable weld quality was achieved with most of the tested parameter combinations. The effect of the wire feeding parameters for the acceptable weld quality was smaller than expected. The variations of weld quality caused by the nonoptimized aiming of filler wire can be compensated by the adjustment of the filler wire feed rate and the heat input. The quality of the welds was compared with developed weld quality index. The increase in heat input will make it possible to accept wire feeding to a lower position or from backwards compared to the welding direction. The misalignment of wire in the transverse direction to welding direction and laser beam optical axis causes problems that may lower the quality of the weld. The comparison of the welds with variable air gap widths showed that the weld width is more dependent on the heat input that on the original gap volume.

Control of weld quality in large welded populations

The use of factorial experimental design during weld procedure development, as a statistical tool for the control of weld quality in large welded populations, is investigated. As an example, it is applied to single pass welds made in the laboratory using the spacer to tube attachment geometry present in AGR boilers. The welds have been deposited using a robotic cold wire TIG technique. A preliminary welding trial has been made to identify a potential working point welding condition and the significant welding parameters. With this experience, a replicated 2 5 factorial welding trial has been designed and evaluated experimentally. Transverse sections have been examined to determine weld penetration quality features. Rapid data accumulation and processing has been achieved by using a digital graphics tablet and a desk top computer. The means and variances of each quality feature have been calculated. Using a standard factorial analysis program the significant effects have been identified and incorporated into control equations. Using these data and having established minimum acceptance criteria, defect rates have been calculated as a function of weld position. The control equations have also been used to revise working point welding conditions, to achieve acceptable weld quality. These equations have also been used in conjunction with the reproducibility measurements to estimate the amount of weld quality variability that can be attributed to process control. The use of these estimates to influence the selection of welding equipment both in the laboratory and on the shop floor is also discussed.