Producing Quality Welds Research Articles

Assessing the Performance of a Dual-Speed Tool When Friction Stir Welding Cast Mg AZ91 with Wrought Al 6082.

A novel dual-speed tool for which the shoulder and pin rotation speeds are separately established was utilized to friction stir weld cast magnesium AZ91 with wrought aluminum 6082-T6. To assess the performance and efficacy of the dual-speed tool, baseline dissimilar welds were also fabricated using a conventional FSW tool. Optical microscopy characterized the weld microstructures, and a numerical simulation enhanced the understanding of the temperature and material flow behaviors. For both tool types, regions of the welds contained significant amounts of the AZ91 primary eutectic phase, Al12Mg17, indicating that weld zone temperatures exceeded the solidus temperature of α-Mg (470 °C). Liquation, therefore, occurred during processing with subsequent eutectic formation upon cooling below the primary eutectic temperature (437 °C). The brittle character of the eutectic phase promoted cracking in the fusion zone, and the "process window" for quality welds was narrow. For the conventional tool, offsetting to the aluminum side (advancing side) mitigated eutectic formation and improved weld quality. For the dual-speed tool, experimental trials demonstrated that separate rotation speeds for the shoulder and pin could mitigate eutectic formation and produce quality welds without an offset at relatively higher weld speeds than the conventional tool. Exploration of various weld parameters coupled with the simulation identified the bounds of a process window based on the percentage of weld cross-section exceeding the eutectic temperature and on the material flow rate at the tool trailing edge. For the dual-speed tool, a minimum flow rate of 26.0 cm3/s and a maximum percentage of the weld cross-section above the eutectic temperature of 35% produced a defect-free weld.

Effect of welding speeds on the metallurgical and mechanical property characterization of friction stir welding between dissimilar aerospace grade 7050 T7651-2014A T6 aluminium alloys

Friction stir welding (FSW) is an effective technique for producing quality welds and resolving solidification issues of aluminium alloys found in fusion welding (FW) techniques. This study seeks to establish the effect of welding speeds (WSs) of 25, 45, 65, and 85 mm/min with a constant tool rotation speed (TRS) of 1000 rpm and a tool tilt angle (TTA) of 20. Furthermore, mechanical testing was performed to determine the tensile strength, microhardness, and residual stresses. In addition, microstructure characterization was performed using optical microscopy, SEM/EDS and EBSD to identify the quality assurance of the dissimilar AA7050 T7651-AA2014A T6 weld joint. The results indicated that while the WS increased from 25 to 85 mm/min, the width of the stir zone decreased from 9.497 mm to 6.315 mm, according to the macrostructure analysis. Moreover, microstructural analysis reveals the direction of material flow and grain structure on the advancing side (AS) and retreating side (RS) of the thermo-mechanical affected zone (TMAZ) in the weldment. EBSD analysis reveals the 65 mm/min weld joint has a finer grain size (the average grain size is 2 µm) compared to other WS. Residual stress analysis also reveals compressive residual stress in all weld zones except for RS. The RS of residual stress values for 25–85 mm/min weld joints is 3, 7, 4, and 5 MPa. At the same time, UTS values were obtained as 274 MPa (25 mm/min), 317 MPa (45 mm/min), 332 MPa (65 mm/min), and 308 MPa (85 mm/min), and fracture occurred on the RS of all the welded joints, which was confirmed by harness values that are (AS and RS) 115–157 HV and 63–154 HV, respectively. The observed results show that WS at 65 mm/min has better mechanical and metallurgical properties than other welding speeds because of proper material mixing and finer grains obtained in the weldment, which resulted in higher UTS values being attained for WS at 65 mm/min of the FSW joint.

Plasma Welding of Aluminum in an Oxygen-Free Argon Atmosphere

Abstract Plasma welding is characterized by a high concentration of energy, which allows for high welding speed and leads to less distortion and residual stresses compared to conventional welding processes. Due to the local and controlled heat input, the process is suitable for sheet metal from ≈ 0.1 mm (micro plasma) up to ≈ 10 mm. In the case of aluminum and its alloys, the natural aluminum oxide layer on the metal surface limits the productivity of the plasma welding process. The electrically isolating and thermally insulating Al2O3 layer has a significantly higher melting point compared to the aluminum (T m(Al2O3) = 2072 °C vs. T m(Al) = 660 °C). The oxide layer hinders the formation of a stable arc and can even impede the joining formation. In order to remove the oxide layer and to produce quality welds with a DC process, it is necessary to weld with reverse polarity to use the principle of cathodic surface cleaning. However, this leads to increased electrode wear and increased penetration depth, which is not always desirable. In the study presented, the use of silane to reduce the oxygen content in the welding atmosphere as well as to remove the natural aluminum oxide layer on the metal surface was investigated. As previous studies have shown that the use of silane-doped plasma-gases is suitable for removing the superficial oxide layer on aluminum components, high-quality welded joints were expected. Quality welds with sufficient dilution were achieved using a transferred arc silane-doped helium plasma. In contrast, welding with an argon-silane mixture led to excessive pores formation. Additionally challenges to stabilize the arc process were identified and ramifications with respect to process optimization are discussed.

Control of Welding Speed and Current in Machine–Human Cooperative Welding Processes

In intelligent manufacturing processes, the dependence of the weld quality on the welder’s skills in manual welding should be minimized. To better control the welding quality, a machine–human cooperative control system was designed in this study, and a skills learning experiment was conducted to correlate the relationship between welding speed and welding current. The obtained skills were then transferred to the control system to control the human welder to achieve the desired welding speed. In addition, to adjust the welding current to control the welding power, the desired full penetration welds were finally obtained. In the present study, full penetration welds with different welding speeds were obtained in a 304 stainless steel pipe having a wall thickness of 2.03 mm and an outside diameter of 113.5 mm using the machine–human cooperative welding process. The back fusion width was 2.3 to 5.5 mm, which met the quality requirements of the weld. This study provides a research direction for effectively solving the problem of the shortage of welders and for helping unskilled welders to produce quality welds, and lays the foundation for developing the next generation of machine–human cooperative intelligent welding system.

Role of Scroll Shoulder and Pin Designs on Axial Force, Material Flow and Mechanical Properties of Friction Stir Welded Al–Mg–Si Alloy

Understanding the flow of plasticized material during friction stir welding was critical for producing quality welds. The tool geometry such as shoulder and pin designs play a significant role in altering the material flow characteristics. In the present investigation, scroll shoulder design with varying pins were addressed in the aspects of tool axial force, material flow and mechanical properties. Marker insert method was used to study the flow of plasticized material with copper as inserts. The results show that the pin geometry variation has showed a significant change in the axial force plot in the welding phase. The taper cylindrical pin tool (TS) facilitated a constant force, hexagonal pin (TS)H and pentagonal pin (TS)P tools facilitated a decreasing force behavior, and triangular pin designed tool (TS)T facilitated an increasing behavior of force in the welding phase. The material flow experiments showed that the scroll shoulder with triangular pin shaped tool (TS)T completely sheared the marker inserts in both parallel and perpendicular to weld conditions compared to other pin shapes. The sheared copper fragments were uniformly distributed on both advancing and retreating sides of the weld for (TS)T tool. Superior mechanical properties were noticed for (TS)T tool with a tensile strength of 184 MPa and Microhardness of 78 HV at stir zone.

Open-Closed-Loop Iterative Learning Control With a Self-Adjustive Factor of the Gas Tungsten Arc Welding (GTAW) Process

Human welder's experiences and skills are critical for producing quality welds in manual gas tungsten arc welding (GTAW) process. For batch welding of the same workpiece, and the welding experience accumulated in same welding track, the welding process has a very high-degree reduplication. In this article, an open-closed-loop iterative learning control algorithm is constructed and implemented as an intelligent controller in automated GTAW process to reach the desired trajectory well. During the welding process, there will encounter external interference, such as random changes in voltage, resulting in pool surface fluctuations. Therefore, we introduce a self-adjustive factor based on ILC algorithm. The self-adjustive factor can adjust the input of the controller according to the error and error rate of change of the system, so that the system body has self-adaptation to improve the ability of anti-interference of the system. The simulation shows that a new proposed ILC control is an effective method for weld penetration in GTAW.

Characteristics of Potential Sources - Vertical Force, Torque and Current on Penetration Depth for Quality Assessment in Friction Stir Welding of AA 6061 Pipes

Friction stir welding of the pipes or curved bodies in aerospace components is tedious due to complex geometry, and hence, research articles related to it are rare. Primarily, this research has investigated the nature of vertical welding force, torque and current on the weld quality during the friction stir welding of Aluminium 6061 (AA 6061) pipe at different penetration depths – 1.4 mm, 1.54 mm, 1.66 mm, 1.7 mm, 1.9 mm and 2 mm. The feed rate and rotational speed have been set at 1 mm/s and 1800 rpm respectively. The weld quality has been evaluated using visual inspection and tensile test. From the results, penetration depths from 1.7 mm to 2 mm have been suitable for producing quality welds on a specimen with 2 mm thickness, which resulted in fewer shoulder marks, and percentage reduction of strength lower than 25.61% compared to the base metal. Friction stir welding using penetration depth of 1.4 mm has resulted in a weld, with excess shoulder marks and 43.84% of reduction in strength, which is not acceptable. In addition, the range of penetration depths (1.7 mm to 2 mm) for which the force remains constant is recommended for producing quality welds. While torque and power have increased linearly with the penetration depth, the distribution of the current has remained the same for different penetration depths. However, the current consumption has varied significantly in each one of the friction stir welding stages clearly distinguishing the tool penetration, the dwell time, the traverse tool movement and the tool pull out. The experimentation has been successful in laying the foundation for the process parameters signifying the importance of penetration depth.

Acoustic horn design for joining metallic wire with flat metallic sheet by ultrasonic vibrations

Ultrasonic Metal Welding is a green manufacturing technique and one of the most advanced solid state welding processes in which similar or dissimilar metallic components are joined by the application of high frequency vibrations (> 20 kHz) and pressure. Ultrasonic metal welding is accompanied by slip and plastic deformation so that the base metals being welded will not melt and in turn forms a homogenous coalescence of two metals at the joining area so that the joint retains the parent metal properties. The major problem faced by the industries using ultrasonic metal welding process is the poor weld quality and weld strength. The design of acoustic horn or sonotrode plays a dominant role in producing quality welds. The primary function of sonotrode is to vibrate at a level required for welding and also to transmit the vibration energy to the point where welding of metals takes place. For producing quality welds, the vibration energy is to be transmitted to the weld interface without much loss. Therefore, there is a need for accurate design of sonotrodes in ultrasonic metal welding process. This work focuses on designing a stepped sonotrode used for joining metallic wire with metallic sheet based on significant design parameters such as amplitude gain and von Mises stress factor using modal and harmonic analysis. Experimental trials are conducted using the stepped sonotrodes and the effectiveness of the designed sonotrodes is evaluated based on improvement of strength of the joint in tension.

Multi-axis force measurements of polymer friction stir welding

In this study, high-molecular-weight-polyethylene sheets were welded in the butt-joint configuration in order to investigate the effect of welding parameters on the generated forces during welding. The welding procedure was performed using newly developed clamping device with the ability to measure and acquire the forces during welding in X, Y and Z directions (FX, FY and FZ). Using a welding tool with a stationary shoulder and position control, the applied force is kept approximately constant during the entire welding extension. The obtained results enabled the identification of the influence of each individual welding parameter on the applied forces. The axial force is the most influential force during this process, which is responsible for forging pressure and frictional heat, while traverse and rotational speeds have the main effect on the traversing and lateral forces. The developed sensitized clamping system was used in support of welding process optimization to produce quality welds without resourcing to costly force control sensors. Using this device, it was possible to use force as a welding parameter and produce strong welds with mechanical behavior close to the parent materials’.

Dynamic Neuro-Fuzzy-Based Human Intelligence Modeling and Control in GTAW

Human welder's experiences and skills are critical for producing quality welds in manual gas tungsten arc welding (GTAW) process. In this paper, a neuro-fuzzy-based human intelligence model is constructed and implemented as an intelligent controller in automated GTAW process to maintain a consistent desired full penetration. An innovative vision system is utilized to real-time measure the specular 3D weld pool surface under strong arc light interference. Experiments are designed to produce random changes in the welding speed and voltage resulting in fluctuations in the weld pool surface. Adaptive neuro-fuzzy inference system (ANFIS) is proposed to correlate the human welder's response to the 3D weld pool surface as characterized by its width, length and convexity. Closed-loop control experiments are conducted to verify the robustness of the proposed controller. It is found that the human intelligence model can adjust the current to robustly control the process to a desired penetration state despite different initial conditions and various disturbances. A foundation is thus established to explore the mechanism and transformation of human welder's intelligence into robotic welding systems.

Three-dimensional vision-based sensing of GTAW: a review

Automated and robotic welding is now widely used in manufacturing industry. The control of the welding process plays a crucial role in producing quality welds in automated and robotic welding where the assistance from skilled welders is no longer available. In gas tungsten arc welding (GTAW) which is the primary arc welding process for precision joining of metals, the weld pool is the major source of information that can be used to assure the production of desired weld penetration which is the most critical factor determining the weld integrity. To meet this challenge, various sensing technologies have been proposed/studied to sense and obtain the feedback for the weld pool state. This paper summarizes the researches on weld pool state sensing: conventional sensing technologies, vision sensing technology, and multi-sensor information fusion technology, with emphasis on the analysis of three-dimensional vision sensing methods. And three-dimensional vision sensing, multi-sensor technology, intelligent modeling, and effective commercial product development show the future trends of GTAW penetration sensing.

Dynamic Control of 3D Weld Pool Surface Based on Human Response Model

Skilled human welders are so far the only to be capable of producing quality welds against perturbations. They adjust welding parameters based on their observation on 3D weld pool surface. Modelling their response to the 3D pool surface thus leads to next generation intelligent welding robots without human physical limitations. This paper first models this response as a human intelligence based controller. Its controlled process, with 3D pool surface as output and welding parameters as input in gas tungsten arc welding (GTAW), is then Hammerstein modelled in order to analyse its effectiveness. Analysis shows that this controller robustly controls the 3D pool surface under various perturbations. Closed-loop control welding experiments further validated this effectiveness. A foundation is thus established to rapidly transform skilled human welder's intelligence into robotic welding systems.

A tutorial on learning human welder's behavior: Sensing, modeling, and control

Human welder's experiences and skills are critical for producing quality welds in manual GTAW process. Learning human welder's behavior can help develop next generation intelligent welding machines and train welders faster. In this tutorial paper, various aspects of mechanizing the welder's intelligence are surveyed, including sensing of the weld pool, modeling of the welder's adjustments and this model-based control approach. Specifically, different sensing methods of the weld pool are reviewed and a novel 3D vision-based sensing system developed at University of Kentucky is introduced. Characterization of the weld pool is performed and human intelligent model is constructed, including an extensive survey on modeling human dynamics and neuro-fuzzy techniques. Closed-loop control experiment results are presented to illustrate the robustness of the model-based intelligent controller despite welding speed disturbance. A foundation is thus established to explore the mechanism and transformation of human welder's intelligence into robotic welding system. Finally future research directions in this field are presented.

소방배관용 강관을 위한 반자동 가스메탈아크용접장치 개발과 용접부 특성평가

본 연구에서는 소방배관용 아연도금 강관을 용접하기 위하여 반자동 가스메탈아크용접장치를 개발하고 최적 용접 조건을 확립하고 용접성을 조사하기 위해 보호가스, 전압을 변화시켜 용접시험을 수행하였다. Ar 가스에 <TEX>$O_2$</TEX>를 첨가함으로써 미려한 비드외관과 기공률이 감소되었다. 반자동 용접부는 기계적 강도와 용접이음효율이 수동용접부에 비해 약 1.8배 우수하였으며 수압시험에 의해 건전성이 확인되었다. 본 연구에서 개발된 반자동용접장치로 비숙련자도 강관용접이 가능하며 용접결함이 적고 용접생산성을 증대시킬 수 있을 것으로 기대된다. The semi-automatic gas metal arc welding equipment was developed and the weldability of zinc coated steel pipes was evaluated in terms of strength, porosity and welding parameters including shielding gas composition. The good bead appearance and the reduction of porosity in the welds could be possibly obtained by adding <TEX>$O_2$</TEX> to Ar. The strength and joint efficiency of welds made by the semi-automatic welding equipment was about 1.8 times higher compared with welds manually made. The integrity of welds was confirmed by the water pressure test as well, Finally, it is expected that the weld productivity will be enhanced even unskilled welders can produce quality welds by operating the semi-automatic welding equipment.

Adaptive interval model control of weld pool surface in pulsed gas metal arc welding

A skilled welder may produce quality welds with a desired weld penetration depth based on his/her observation on weld pool surface. In a recent study the authors found that the arc voltage change during the peak current period in pulsed gas metal arc welding is a relevant measurement of the weld pool surface that can estimate the penetration depth. A system is thus developed to measure the arc voltage and control its change as output using the base current period as input and the necessity to use relatively complex sensors such as imaging systems is avoided. Analysis shows that the resultant process being controlled is time-varying, noisy, and nonlinear. After simplification into a linear system, an adaptive interval model control system has been designed. Experimental results confirmed the effectiveness of the adaptive interval model control algorithm and the developed control system.

Adaptive Nontransferred Plasma Charge Sensor and Its Applications

Practical welding control systems require durable/compact sensors to sense the welding process and appropriate control algorithms to produce smooth welds. A novel arc welding sensor, referred to as nontransferred plasma charge sensor, which requires no additional attachment to the torch, has been proved to be reliable for weld pool surface sensing. Aiming at eliminating the effect of manufacturing conditions on the sensor performance, this paper proposes two simple yet effective methods. Specifically, reference signals are sampled either from the bottom or the top surface of the work-piece and used to define relative signals, which can measure the depth of the weld pool with better accuracy. Using improved sensing methods, two groups of welding control experiments, keyhole plasma arc welding and all-position pipe welding, have been conducted, and the effectiveness of the developed sensing/control systems in producing quality welds under various conditions is verified.

Sensing of weld pool surface using non-transferred plasma charge sensor

Gas tungsten arc welding (GTAW) is the primary process for precision joining of metals due to its capability for accurate control of heat input. As a close relative and modification of GTAW, plasma arc welding (PAW) has higher energy density and can penetrate thicker workpieces while maintaining the desired capability for accurate control of heat input. In order to produce quality welds consistently using PAW, in addition to accurate control of heat input, the sensing and control of the weld pool surface are also critical. It has been found that the non-transferred plasma arc, in the absence of the transferred arc or the main plasma arc for welding, can establish a plasma charge potential which is indicative of the arc length. In order to take good advantage of this intrinsic characteristic of the non-transferred arc and eliminate the influence of the transferred arc in a normal PAW process, a power module is used to cut off the main arc current periodically for a very short period of time to acquire accurate information for monitoring the weld pool surface. A non-transferred plasma charge sensor is proposed based on this mechanism and experiments verified its effectiveness.

Welding current effect on diffusible hydrogen content in flux cored arc weld metal

The application of flux cored arc welding (FCAW) has increased in manufacturing and fabrication. Even though FCAW is well known for its good capability in producing quality welds, few reports have been published on the cause of the relatively high diffusible hydrogen content in the weld metal and its relation with the ingredients used in the wire production and with the welding parameters (mainly welding current). This paper describes experiments where data obtained from weld metal diffusible hydrogen analysis, metal droplet collection, and high-speed recording of metal droplet transfer were used to evaluate the effect of welding current on diffusible hydrogen content in the weld metal. The results from gas chromatography analysis showed that weld metal hydrogen content indeed increased with welding current. A polynomial regressional analysis concluded that hydrogen increase with current was better described by a linear function with proportional constant of approximately 0.7 or 70%. Different from the GMA welding transfer behavior, statistical analysis showed only a small increase in metal droplet size with increasing current. The metal transfer mode remained in the globular range for currents between 100 and 150 A. The most surprising findings were with the high-speed cinematography recording. Observing the high speed movies, it was possible to see that at low current, "unmelted" flux sporadically touched the weld pool but at higher current, the flux remained touching the weld pool during the whole time of droplet formation and transfer. It is believed that since the flux has ingredients that contain hydrogen, hydrogen passes through the arc undisturbed, going to the weld bead intact and increasing the hydrogen content in the weld metal. Another important observation is regarding to droplet size. Droplet size increased with increasing current because forces from decomposed gases from the flux could sustain the droplets, retarding their transfer and allowing them to grow.

Stability of keyhole in plasma arc welding

Stability of the keyhole plays a fundamental role in producing quality welds in keyhole plasma arc welding. Some studies have assumed that keyhole diameter is a stability measurement. However, recent experimental observation by the authors revealed that width is not correlated with stability. To measure stability, this study uses the keyhole width as the lower limit of the weld pool width required to prevent keyhole collapse. Because the maximal width of the weld pool before burn-through is approximately fixed for particular applications, it can be used as the upper limit of the weld pool width for preventing burn-through. Hence, the margins of the keyhole process from collapse and burn-through can be measured based on how close the pool width is to its upper and lower limits. The stability distance and stability factor are then defined based on the margins to quantity the state of the stability of the keyhole process.

Urinary excretion of chromium as an indicator of exposure to various chromium forms in MMA/SS welders

Human welder's experiences and skills are critical for producing quality welds in manual GTAW process. Learning human welder's behavior can help develop next generation intelligent welding machines and train welders faster. In this tutorial paper, various aspects of mechanizing the welder's intelligence are surveyed, including sensing of the weld pool, modeling of the welder's adjustments and this model-based control approach. Specifically, different sensing methods of the weld pool are reviewed and a novel 3D vision-based sensing system developed at University of Kentucky is introduced. Characterization of the weld pool is performed and human intelligent model is constructed, including an extensive survey on modeling human dynamics and neuro-fuzzy techniques. Closed-loop control experiment results are presented to illustrate the robustness of the model-based intelligent controller despite welding speed disturbance. A foundation is thus established to explore the mechanism and transformation of human welder's intelligence into robotic welding system. Finally future research directions in this field are presented.