Wire‐arc additive manufacturing is a metal additive manufacturing process that enables the production of large components at a high deposition rate. This process transfers a large amount of heat to the workpiece, requiring the introduction of idle times between the deposition of subsequent layers so that the workpiece cools down. This procedure prevents the workpiece from collapsing and ensures a suitable interpass temperature. The main challenge is the selection of such an idle time capable of ensuring the required interpass temperature, because the cooling rate of the workpiece changes throughout the process, entailing the need for a different idle time between the deposition of subsequent layers to achieve a constant interpass temperature. This paper proposes an innovative approach to schedule the deposition of interlayer idle times for wire‐arc additive manufacturing process. The technique is based on a finite element analysis of the thermal behavior of the workpiece, by solving the heat transfer equations. The simulation data are processed using the developed algorithm to compute specific idle times for the deposition of each layer, thereby ensuring a constant interpass temperature. The effectiveness of the proposed technique is validated by experiments performed on a test case component. The idle times are calculated using the proposed technique, by simulating the process, and used to manufacture the test case. The temperature data measured during the process are compared with the FE simulation results to verify the accuracy of the model. An analysis of the geometry of the manufactured workpiece confirms that the adoption of the idle times obtained by the proposed technique prevents the occurrence of major structural collapses.

Idle time selection for wire-arc additive manufacturing: A finite element-based technique

Welding is a major manufacturing process that joins two or more pieces of materials together through heating/mixing them followed by cooling/solidification. The goal of welding manufacturing is to join materials together to meet service requirements at lowest costs. Advanced welding manufacturing is to use scientific methods to realize this goal. This paper views advanced welding manufacturing as a three step approach: (1) pre-design that selects process and joint design based on available processes (properties, capabilities, and costs); (2) design that uses models to predict the result from a given set of welding parameters and minimizes a cost function for optimizing the welding parameters; and (3) real-time sensing and control that overcome the deviations of welding conditions from their nominal ones used in optimizing the welding parameters by adjusting the welding parameters based on such real-time sensing and feedback control. The paper analyzes how these three steps depend on process properties/capabilities, process innovations, predictive models, numerical models for fluid dynamics, numerical models for structures, real-time sensing, and dynamic control. The paper also identifies the challenges in obtaining ideal solutions and reviews/analyzes the existing efforts toward better solutions. Special attention and analysis have been given to (1) gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW) as benchmark processes for penetration and materials filling; (2) keyhole plasma arc welding (PAW), keyhole-tungsten inert gas (K-TIG), and keyhole laser welding as improved/capable penetrative processes; (3) friction stir welding (FSW) as a special penetrative low heat input process; (4) alternating current (AC) GMAW and double-electrode GMAW as improved materials filling processes; (5) efforts in numerical modeling for fluid dynamics; (6) efforts in numerical modeling for structures; (7) challenges and efforts in seam tracking and weld pool monitoring; (8) challenges and efforts in monitoring of keyhole laser welding and FSW; and (9) efforts in advanced sensing, data fusion/sensor fusion, and process control using machine learning/deep learning, model predictive control (MPC), and adaptive control.

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Advanced Welding Manufacturing: A Brief Analysis and Review of Challenges and Solutions

The metal transfer is a fundamental process in gas metal arc welding, which substantially determines the shape of the weld seam and strongly influences arc formation and stability. In this investigation the material transfer from the wire electrode (anode) to the workpiece (cathode) is analysed experimentally with high accuracy using various innovative diagnostic techniques for a pulsed gas metal arc welding (PGMAW) process. A high-speed two-colour pyrometer, a calorimeter, thermocouples, a stereo optical setup and a droplet oscillation technique are used to analyse a precisely defined PGMAW process. Thus, results obtained are verified by different measurement techniques and enable a comprehensive description of the material transfer procedure.The surface temperature of both electrodes as well as the droplet temperature, enthalpy and surface tension were determined. Furthermore, the geometry of the arc, wire, droplets and weld pool were extracted in three dimensions in order to describe the interaction between the material transfer and the formation of the weld seam. The experiments are performed using argon as shielding gas and pure iron as filler and base material to reduce complex chemical processes. It turned out that the wire feed rate has the biggest influence on droplet temperature and detachment. A correlation between weld pool formation and weld pool surface temperature gradient was observed, which is mainly a function of welding speed and wire feed rate. The experimental results obtained provide a detailed data pool for use in modelling.

https://iopscience.iop.org/article/10.1088/0022-3727/46/22/224008/pdf

Determination of enthalpy, temperature, surface tension and geometry of the material transfer in PGMAW for the system argon-iron

Molten pool behaviors and their influences on welding defects in narrow gap GMAW of 5083 Al-alloy

In the present study, the mechanism to control droplet temperature in the plasma MIG welding was discussed based on the measurements of the droplet temperature for a wide range of MIG currents with different plasma electrode diameters. The measurements of the droplet temperatures were conducted using a two color temperature measurement method. The droplet temperatures in the plasma MIG welding were then compared with those in the conventional MIG welding. As a result, the droplet temperature in the plasma MIG welding was found to be reduced in comparison with the conventional MIG welding under the same MIG current. Especially, when the small plasma electrode diameter was used, the decrease in the droplet temperature reached maximally 500 K. Also for a particular WFS, the droplet temperatures in the plasma MIG welding were lower than those in the conventional MIG welding. It is suggested that the use of plasma contributes to reduce the local heat input into the base metal by the droplet. The presence of the plasma surrounding the wire is considered to increase the electron density in its vicinity, resulting in the arc attachment to expand upwards along the wire surface to disperse the MIG current. This dispersion of MIG current causes a decrease in current density on the droplet surface, lowering the droplet temperature. Furthermore, dispersed MIG current also weakens the electromagnetic pinch force acting on the neck of the wire above the droplet. This leads to a larger droplet diameter with increased surface area through lower frequency of droplet detachment to decrease the MIG current density on the droplet surface, as compared to the conventional MIG welding at the same MIG current. Thus, the lower droplet temperature is caused by the reduction of heat flux into the droplet. Consequently, the mechanism to control droplet temperature in the plasma MIG welding was clarified.

Study on factors affecting the droplet temperature in plasma MIG welding process

Average temperature was calculated from calorimetric measurements in free-flight gas metal arc welding (GMAW) for ER70S-6 (carbon steel), ER316L (stainless steel), and ER4030 (aluminum) electrodes. Measurements were conducted using a constant-pressure water calorimeter to capture the droplets and a flow-through copper cathode/calorimeter to carry the arc. Thermocouples were used to monitor the temperature change of the water flowing through the cathode as well as in the constant pressure calorimeter. Results show a local minimum in temperature during the transition from globular to spray transfer modes.

Calorimetric Measurement of Droplet Temperature in GMAW


 Elastic modulus of 316L and XM-19 austenitic stainless steel was characterized before and after hydrogen charging using impulse excitation. Four coupons of each alloy were subsequently charged at 300 C and 138 MPa for 13 days with gaseous hydrogen to ensure an equilibrium concentration of hydrogen was achieved. Elastic modulus was re-characterized after charging. In 316L, the elastic modulus increased by 0.28% (0.546 GPa) on average on three of the samples, while one sample did not show a significant change. In XM-19, the elastic modulus increased by 0.38% (0.734 GPa) on average. The increase in both XM-19 test coupons and three 316L test coupons was statistically significant, while the increase in the other 316L test coupon was not.

K. M. Scott

Papers

Calorimetric Measurement of Droplet Temperature in GMAW

The Influence of Hydrogen on the Elastic Modulus of 316L and XM-19 Austenitic Stainless Steels