Study of parametric influence and welding performance optimization during regulated metal deposition (RMD™) using grey integrated with fuzzy taguchi approach

Abstract

Regulated Metal Deposition (RMD™) welding process is fundamentally a modified short-circuit gas metal arc welding (GMAW) process wherein a precisely-controlled metal transfer provides uniform droplet deposition, making it easier for the welder to control the puddle and hence achieve an enhanced quality of welded joints. RMD technique was experimented and implemented on a variety of steels such as stainless steel, carbon steels, etc. However, obtaining satisfactory welding performance is indeed a challenging task during RMD. Hence, this study aims to assess the favorable parameters settings in order to optimize welding performance characteristics such as Heat affected zone (HAZ), depth of penetration (DOP) and bead width (BW) of Regulated Metal Deposited arc weldment using grey relation analysis integrated with fuzzy inference system with Taguchi approach. From the adopted methodology the optimal parameter settings for current (135 A), voltage (14 V), and gas flow rate (13 L/min) have been achieved. The study also tells that the voltage is the most influencing parameter during RMD welding.

Introduction

Gas metal arc welding (GMAW) has extensive applications in various sectors namely marine, power plants, aerospace, automobiles, and petrochemicals. It has been noticed that aforesaid sectors face a lot of problems in order to obtain efficient and effective welding with a good rate of productivity while performing welding in conventional ways. The most common problems associated with the GMAW process are spattering, cold laps, cold shut, shallow penetration, porosity, cracking, insufficient strength [[1], [2], [3]]. The components welded by this method require to rework like cleaning of the welded surface due to excessive spattering. Due to rapid development, these days, the demand for gas and oil is also increasing. To fulfill the demand, manufacturers are struggling very hard to produce pipelines with good weld strength and leak-free characteristics. The superior quality of the weld joints along with a good rate of production has become a tough task for them. One of the main reasons behind all these problems is the lack of skilled operators [2, [4], [5], [6]]. In order to eliminate the aforesaid problems besides maintaining an excellent weld quality and productivity, Miller Electric Mfg. Co. developed a new welding process in 2004 named as Regulated Metal Deposition (RMD) welding. This process is an enhancement over the GMAW and therefore it is also known as the advance GMAW process. Actually, it is a software-based, digitally controlled, modified short circuit GMAW process in which the current waveform is controlled for the metal transfer phase [4, [6], [7], [8], [9]].

RMD process consists of seven distinct phases based on the current waveform as shown in Fig. 1. The combination of all these seven phases forms a cycle known as the RMD cycle. The various phases of the RMD cycle have been elaborated in Fig. 2 [10].

The RMD cycle begins with the ‘ball’ phase where current increases and melt the electrode tip for the occurrence of a short-circuit. In the next phase of ‘background’, the current decreases and allows the short circuit to occur. After it, the ‘pre-short’ phase comes where the current reduces to a low level for the protection of the stable pool from the arc force. In the ‘wet' phase, the molten edge of the filler metal touches the workpiece at low current. In the ‘pinch' phase, the pinch effect is obtained at the tip with a rapid increase in current and a short circuit occurs. At the intermediate stage where the ‘pinch' phase ends and the ‘clear' phase begins, the pinch effect is detected. At the end of the ‘clear’ phase, the droplet detaches from the electrode. The cycle gets completed with the ‘blink’ phase in which short circuit breaks and current rapidly decreases to a lower level [4, 6, 8, 11].

This process is advantageous over the other welding processes like tungsten inert gas (TIG), metal inert gas (MIG), etc. It has a high welding travel speed of 6–12 inches per minute (ipm), which is approximate twice the welding travel speed of TIG and MIG processes. This leads to an approximately three times faster production as compared to the conventional processes. This process reduces the reworking and cleaning. It also eliminates the requirement of the hot pass as it creates a root pass of 1/8 in. to 1/4 in. It has very low heat input which significantly eliminates the distortion of weld material. It also eliminates the chances of a cold lap and cold shut and creates welds with very less spatter to spatter-free welds. All these aforesaid advantages of the RMD process lead to superior weld quality with higher productivity [[12], [13], [14], [15]].

Due to its numerous advantages, the applications of RMD welding cover a wide range of sectors namely chemical, petrochemical, power plants, marine, etc. In chemical and petrochemical sectors, the pipes, which are used to transfer gas and oil, are usually welded by this technique. It is used to weld the components of pressure vessels, heat exchangers, condensers, and other piping systems for power plants. It is also used in shipbuilding and other marine applications [6, 11,16].

Costa and Vilarinho [8] performed RMD welding on low carbon steel pipes. The process parameters like wire feed speed, travel speed, trim, arc control, and weaving have been taken into consideration for the experiments. After analyzation, it has been observed that there were no internal discontinuities like porosity, lack of fusion, and cracking. Macroscopic analyzation has been done on the samples and it was examined that when the wire feed speed increases, the penetration, and root reinforcement also increase but the face reinforcement decreases. A study has been also done on the trim parameter. When the trim parameter increases, the width of the weld bead also increases. Due to this, face reinforcement decreases. Kah et al. [4] executed RMD welding on steel pipes. Various process parameters like voltage, welding current, wire diameter, wire feed rate, and travel speed were taken into consideration for the experiments. A study has been done on the responses like heat input and productivity and it was found that when the welding speed increases 2–3 times higher as compared to MIG, the heat input drops down and the production rate increases for the root pass. Liskevych & Scotti [17] examined the effect of shielding gas with varying percentages of CO₂ in the short circuit GMAW process. By varying CO₂ percentage in shielding gas, a study has been done to determine its effect on the metal transfer behavior, spatter generation, and bead geometry of the samples. It has been observed that with an increasing percentage of CO₂, metal transfer regularity decreases and spattering increases. Due to this, the formation of an unregular bead geometry takes place. The best result has been obtained with the availability of 10–30 % of CO₂ in the shielding gas mixture. Vilrinho et al. [18] compared the conventional short circuit GMAW process with modified short circuit GMAW processes like RMD, cold metal transfer (CMT), and surface tension transfer (STT). Experiments were conducted on API 5 L X65 pipes of 8 mm thickness and 8 inch diameter. 15° of v-shaped bevel angle was provided on pipes for the purpuse of reducing the volume of metal deposition. AWS ER70S-6 of 1.2 mm diamter was used as a wire electrode. Ar+25 %CO₂ was used as the shielding gas for inert environment. The contact-tip to base metal distance was 12 mm. For root pass and cap pass, the wire feed speed was set 5–5.75 m/min and 3.5−5 m/min respectively. The travel speed for root pass and cap pass was 25−35 cm/min and 20−24 cm/min respectively. Several tests such as tension, root and face bending, nick break, microhardness, and toughness were conducted, it was observed that the modified short circuit GMAW processes provide much better results as compared to the conventional short circuit GMAW process. Singaravelu D et al. [16] studied the modified short circuit GMAW process for the root pass and compared it with the conventional GMAW process. The comparison has been done on the parameters like current, voltage, wire feed rate, and travel speed and it was found that the weld quality in modified short circuit GMAW has been improved with the smooth appearance of the weld bead along with less spattering. Kumari et al. [19] performed GMAW and studied the effect of various parameters namely arc voltage, welding speed, and gas flow rate on the weld bead geometry. It was observed that when the arc voltage increases, penetration, bead width, and dilution increase but bead height decreases whereas the bead width decreases when the welding speed increases. The effect of gas flow rate has been also examined and it was found that the bead appearance improves with an increase in gas flow rate.

It has been noticed from the literature that less effort has been made by the researchers to study the weldability aspects of the advanced GMAW processes specifically in short circuit mode. Therefore, the present study highlights the weldability aspects of the modified short circuit GMAW process. The study also examines the effects of welding parameters on the welding performance characteristics such as heat-affected zone (HAZ), depth of penetration (DOP), and bead width (BW) using ANOVA methodology. It is also essential to understand the process behavior followed by a proper selection of welding parameters of critical importance towards achieving satisfactory welding yield in an economic way. However, optimization aims to minimize or maximize a single objective function, but in real-world situations, several conflicting functions simultaneously may be required to be optimized for selecting the best option from the number of possible choices. Therefore, grey integrated with the fuzzy inference system has also been adopted here to find out the optimal parametric combination and to improve the welding characteristics during RMD.