Tensile Fracture in Laser Welding Joints of Al-Cu-Li-Mg-X Alloy Bearing Gaseous Porosity

Tensile Fracture in Laser Welding Joints of Al-Cu-Li-Mg-X Alloy Bearing Gaseous Porosity

Naveed Akhtar (School of Material Science and Engineering, Beihang University (BUAA), Beijing, China) and SuJun Wu (School of Material Science and Engineering, Beihang University (BUAA), Beijing, China)
DOI: 10.4018/IJMMME.2015100104
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This article measures the impact of gaseous porosity on the mechanical performance of the laser welded thin sheet of an aerospace-grade Al-Cu-Li-Mg-X material. The laser fabricated joints were characterized by optical microscopy and scanning electron microscopy to reveal the metallurgical features. A uniaxial tensile test was carried out to measure the mechanical strength of the porosity free welds, and porosity bearing welds. The results showed that the welds bearing gaseous porosity performed nearly 24% lower mechanically than the porosity free welds. Fractography exposed the presence of fine spherical holes in porosity bearing welds, which acted as crack nucleation sites, and their further growth lead to a sudden failure at higher strain rates. Microstructural observations revealed that the laser processing has transformed the superior age-hardened microstructure of the base metal into a new as-cast microstructure. The freshly evolved microstructure was held responsible for the low mechanical properties of the joints with respect to the base metal.
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In the present century the laser beam welding (LBW) has been grown rapidly because it offers certain benefits over the conventional welding techniques (Freeman, 2012; Quintino et al., 2012). The most widely used aerospace-grade aluminium alloys of 2xxx, 7xxx and Al-Li variants were remained difficult to weld by old welding methods such as the gas tungsten arc welding (GTAW) and the gas metal arc welding (GMAW). However, with the evolution of the laser beam welding technique, these aerospace-grade aluminium alloys could easily be welded (Cao, Wallace, Poon, & Immarigeon, 2003; Ion, 2000; Mendez & Eagar, 2001) and the resulted joints also preserve the higher mechanical strength of the base metal. Thus, the extra weight of fastenings, riveting or big bolted joints could be avoided in the fabrication of primary structural parts (Soetens & van Hove, 2003; W. Akhtar, N. Akhtar, & S. J. Wu, 2014). Moreover, with the evolution of industrial lasers the use of aluminium material has been extended to the automobiles, railway cars, ship building industry, and other high strength and weight critical applications. Other advantages associated with laser beam welding process include are higher production rates and high accuracy and reliability. However, there may be some difficulties during the laser beam welding of the aluminium alloys mostly due to their inherent properties such as high reflectivity, high thermal conductivity and low boiling constituents etc (Lathabai, 2011). Apart from these hurdles laser beam welding is making its utility in high strength and weight cognizant applications.

During the laser welding process radiant energy is used to produce the heat, which melt the materials to be joined together. The laser produces a concentrated beam of monochromatic light that is focused like a small spot on the workpieces. The wavelength and the power density of the laser beam is selected such that it would produce the heat required to melt the material (Cao, Wallace, Poon, et al., 2003), which on solidification produces a homogeneous joint. As compared to the other welding techniques a very narrow fusion zone (weld zone) is formed in the laser beam welding, that give a smart look to the resultant joint. Laser beam welding could be done with or without filler material, and in various types of environments such as air, vacuum or under shielding gases etc.

Typically, the case of aluminium alloys, the mechanical properties of the fusion welding joints are lower than that of the base metal (i.e. GTAW, GMAW). The reason behind is the loss of strengthening precipitates and disturbance of superior work-hardened microstructure of these alloys (Cui, Li, He, Chen, & Gong, 2012). Actually, the heat produced during the fusion welding quickly moves away from the weld pool, and get absorbed into the contiguous metal and therefore, deteriorates the base metal properties. On the other hand, high-strength joints could be fabricated by the laser welding technique as it produces a narrow heat affected zone (HAZ), and fine microstructure inside the weld zone. There are certain factors that control the hardness and the mechanical performance of the fabricated joints such as pre-treatment of the joining surfaces, welding parameters, laser related parameters, filler material, and post-weld heat treatment practices. A summary of the advantages, disadvantage, processing parameters and common defects associated with the laser beam welding process have been put together in the Figure 1. According to the Fig.1 laser related parameters that can severely affect the weld quality are the laser type, wavelength, power density, focal length, spot diameter, beam alignment etc. Similarly, process parameters such as welding speed, cooling rate, shielding gases and filler metal has obvious effects on the formation of fusion zone microstructure. Further, chemical composition, thickness and surface condition of the material, and the joint type will also determine the final weld quality. There are certain defects that are particularly associated with the laser welding of aluminium and its alloys e.g. lack of penetration, weld cracking, hydrogen gas porosity, and loss of low boiling elements like Mg, Zn and Li (Xiao & Zhang, 2014). Moreover, some defects such as irregular roots, blow roots and cavities are caused by practicing bad procedures (Cao, Wallace, Immarigeon, & Poon, 2003).

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