Plasma-MIG Hybrid Welding of 7075 Aluminum Alloy Hot Cracking Sensitivity

Literature Overview

This study by Shao Yingkai and colleagues at Dalian Jiaotong University, published in Acta Metallurgica Sinica (2018), investigates the hot cracking sensitivity of 7075 aluminum alloy when welded using plasma-MIG hybrid welding with optimized weld penetration depth. The research was funded by the Liaoning Provincial Department of Education General Project (No. JDL2016009). The work addresses one of the most persistent challenges in aluminum alloy welding: hot cracking in high-strength 7xxx series alloys, which are widely used in aerospace and high-performance structural applications.

Core Technical Content and Process Parameters

The plasma-MIG hybrid welding process combines the deep penetration of a plasma arc with the heat input and shielding of a MIG arc. For 7075 aluminum alloy, the following process parameters were investigated:

The optimization of weld penetration depth is central to this research. Deeper penetration leads to a longer solidification time in the final stages of weld solidification, which can increase the window for hot crack formation. However, shallower penetration may result in insufficient fusion and reduced joint strength. The optimal penetration depth represents a balance between these competing requirements.

Hot Cracking Mechanism in 7075 Aluminum Alloy

Hot cracking in 7075 aluminum alloy is primarily due to the wide solidification range of the weld metal, which contains significant amounts of zinc and magnesium. As the weld solidifies, a mushy zone forms where liquid films between solid dendrites become susceptible to cracking under thermal stresses. The zinc content in 7075 alloy (5.1–6.1 wt%) is particularly problematic, as it lowers the melting point of the liquid films and increases the susceptibility to hot cracking.

The plasma-MIG hybrid process affects hot cracking through several mechanisms:

1. The plasma arc provides a concentrated heat source that creates a steeper thermal gradient, promoting columnar grain growth and potentially reducing hot crack susceptibility by directing liquid flow away from crack initiation sites.
2. The MIG arc provides additional heat at the weld pool surface, which can either increase or decrease hot cracking depending on the specific parameter combination.
3. The synergistic interaction between the two heat sources can modify the weld pool geometry, affecting the solidification pattern and stress distribution.

Weld Penetration Optimization and Cracking Sensitivity

The research demonstrates that penetration depth has a non-linear relationship with hot cracking sensitivity. At shallow penetration depths, the weld pool is narrow and the solidification rate is high, which can actually reduce hot cracking by limiting the time available for crack propagation. At very deep penetration depths, the weld pool is larger and the solidification time is longer, increasing the window for hot crack formation.

The optimal penetration depth for 7075 aluminum alloy plasma-MIG hybrid welding was found to be in the range of 2-3 mm for typical plate thicknesses of 3-6 mm. At this penetration depth, the hot cracking sensitivity is minimized while maintaining adequate fusion and joint strength.

Filler Metal Selection and Cracking Resistance

The selection of filler metal is critical in controlling hot cracking in 7075 aluminum alloy. The research compared several filler metal options:

The ER4043 filler metal, with its high silicon content, was found to be most effective in reducing hot cracking. The silicon reduces the solidification range of the weld metal and promotes a more uniform solidification pattern, which reduces the tendency for liquid films to form between dendrites.

Engineering Practice and Standards Compliance

For engineers working in bimetal pressure vessel fabrication, the hot cracking sensitivity of 7075 aluminum alloy has direct implications for design and fabrication qualification. Under ASME VIII Div.1 and Div.2, as well as NB/T 47002, weld procedures must be qualified to demonstrate freedom from cracking and other critical defects. The plasma-MIG hybrid welding process, with its ability to optimize penetration depth and reduce hot cracking, offers a viable welding method for 7075 aluminum alloy applications where high strength and corrosion resistance are required.

In cladding applications, the hot cracking resistance of the weld overlay is equally important. When applying corrosion-resistant cladding layers to aluminum alloy substrates, the same principles of penetration depth optimization and filler metal selection apply. The plasma-MIG hybrid process's ability to produce narrow, deep welds with minimal dilution is advantageous for cladding applications where maintaining the base metal's properties is critical.

Defect Analysis and Countermeasures

Study Insights and Implications

The research provides valuable insights into the complex relationship between weld penetration depth and hot cracking sensitivity in 7075 aluminum alloy. The finding that an optimal penetration depth exists, rather than a simple monotonic relationship, has important implications for process optimization. Engineers must consider not only the mechanical properties of the joint but also the cracking resistance when selecting welding parameters.

For cladding and bimetal manufacturing, this work highlights the importance of understanding the fundamental metallurgical mechanisms that govern weld quality. The same principles of penetration depth optimization and filler metal selection that apply to structural welding of 7075 aluminum alloy are directly relevant to overlay cladding applications. The plasma-MIG hybrid process offers a flexible and controllable welding method that can be adapted to a wide range of cladding and bimetal applications.