Low carbon steel has a
Low carbon steel has a different hardness distribution compared to IF steel, as illustrated in Fig. 6. A sharp increase in hardness around the weld beam is observed. It is a fact that the carbon element increases the hardnesses of steels. Similar to hardening process, rapid heating and cooling around weld beam arise with an increase in hardness. In addition to the rate of hardening, the rapid heating and cooling cycles during the welding process result in gathering the carbon atoms around the heated zone and the hardness increase in that zone.
Ultrasonic and Rockwell hardness distributions in low carbon steel are illustrated in Fig. 7.
Introduction AA2219 is basically Al–Cu–Mn ternary alloy and has a unique combination of properties such as good weldability and high strength to weight ratio . The alloy is extensively used for fabrication of cryogenic tanks and pressure vessels due to high strength, superior resistance to cracking and corrosion resistance . The AA2219 aluminium alloy contains a major alloying addition of copper and minor additions of manganese, titanium, vanadium and zirconium. Generally, the alloy is produced in the T87 temper condition (solution treatment + 7% cold working + aging) . One of the drawbacks of most of the high strength Al alloys is that they suffer from poor weldability. However, AA2219 is an exception due to the presence of more Cu that helps in healing the cracks by providing extra eutectics. Lots of studies have been carried out in order to assess the effect of copper content and the distribution of second phase intermetallic particles on the properties of AA 2219 alloy . The preferred welding processes for AA 2219 aluminum alloy are frequently gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW) due to their comparatively easier applicability and better economy. The gas tungsten arc welding (GTAW) process for aluminium alloy AA2319 as filler metal has generally been used . Although the AA2219 alloy has better weldability compared to other grades of precipitation hardenable aluminium alloy, it has inferior weld joint strength than NVP-TNKS656 cost material . It is well known that the weld strength of the alloy is characterized by the weldment microstructure, which largely depends on the welding processes. Several researchers have investigated the weld strength of the alloy and have confirmed that it has low weld strength after welding [6–8]. However, it is reported that the electron beam welding (EBW) provides strong and sound welds for AA2219 with high weld efficiency [1,9]. But, the application of EBW is practically difficult for certain weld joints. The observation of the high weld efficiency of the EBW process indicates the possibility of improving weld property through an appropriate process design using gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW) processes and their comparisons have not been studied in detail. In addition, thick section by multi-pass arc welding procedure may generate high shrinkage stresses due to differential contraction under cooling thermal cycle of the welding process. Thus, it needs to study the various properties of the AA2219 weld joints under different welding processes. Hence, the present work describes the comparative studies on transverse shrinkage, mechanical and metallurgical properties of 25 mm-thick AA2219-T87 aluminium alloy weld joints prepared by GTAW and GMAW processes.
Results and discussion
Conclusions The following conclusions are drawn from the present investigation:
Introduction The gas metal arc welding (GMAW) process has been widely investigated and reported since 1950s . A number of variants of GMAW have been developed in an attempt to improve the performance and productivity of the process . In this regard, John Norrish et al. [2,3] reported the evolution of the GMAW process starting from standard operating modes, such as surface tension, globular and spray, of metal transfer behavior to waveform control technology up to the various hybrid techniques. From these literatures, it is well understood that the conventional GMAW process can be modified to enhance productivity and quality by manipulating the operating parameters, such as the electrical extension of wire/polarity, and the improved process control can be achieved by modifying the current waveform. Dual pulse is also introduced in the GMAW process to improve the energy transfer efficiency in comparison to the conventional and pulsed GMAW processes. Praveen et al.  reported that the dual pulse GMAW (DP-GMAW) process operates at low heat input. In the DP-GMAW process, the low frequency current pulsation is superimposed on high frequency pulsed current for better control of arc and metal transfer behavior as reported by Anhua Liu et al.  in the case of welding of AA5754 aluminum alloy. Celina Leal Mendes da Silva et al.  concluded that DP-GMAW technique maintains the capability of porosity minimization in aluminum weldment attributed to the pulsed current GMAW (P-GMAW) technique. The process characteristics of inverter type GMAW process under static and dynamic operating conditions were reported by Devakumaran et al. . It was concluded that the DP-GMAW process operates at low heat input compared to the conventional and pulsed GMAW processes, which is in agreement to the earlier work reported by Praveen et al. . From the above literature, it is understood that very little work has been carried out with respect to the process characteristics of DP-GMAW process, but the utilization of DP-GMAW process for various applications is not well known to the users because of the limited understanding of the mechanism of DP-GMAW and its influence on weld joint quality. Hence, it is felt that a systematic understanding of the DP-GMAW process in welding of various ferrous and nonferrous materials are very much important.