Green Welding: eco-friendly, cost-effective

April 12, 2014 5:48 am

The Centre for Materials Joining and Research of Annamalai University has indigenously developed an eco-friendly and cost-efficient welding technology that offers benefits in terms of welding of difficult to weld alloys, better retention of baseline material properties, fewer weld defects, low residual stresses, and better dimensional stability of the welded structure
Eco-friendly welding (EFW) was invented at The Welding Institute (TWI), UK, which is popularly known as Friction Stir Welding (FSW) process. Compared to many of the fusion welding processes that are routinely used for joining structural alloys, FSW is an emerging solid state joining process in which the material that is being welded does not melt and recast. Due to the absence of parent metal melting, the new FSW process is observed to offer several advantages over fusion welding.  The benefits that stand out most are welding of difficult to weld alloys, better retention of baseline material properties, fewer weld defects, low residual stresses, and better dimensional stability of the welded structure.  Above all, EFW is an environmentally cleaner process, due to the absence of a need for the various gases that normally accompany fusion welding.
PrincipleEFW is a continuous, hot shear, autogenous process involving non-consumable rotating tool of harder material than the substrate material. Fig 1 explains the working principle of EFW process. When alloys are friction stir welded, phase transformations that occur during the cool down of the weld are of a solid-state type.  Due to the absence of parent metal melting, the new EFW process is observed to offer several advantages over fusion welding. The benefits that stand out most are welding of difficult to weld, better retention of baseline material properties, fewer weld defects, low residual stresses, and better dimensional stability of the welded structure.    EFW joints usually consist of four different regions
• Unaffected base metal• Heat affected zone (HAZ) • Thermo-mechanically affected zone (TMAZ)• Friction stir processed (FSP) zone or dynamically recrystallised zone (DRX).
The formation of above regions is affected by the material flow behaviour under the action of rotating non-consumable tool. However, the material flow behaviour is predominantly influenced by the FSW tool profiles and FSW process parameters.
ApplicationsEFW gives rise to softening in the joints of the AA7075, a heat-treatable aluminium alloy, because the dissolution or growth of strengthening precipitates during the welding thermal cycle, thus resulting in the degradation of the mechanical properties of the joints. EFW creates the weld joint without bulk melting.  In addition, the extensive thermo mechanical deformation induces dynamic recrystallisation and recovery that refine the microstructure of the stir region. Therefore, welds made by EFW are shown to have much improved mechanical properties than the corresponding fusion welds. EFW is capable of joining magnesium alloys without melting and thus it can eliminate problems related to the solidification. As EFW does not require any filler material, the metallurgical problems associated with it can also be reduced and good quality weld can be obtained. Higher thermal conductivity and thermal expansion of copper result in greater weld distortion than in comparable steel welds. On the other hand, EFW would alleviate most of the problems caused by the fusion welding processes because it does not result in the melting and resolidification of the material to be welded. EFW efforts to date have involved joining of aluminium alloys there is considerable interest in extending the technology to other materials, including steels. EFW appears to offer several advantages over arc welding of steels. The lower apparent energy inputs of EFW are expected to minimise grain growth in the HAZ, limit distortion and residual stress in steels, eliminate welding fumes and hydrogen induced cracking etc.
Environmental benefitsWelding is an important occupational activity, in part because from 0.2 to 2.0 per cent of the working population in industrialised countries has been reported to be engaged in welding. Although there are number of processes, it has been estimated that shielded metal arc welding (SMAW) and gas metal arc welding (GMAW) applied to carbon steels, stainless steels and aluminium alloys account for combinations practiced by 70% of welders. The welding environment is very complex, with the presence of numerous gaseous and particulate components. The extremely high temperatures (>4,000 C) of arc welding processes heat both the base metal pieces to be joined and a consumable electrode fed into the weld.

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