Invar Alloy Laser-MIG Hybrid Multi-Layer Welding Weld Profile and Microstructure Analysis

Literature Overview

This paper by Chen Jie, Zhan Xiaohong, Chen Jicheng, Wang Yubo, Yan Dongxiu, Wang Yuhua, and Liu Hongbing, published in the Chinese Journal of Nonferrous Metals (Vol. 26, No. 5, 2016, pp. 1010–1018), presents a comprehensive investigation of laser-MIG hybrid welding applied to Invar alloy (Fe-36Ni low expansion alloy) plates with a thickness of 19.05 mm. The study employed Invar M93 filler wire and systematically examined how welding parameters influence weld profile characteristics including penetration depth, weld width, and depth-to-width ratio, as well as the microstructural evolution across different weld zones from root pass to cap pass. The research was supported by the Shanghai Economic Commission Project (15XI-1-15) and represents collaborative work between Nanjing University of Aeronautics and Astronautics and the Shanghai Aircraft Manufacturing Company of COMAC.

Weld Profile Evolution Across Layers

The most striking finding in this study is the progressive transformation of weld profile morphology from the root pass to the cap pass. The average depth-to-width ratio decreases from 2.6 at the root layer to 0.86 at the cap layer, indicating a transition from deep, narrow penetration to shallow, wide deposition. This trend has significant implications for residual stress distribution, distortion control, and mechanical property uniformity in multi-layer welds.

The depth-to-width ratio of 2.6 at the root layer indicates that the laser component dominates the heat input distribution, creating a narrow, deep molten pool characteristic of laser welding. As subsequent layers are deposited, the thermal accumulation from previous layers preheats the base metal, effectively reducing the relative contribution of the laser energy to the overall thermal input. The MIG arc, which has a broader heat distribution, becomes increasingly dominant in determining the molten pool geometry, resulting in the progressive widening and shallowing of the weld profile.

The transition from nail-head to high-cup weld profile is consistent with the well-established principle that in multi-layer welding, the thermal history of the previous layers fundamentally alters the solidification conditions of subsequent layers. The HAZ width increase from root to cap is directly correlated with the cumulative thermal input, as each successive layer subjects the previously solidified weld metal and base material to additional thermal cycles.

Microstructural Characteristics and Formation Mechanisms

The weld metal microstructure exhibits a distinctive columnar dendrite morphology at the weld center, with elongated austenitic dendrites oriented longitudinally along the weld axis. Flanking this central columnar zone are cellular dendrites of austenite that grow at oblique angles, creating a characteristic V-shaped or chevron pattern typical of weld solidification. The presence of austenite as the primary phase is consistent with the Fe-36Ni composition of Invar alloy, which has a wide austenite stability range due to the high nickel content.

From root to cap, the grain size progressively increases in all regions of the weld metal. This coarsening is attributed to two mechanisms: first, the increasing thermal input per layer reduces the cooling rate, allowing more time for grain growth during solidification and post-solidification; second, the preheating effect of previous layers reduces the undercooling at the solidification front, which decreases the nucleation rate and favors fewer, larger grains.

The phenomenon of coupled crystallization (联生结晶) observed at the boundaries between adjacent weld passes is particularly noteworthy. This occurs when the solidification front of a new pass encounters the partially melted or reheated boundary of the previous pass, leading to a cooperative growth of dendrites from both the previous and current pass. This coupled crystallization can create preferential paths for crack propagation and intergranular corrosion, representing a potential quality concern in production welding.

Engineering Practice Considerations

For engineers working with Invar alloy weldments, particularly in aerospace applications where dimensional stability under thermal cycling is critical, the progressive grain coarsening from root to cap has direct implications for thermal expansion uniformity. Invar alloy is prized for its near-zero coefficient of thermal expansion (approximately 1.2 × 10⁻⁶/°C at 20°C), and any microstructural non-uniformity can lead to localized differences in thermal expansion behavior. The coarser grains in the cap layers may exhibit slightly different thermal expansion characteristics compared to the finer-grained root layers, potentially introducing internal stresses during temperature cycling.

The coupled crystallization phenomenon at pass boundaries warrants careful attention in production welding procedures. Engineers should consider implementing welding sequences that minimize the number of pass boundaries in critical load-bearing regions, and may need to incorporate post-weld heat treatment to homogenize the microstructure. The progressive HAZ widening from root to cap also suggests that distortion control strategies should account for the asymmetric thermal input distribution, potentially requiring asymmetric back-gassing, backing strip selection, or拘束 (constraint) strategies that differ between the first and last layers.

Key Questions and Reflections

The study raises an important question about the optimal number of layers for thick-section Invar alloy welding. While the root layer benefits from the deep penetration of the laser, the cap layer suffers from excessive grain coarsening and a wide, shallow profile that may have inferior mechanical properties. An engineer might consider whether a different layering strategy—such as using a higher laser power for the cap layer to restore deeper penetration and finer grain structure—could improve the overall weld quality.

The depth-to-width ratio variation from 2.6 to 0.86 represents a factor of three change in weld geometry, which is substantial. In production settings, this variation must be accounted for in weld design calculations, particularly for fatigue-critical applications where weld profile geometry directly influences stress concentration factors. The transition from nail-head to high-cup profile may actually be beneficial in terms of fatigue performance, as the high-cup profile with its wider, shallower geometry typically exhibits lower stress concentrations at the weld toe.

This research demonstrates that laser-MIG hybrid welding is a viable and controllable process for thick-section Invar alloy fabrication, but the inherent microstructural non-uniformity across layers requires careful process design and potentially post-weld treatment to meet the stringent requirements of aerospace-grade Invar components.