EFFECT OF MICROPLASMA ARC WELDING PROCESS PARAMETERS ON GRAIN GROWTH AND POROSITY DISTRIBUTION OF THIN SHEET Ti6AL4V ALLOY WELDMENT

ABSTRACT

In the present investigation, the effect of microplasma arc welding (MPAW) process parameters on grain growth and porosity distribution of thin sheet Ti6Al4V alloy weldment has been examined. The MPAW procedure was performed at different current, welding speed, and flow rates of shielding and plasma gas. Metallographic characterization was carried out by light microscopy and scanning electron microscopy (SEM). The results have shown that beyond a threshold energy, burn of the metal in Ti6Al4V thin sheet alloy can occur. This case should be considered for any selection of welding process parameters. Furthermore, in MPAW there are a number of threshold parameters that can control weldment size. Metallographic characterization has shown that coarse primary beta grains exist in the fusion zone. This is due to the fact that these grains have nucleated epitaxially upon coarsened beta grains in the near heat affected zone (NHAZ) and grown competitively into the molten weld pool. Evaluation of grain size indicates that there is a threshold energy input for phase transformation of α  →  β in the HAZ. In the case of lower energy input, grain growth of β phase in the HAZ could be restricted by α phase. The presence of small quantities of this phase at high peak temperatures in the weld cycle is sufficient to prevent beta grain growth of HAZ and fusion zone (FZ). The SEM and optical metallographic examinations also indicated that porosity occurs in FZ. This could be due to a mechanism of hydrogen rejection that is already present in the starting base metal. Pore nucleation can be homogenous or heterogenous. Calculations have shown that homogenous nucleation is not a feasible mechanism for hydrogen pore creation in this alloy.

Key Words:

• Burning
• Epitaxy
• Fusion zone
• Grain growth
• Heat affected zone
• Heat input
• Microplasma arc welding
• Porosity
• Ti6Al4V alloy
• Weldment size

ACKNOWLEDGMENT

Special thanks are extended to Dr. Amjadi, Mr. Tahvilian, and Mr. Sirati for their encouragement and support. We are thankful to one and all who were either directly or indirectly helpful in the successful completion of the program.