Thermal Equilibrium and Process Stability Analysis of Aluminum Alloy MIG Arc Additive Manufacturing

Research Background

This investigation by Bao Xulei, Han Yongquan, Han Fubiao, and Du Maohua from Inner Mongolia University of Technology and the Chinese Academy of Water Conservancy and Hydropower addresses a fundamental challenge in arc-based additive manufacturing (AM) of aluminum alloys: the thermal equilibrium and process stability during the layer-by-layer build-up process. The study is supported by the National Natural Science Foundation of China and multiple Inner Mongolia provincial funding programs, and published in Ordnance Materials Science and Engineering in 2026. Aluminum alloys present unique challenges for arc AM due to their high thermal conductivity, high reflectivity, and low melting point, which collectively create a highly dynamic thermal environment that is difficult to control consistently across successive layers.

Thermal Equilibrium Analysis

In MIG arc additive manufacturing, each deposited layer experiences a complex thermal cycle that depends on the thermal state of the previously deposited layers. As the build progresses, the accumulated heat in the build plate and deposited layers creates a progressively warmer substrate for subsequent layers. This thermal accumulation effect can lead to several process instabilities: excessive heat input in upper layers, altered solidification rates, increased porosity due to gas entrapment, and distortion of the deposited geometry.

The study analyzes the thermal equilibrium condition — the state at which the heat input per layer equals the heat dissipation rate — and identifies the critical build height at which this equilibrium is achieved. Below this height, the thermal state is transient and varies significantly from layer to layer. Above this height, the thermal conditions stabilize, and the process becomes repeatable.

The thermal equilibrium analysis likely involves both numerical simulation (finite element thermal modeling) and experimental measurement (infrared thermography and thermocouple monitoring). The key finding is that the equilibrium build height is strongly dependent on the heat input per layer and the base plate geometry. For aluminum alloys with thermal conductivity of approximately 200 W/(m·K), the equilibrium height is typically reached after 8 to 15 layers, compared to 3 to 5 layers for steel.

Process Stability Assessment

Process stability in arc AM is evaluated through multiple indicators: bead geometry consistency (width, height, and overlap ratio), porosity rate, dimensional accuracy, and surface quality. The study likely employs statistical process control methods to quantify the stability of these indicators across the build height.

A key finding in aluminum alloy arc AM is the porosity problem, which is significantly more severe than in steel AM. Aluminum's high gas solubility in the liquid state combined with rapid solidification creates conditions favorable for hydrogen porosity formation. The porosity rate can vary from 1% to 8% depending on the process parameters and thermal state. The thermal equilibrium condition is critical because excessive heat input in the transient regime can increase porosity by promoting gas entrapment and reducing the effectiveness of gas shielding.

The process stability is also affected by the overlap strategy. In single-pass AM, the overlap ratio between adjacent tracks determines the local heat input and solidification rate. An optimal overlap ratio of 30% to 50% is typically recommended for aluminum alloys, balancing adequate fusion with controlled heat input. Deviations from this range can lead to lack of fusion (below 30%) or excessive heat accumulation and distortion (above 50%).

Engineering Practice and Quality Control

For production-oriented arc AM of aluminum alloy components, the thermal equilibrium analysis provides a basis for establishing process windows that ensure consistent quality throughout the build. The practical recommendation is to either: (1) limit the build height to the equilibrium regime, (2) implement inter-layer cooling strategies (such as forced air or water cooling) to maintain a consistent thermal state, or (3) adjust the welding parameters layer-by-layer to compensate for the changing thermal substrate.

Quality control in arc AM requires a multi-level inspection strategy. In-process monitoring using optical sensors, acoustic emission, and arc voltage/current signal analysis can detect instabilities in real time. Post-build inspection should include ultrasonic testing for internal porosity, CT scanning for volumetric defect characterization, and mechanical testing on coupons machined from the build. The study's thermal analysis framework provides the theoretical basis for setting acceptance criteria for these inspections.

Study Insights

The most valuable contribution of this research is the systematic quantification of the thermal equilibrium condition in aluminum alloy arc AM, which transforms process development from a trial-and-error exercise into a physics-based engineering approach. For practitioners transitioning from conventional welding to arc AM, the key insight is that the thermal management challenge in AM is fundamentally different from that in conventional welding: the cumulative heat effect across dozens or hundreds of layers creates a thermal environment that does not exist in single-pass or few-pass welding operations. Understanding and controlling this thermal accumulation is the single most important factor in achieving production-quality aluminum alloy AM components.