Comparative Analysis of Microstructural Assessment and Weld Defects in GMAW and GTAW Techniques

Abstract

The project aims to provide a comprehensive understanding of weld defects, their causes, and the influence of welding processes on microstructure. Attempts have been made to cover almost all necessary aspects of weld defects caused by welding processes and their parameters, material properties, operator proficiency etc. and their effects on weld quality and appearance. Started with the literature review made on weldability of various steels using GMAW, GTAW etc.; metallurgical properties, welding factors, impact on weld due to various allowing elements; influence of filler wire composition on defect-free weld; impacting welding parameters, microstructure and mechanical properties of welded joints and HAZ obtained by various processes, materials, parameters; various common but potential weld defects, their causes and remedies. Lastly, a conclusion has been made on the captioned subject which has provided insights into various weld defects detected through radiographic testing. Additionally, the study analyzed the microstructure of welded joints. The findings shed light on the causes and consequences of weld defects and microstructural changes. The study finally concluded that GTAW is preferable for welding of mild steel plates having lesser thickness (i.e. ≯ 2 mm) in terms of fewer defects, better weld quality, clean and precise appearance over the GMAW process.

1. Introduction

The 1940s saw the invention of GMAW/MIG welding; while other new welding techniques have since been adopted, GMA welding remains one of the most significant processes for stainless steel when it comes to productivity and quality. It is true that studies on GMA welding of ferritic and austenitic stainless steel have been conducted. Numerous facets within this particular environment have been examined. However, the literature review provided above shows that study is still ongoing. It implies that more in-depth research in the field of gas metal arc welding is required. Numerous GMAW process factors interact in a complex way, affecting different aspects of the weld quality either directly or indirectly. Various studies have attempted multiple times to determine the parametric influence on the intended welding properties. Additionally, a number of studies were conducted to create mathematical models of metal transmission, heat transfer, etc. Additionally, neural-network models have been developed to forecast how process parameters will affect the replies. Numerous studies had looked on process improvement and other related GMAW aspects. To determine the scope and goal of the current effort, a review of the literature is conducted on the subject. With a focus on GMA welding of stainless steels, particularly Austenitic and Ferritic stainless steels, and welding of dissimilar metals, particularly dissimilar stainless steels, a review of the literature is made and provided below. But in order to determine the scope of the current task, several other pertinent fields are also taken into account in addition to the ones that were previously mentioned. These include welding different materials, optimizing work done on different metals and welding kinds, welding different materials other than Austenitic and Ferritic stainless steel, etc. Austenitic-to-ferritic stainless steel butt joints are welded in this study in order to do an inquiry. By working together to optimize the experiments, building the butt joints in accordance with the Taguchi Design of Experiment, using X-rays and visual inspection to examine the welded specimens, and analyzing the microstructure in different areas of the weldment. This inquiry has produced several useful conclusions.

1.1 Background and motivation

This project encompasses a comprehensive investigation of weld defects, including blow holes, undercut, cracks, incomplete fusion, spatter, and excessive deposition. It also includes defects detected through radiographic testing, such as incomplete penetration, slag, and porosity. Additionally, the study delves into the microstructure of welded joints, with a specific emphasis on the HAZ. The research focuses on mild steel weld specimens produced through GMAW and GTAW processes. The study does not cover other welding methods or materials. By concentrating on these specific processes and materials, the project aims to offer a detailed understanding of defect formation and microstructural changes in these welding scenario.

1.2 Objectives

The primary objectives of this study are as follows:

• To fabricate mild steel weld specimens using both GMAW and GTAW processes.

• To conduct a thorough analysis of various weld defects, including blow holes, undercut, cracks, incomplete fusion, spatter, and excessive deposition.

• To utilize radiographic testing for the detection and characterization of defects such as incomplete penetration, slag, and porosity.

• To perform detailed metallographic analysis to evaluate the microstructure of welded joints and HAZ.

• To compare and contrast the types and severity of weld defects and microstructural changes between GTAW and GMAW processes.

• To identify and discuss the factors that influence the formation of defects and microstructural alterations.

2. Literature review

GTAW (TIG welding) and GMAW (MIG welding) are two widely used welding processes. GMAW utilizes a consumable wire electrode and a combination of inert and active shielding gases while GTAW employs a non-consumable tungsten electrode, optional filler metal and inert shielding gas. Ramkumar et al. [1] investigated AISI 430 weldability using TIG and activated fluxes. SiO₂ and Fe₂O₃ were employed in the A-TIG procedure to examine AISI 430 weld beads’ geometric and metallurgical properties. Kumar et al. [2] evaluated Yb-fiber laser-welded austenitic stainless steel’s thermo-mechanical performance (SS304). Ghosh et al. [3] explore the impacts of welding factors on the final product’s UTS and percentage elongation (PE). An investigation on the impacts on as-welded AISI 430 FSS welds by Mallaiah et al. [4] found that copper, titanium, and aluminum had a significant impact. Shanmugam et al. [5] studied fatigue fracture development in AISI 409 M joints. Friction stir welding on 409 FSS generated high-quality, defect-free welds studied by Cho et al. [6]. Microstructures of GMAW welds in 444 FSS are influenced by filler wire composition, according to research by Villaret et al. [7]. Ambade et al. [8] conducted a study on welding FSS 409 M using three different welding methods: shielded metal arc welding (SMAW), gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW) at current levels of 90 A, 100 A and 110 A. They used 4-mm-thick plates to weld a V-joint and performed single pass welding. The study found that the base metal, heat affected zone (HAZ) and weld joint all had transverse microstructures. Ambade et al. [9] employed SMAW to evaluate AISI 409M mechanical, intergranular corrosion and microstructure properties. Ahmed et al. [10] studied FSWeld FSS AISI 409. The aim of the research was to investigate the microstructure and mechanical properties of the FSS. The researchers used optical microscopes, scanning electron microscopes (SEM).

2.1 Radiographic testing for defect detection

Radiographic inspection is a critical non-destructive testing (NDT) technique used for detecting weld defects. It involves exposing the weld to X-rays or gamma rays and capturing the resulting image on radiographic film. Weld defects appear as variations in film density, allowing their detection and characterization.

2.2 Microstructure of welded joints and HAZ

The microstructure of welded joints, particularly the HAZ, significantly affects the mechanical properties and performance of welds. Changes in grain structure, hardness, and other microstructural features can occur due to the welding process.

2.3 Previous studies on weld defects and microstructure

Past research has extensively explored the detection and analysis of weld defects using radiographic inspection. Moreover, studies have examined the microstructural changes within welded joints and the HAZ. However, comprehensive studies that integrate both defect analysis and microstructural examination in the context of GMAW and GTAW processes on mild steel are limited. This project aims to bridge this research gap by offering a comprehensive analysis of defects and microstructural changes in these specific welding scenarios.

3. Material and methods

The materials employed in this study comprise:

• Mild steel specimens

• GTAW welding equipment

• GMAW welding equipment

• Radiographic inspection equipment

• Metallographic analysis equipment

• Radiographic film

Base metal composition is shown in Table 1 [11].

SAE/AISI 1017 composition
Element	Content (%)
Iron, Fe	99.11–99.56
Manganese, Mn	0.30–0.60
Carbon, C	0.14–0.20
Sulfur, S	≤0.050
Phosphorous, P	≤0.040
Silicon, Si	≤0.30
Zinc, Zn	≤0.25

Table 1.

Base metal composition [11].

3.1 Welding procedures

Elaborate welding procedures were meticulously adhered to for both GTAW and GMAW processes to create weld specimens. Critical parameters such as current, voltage, welding speed, and shielding gas composition were rigorously controlled to ensure consistency. The welding parameters for GMAW and GTAW is shown in Table 2.

Parameter	GMAW (MIG)	GTAW (TIG)
Method	Conventional	Conventional
Parent metal	Mild steel	Mild steel
Parent metal grade	SAE/AISI 1017	SAE/AISI 1017
Job dimension (mm)	120 × 60 × 2 (L × B × T)	100 × 50 × 2 (L × B × T)
Weld length (mm)	60	100
Type of joint	Butt	Butt
Root gap (mm)	1.5	1.5
Bevel angle (°)	60°	60°
Consumable electrode ⌀ (mm)	0.8 (copper coated)	Not applicable
Non-consumable electrode ⌀ (mm)	Not applicable	2.4
Non-consumable electrode grade	Not applicable	EN26848
Filler metal grade	ER70S-6	ER70S-6
Filler metal feed rate (m/min)	2–10	0.3–20
Welding speed (mm/min)	350	300
Welding voltage range (V)	18–31	10–35
Welding current (A)	90–220 (DC/RP)	90–120 (DC/SP)
Shielding gas	100% CO2	100% Argon
Gas flow rate (liter/min)	6–12	5–6

Table 2.

Weld parameters.

3.2 Radiographic inspection

Radiographic inspection, as a non-destructive testing technique, was carried out on both the weld specimens. The specimens underwent exposure to X-rays, and the resulting radiographic images were analyzed to detect and characterize weld defects.

3.3 Metallographic analysis

Metallographic analysis was performed to examine the microstructure of welded joints and the HAZ. Specimens were prepared, polished, etched, and scrutinized under a microscope to assess changes in grain structure, hardness, and other microstructural features.

3.4 Data collection

Radiographic images, along with comprehensive documentation of welding process parameters and metallographic analysis results, were recorded for each specimen. The types, locations, and severity of defects, as well as microstructural changes, were meticulously documented for subsequent analysis. Radiographic test report for the welded samples are shown in Table 3.

Radiography test report	GMAW (MIG)	GTAW (TIG)
Job description	Weld test	Weld test
Material	Mild steel	Mild steel
Thickness	2 mm	2 mm
Method	Conventional	Conventional
Equipment	XXQ-2005	XXQ-2005
Voltage	150 kV	150 kV
Current	5 mA	5 mA
Film	D7	D7
Density	2.2	2.2
Sensitivity	2%	2%
FFD	28″	28″
Screen	0.15 mm	0.15 mm
Technique	SWSI	SWSI
IQI	ASTM-1A	ASTM-1A
Exposure	1 minute	1 minute
Processing temperature	20°C	20°C
Film	D7	D7
Density	2.2	2.2
Segment	A–B	A–B
Film Size	3″ × 5″	3″ × 5″
Observation	Incomplete penetration	Slag and porosity

Table 3.

Radiography test report.

4. Welding defects

Any fault that reduces a weldment’s functionality is called a weld defect. Weld lifespan and performance can be significantly impacted by welding flaws. The following is a breakdown of welding fault causes, per ASME: 32% operator mistake, 12% incorrect method, 8% incorrect consumables, 3% bad weld grooves, and 45% poor process conditions. The following list includes common welding flaws:

• Overlap: this is a result of bad welding processes and is usually avoidable with a better weld process. By surface grinding to the base metal and removing extra weld metal, the overlap can be fixed.

• Undercut: an empty groove located along the weld’s edge. Incorrect electrode angle, high current, and travel speed are typically linked to the reasons. By paying close attention to edge preparation and streamlining the welding process, undercutting can be prevented.

• Cracking: when weld joints are exposed to fatigue stress conditions, cracks and planar discontinuities become some of the most severe flaws. Longitudinal cracks are typically produced by an issue with the hardness of the weld metal and run parallel to the direction of the weld. Cold cracking happens after the metal is fully cemented, which happens after welding [12, 13].

• Lamellar tearing: at locations where there is a large concentration of stress, this kind of defect is most likely to appear beneath a welded connection. It is produced during the production process when non-metallic inclusions are rolled into the hot plate metal. This fault is minimized by special joint design.

• Porosity: during solidification, molten metal can trap gas and non-metallic materials, creating cavities or pores that are known as porosity. There are numerous reasons for this, including as pollution, insufficient shielding, an excessively small arc gap, and subpar welding technique. By using the right electrode, filler material, and slower speed to provide gases enough time to escape, porosity can be reduced [12, 13].

• Misalignment: this kind of geometric flaw is typically brought about by combining plates of varying thicknesses or by set up or fit up issues.

• Lack of fusion: when the weld metal fails to extend into the base metal to the necessary depth or fails to make a cohesive bond with it, incomplete fusion takes place. Low current, inadequate preheating, excessive welding speed, improper edge preparation, and an arc that is not in the center of the seam are the causes of this kind of flaw.

• Spatter: metal droplets released from the weld adhere to nearby surfaces and cause scattering. If there is splatter, it should be removed by grinding and can be reduced by repairing the welding condition.

• Inclusions: extraneous elements including slag, tungsten, and sulfide and oxide inclusions produce inclusions. Slag incorporation decreases the joint’s strength by reducing its cross sectional area [14].

5. Results and discussion

A comprehensive analysis of various weld defects was conducted.

5.1 Excessive deposition

Found in GMAW joint. Excessive deposition of weldmetal (Figure 1) was analyzed, and its effects on weld properties and distortion were discussed.

Figure 1.

GMAW joint (top) showing excessive deposition.

5.2 Incomplete penetration/fusion

Found in GMAW joint. The extent of incomplete penetration/fusion (Figure 2) was determined and documented, along with potential contributing factors.

Figure 2.

GMAW joint (bottom) showing incomplete penetration and fusion.

5.3 Cracks

Found in GMAW joint. Cracks in welds were scrutinized, and their types and locations were documented. The causes of crack formation were explored.

5.4 Slag

A trace amount of slag was found in GTAW. The presence and distribution of slag inclusions were assessed, and their impact on weld quality was discussed. GTAW JOINT for top and bottom side are shown in Figures 3 and 4 respectively.

Figure 3.

GTAW joint (top).

Figure 4.

GTAW joint (bottom).

5.5 Porosity

A trace amount of porosity was found in GTAW. The density and distribution of porosity were examined, and their significance in relation to weld integrity was explored.

5.6 Radiographic inspection results

Radiographic inspection results for each specimen were reviewed and analyzed. Radiograph of MIG weld shows The lack of penetration/fusion. Lack of penetration/fusion could have arisen because of either any of the causes like too much welding speed, incorrect welding angle, surface contamination, and poor heat input. On the other hand, radiograph of TIG weld shows the slag and porosity discovered during the Radiography of weldment. Slag and porosity could have arisen because of either any of the causes like presence of contaminants/impurities on the job surface or on electrode flux or presence of high sulfur in the job or electrode materials or trapped moisture between joining surfaces, fast freezing of weld metal or improper cleaning of the edges.

5.7 Incomplete penetration/fusion

Found in GMAW joint. The extent of incomplete penetration/fusion (Figure 5) was determined and documented, along with potential contributing factors.

Figure 5.

Radiograph of GMAW joint showing incomplete penetration and fusion.

5.8 Slag

A trace amount of slag was found in GTAW. The presence and distribution of slag inclusions were assessed, and their impact on weld quality was discussed.

5.9 Porosity

A trace amount of porosity (Figure 6) was found in GTAW. The density and distribution of porosity were examined, and their significance in relation to weld integrity was explored.

Figure 6.

Radiograph of GTAW joint showing slag and porosity.

6. Comparative analysis of defects and microstructure

A comparative analysis was performed to evaluate the relationship between weld defects and microstructural changes in the context of GTAW and GMAW processes are shown in Figures 7 and 8.

Figure 7.

Microstructure of GMAW specimen.

Figure 8.

Microstructure of GTAW specimen.

Weld and HAZ found to be of not a quite fine grain structure. Strength of joints of both the specimens under study might have decreased due to various defects developed during the processes.

7. Factors influencing defect formation and microstructural changes

The study investigated factors that influence the formation of weld defects and microstructural alterations. This included welding processes and their parameters, material properties, and operator proficiency.

8. Conclusions

The primary objective was to study the various defects found during welding of mild steel by GMAW and GTAW processes while keeping all the process parameters constant in both the processes. This comprehensive study provided insights into various weld defects detected through radiographic testing. Additionally, the study analyzed the microstructure of welded joints. The findings shed light on the causes and consequences of weld defects and microstructural changes. The study finally concluded that GTAW is preferable for welding of mild steel plates having lesser thickness (≯2 mm) in terms of fewer defects, better weld quality, clean and precise appearance over the GMAW process.

Acknowledgments

This study’s authors like to thank all participants in the Blue Earth workshop at Jadavpur University Kolkata has been extremely helpful.

Competing interests

There are no potential conflicts of interest for the authors.

Data and materials are readily available

In this article, all data created or analyzed during this investigation are available to the author on reasonable request.

Participant agreement and ethical clearance

This does not apply.

Obtaining permission to publish

Not the case here.