PSI - Issue 33

Rizki Dwi Ardika et al. / Procedia Structural Integrity 33 (2021) 171–180 Author name / Structural Integrity Procedia 00 (2019) 000–000

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2.3. Cold metal transfer CMT is a modification of the MIG welding process has a low heat input process and uses the mechanical droplet cut method (Furukawa, 2006). The CMT welding process parameters consist of temperature, surface tension, and filler viscosity (Sravanthi et al., 2019). CMT welding works by joining material with a high deposition drop transfer method and low heat input, resulting in a very low spark (Kannan et al., 2019). Low heat input can increase the joint's efficiency by reducing the intermetallic layer's thickness (Rajeev et al., 2019). CMT welding's advantage is that it can increase welding efficiency, flexibility, welding speed, and tensile strength (Guojin et al., 2018).

3. Porosity defect form 3.1. Microporosity

Microporosity is formed due to two mechanisms, shrinkage porosity caused by shrinkage during the metal solidification process and gas porosity caused by air trapping and undissolved hydrogen (Z. Li et al., 2019). Microporosity can cause defects in weld joints. The pore formation at microporosity is affected by H2 gas and shrinkage (Gu et al., 2021). Microporosity sizes are generally below 5 mm (Toda et al., 2011). The presence of porosity can affect microstructure growth, while microporosity does not affect microstructural compaction (Pham et al., 2020). Microporosity can cause reduced tensile strength and fatigue. 3.2. Macroporisity Macroporosity has a larger size above 5 mm. Macroporosity can cause welding defects that result in decreased strength in the weld joint. Macroporosity forms the insufficient feed of molten metal to compensate for the volumetric shrinkage associated with solid to liquid transformation (B. Zhang et al., 2005). The cause of macroporosity is the high solubility of hydrogen. Macroporosity can be minimized if the hydrogen content, air flow velocity, and room temperature are controlled (Çolak et al., 2020). The form of porosity can be shown in Fig. 3.

Fig. 3. Fractography porosity in welding (Liu et al ., 2015)

4. Porosity formation mechanism The porosity on aluminum is influenced by the presence of hydrogen and air from the welding process, filler/filler wire, and environmental influences, including temperature and air humidity (Huang et al., 2020). During the MIG hybrid laser welding process, H₂O reacts with the molten metal to produce hydrogen atoms. Hydrogen atoms are easily dissolved in the molten pool at high temperatures. As the welding process progresses, the hydrogen atoms can decrease as the temperature decreases. The solubility of hydrogen atoms in the molten pool can affect porosity formation (X.