Investigation of Hot Cracking Behavior in Transverse Mechanically Arc Oscillated Autogenous AA2014 T6 TIG Welds

Abstract

Hot cracking studies on autogenous AA2014 T6 TIG welds were carried out. Significant cracking was observed during linear and circular welding test (CWT) on 4-mm-thick plates. Weld metal grain structure and amount of liquid distribution during the terminal stages of solidification were the key cause for hot cracking in aluminum welds. Square-wave AC TIG welding with transverse mechanical arc oscillation (TMAO) was employed to study the cracking behavior during linear and CWT. TMAO welds with amplitude = 0.9 mm and frequency = 0.5 Hz showed significant reduction in cracking tendency. The increase in cracking resistance in the arc-oscillated weld was attributed to grain refinement and improved weld bead morphology, which improved the weld metal ductility and uniformity, respectively, of residual tensile stresses that developed during welding. The obtained results were comparable to those of reported favorable results of electromagnetic arc oscillation.

Appendices

Appendix: Calculation of effective welding speed (w)

$$ \begin{aligned} w & = \left( {u^{2} + v^{2} } \right)^{1/2} \\ &= \left( {3.6^{2} + 1.8^{2} } \right)^{1/2} \\ & = 4.02\,{\text{mm}}/{\text{s }}\left( {\text{linear welding}} \right) \\ & = \left( {5.3^{2} + 1.8^{2} } \right)^{1/2} \\ & = 5.6{\text{ mm}}/s \, \left( {\text{CWT welding}} \right) \\ \end{aligned} $$

where u = linear welding speed (3.6 mm/s for linear welding and 5.3 mm/s for CWT welding) and v = transverse welding speed of the torch in mm/s. Effective welding speed for the TMAO parameter (amplitude = 0.9 mm and frequency = 0.5 Hz):

$$ \begin{aligned} v & = \left( {0.9 \times 4 \times 0.5} \right){\text{ for one cycle of oscillation}} \\& = 1.8{\text{ mm}}/{\text{s}} \\ \end{aligned} $$

Heat input calculations (H _Net) used for tungsten inert gas welding with and without TMAO (amplitude = 0.9 mm and frequency = 0.5 Hz)

For linear TIG welding:

$$ \begin{aligned} H_{{{\text{Net }}\left( {\text{without\,TMAO}} \right)}} &= V \times I \times \eta /u \\ & = 165 \times 13.4 \times 0.40/3.6 \\ & = 245.6 \, {\text{J}}/{\text{mm}} \\ \end{aligned} $$

$$ \begin{aligned} H_{{{\text{Net }}\left( {\text{with\,TMAO}} \right)}} & = V \times I \times \eta /w \\ & = 165 \times 13.4 \times 0.40/4.02 \\ & = 220{\text{ J}}/{\text{mm}} \\ \end{aligned} $$

For CWT TIG welding:

$$ \begin{aligned} H_{{{\text{Net }}\left( {\text{without\,TMAO}} \right)}} & = V \times I \times \eta /u \\ & = 175 \times 13.4 \times 0.40/5.3 \\ & = 176.9 \, {\text{J}}/{\text{mm}} \\ \end{aligned} $$

$$ \begin{aligned} H_{{{\text{Net }}\left( {\text{with\,TMAO}} \right)}} & = V \times I \times \eta /w \\ & = 175 \times 13.4 \times 0.40/5.6 \\ & = 167.5{\text{ J}}/{\text{mm}} \\ \end{aligned} $$

where V = voltage in volts, I = current in Amps, η = arc efficiency (assumed as 0.40 for TIG),[32] u = welding speed in mm/s (without arc oscillation), and w = effective welding speed due to TMAO in mm/s.

Cooling rate calculations, R, for TIG welds without and with TMAO (amplitude = 0.9 mm and frequency = 0.5 Hz)

For Linear TIG welding:

R = 2πkρC (τ/H _Net)² (T _c – T ₀)³ for thin plates at full penetration were produced.[45]

$$ \begin{aligned} & R_{{\left( {\text{without\,TMAO}} \right)}} \\ &\quad = 2\pi \times 0.23 \times 0.0027\left( {4/245.6} \right)^{2} \times \left( {911.15 - 303.15} \right)^{3} \\ & \quad = 505.75{\text{ K}}\left( {232.6^\circ {\text{C}}/{\text{s}}} \right) \\ \end{aligned} $$

$$ \begin{aligned} & R_{{\left( {\text{with\,TMAO}} \right)}} \\&\quad = 2\pi \times 0.23 \times 0.0027\left( {4/220} \right)^{2} \times \left( {911.15 - 303.15} \right)^{3} \\& \quad = 563.05{\text{ K}}\left( {289.9^\circ {\text{C}}/{\text{s}}} \right) \\ \end{aligned} $$

For CWT TIG welds:

$$ \begin{aligned} & R_{{\left( {\text{without\,TMAO}} \right)}} \\ & = 2\pi \times 0.23 \times 0.0027\left( {4/176.9} \right)^{2} \times \left( {911.15 - 303.15} \right)^{3} \\ & = 721.45{\text{ K}}\left( {448.3^\circ {\text{C}}/{\text{s}}} \right) \\ \end{aligned} $$

$$ \begin{aligned} & R_{{\left( {\text{with\,TMAO}} \right)}} \\ & \quad = 2\pi \times 0.23 \times 0.0027\left( {4/167.69} \right)^{2} \times \left( {911.15 - 303.15} \right)^{3} \\ & \quad = 771.35{\text{ K}}\left( {498.2^\circ {\text{C}}/{\text{s}}} \right) \\ \end{aligned} $$

where k = thermal conductivity (J/mm² K), ρC = thermal capacity (J/mm³ K), τ = thickness of the plate (mm), H _Net = effective heat input (J/mm), T _c  = liquidus temperature (K), and T ₀ = initial temperature (K).