Thermal Process and Microstructure-Mechanical Properties of 960 MPa High-Strength Steel Pulse TIG Arc Additive Manufacturing

Literature Overview

This 2023 study published in Materials Reports by researchers from Lanzhou University of Technology investigates the thermal process, microstructure evolution, and mechanical properties of 960 MPa high-strength steel fabricated using pulse TIG arc additive manufacturing (AM). The research, supported by the National Natural Science Foundation of China and the State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals, addresses the growing demand for high-strength steel components produced through additive manufacturing for applications in heavy machinery, pressure vessels, and structural components. The 960 MPa strength level represents a significant improvement over conventional structural steels, offering potential for weight reduction and improved performance in demanding applications.

Additive Manufacturing Process Description

The pulse TIG arc additive manufacturing process combines the advantages of pulse TIG welding with the layer-by-layer deposition strategy of additive manufacturing. The process uses a robotic system to deposit molten metal in a controlled manner, building up the component layer by layer. The pulse welding mode provides precise control over heat input, enabling the management of thermal gradients and residual stresses that are critical for high-strength steel applications.

The process parameters were optimized through a combination of experimental trials and thermal simulation. The pulse current was varied between 150 and 300 A, with a background current of 50-80 A and a pulse frequency of 8-15 Hz. The wire feed speed was adjusted between 300 and 600 mm/min, and the travel speed was controlled between 100 and 200 mm/min. The shielding gas was argon at a flow rate of 20-25 L/min, and the interlayer temperature was maintained between 150 and 250°C to promote bonding between layers while avoiding excessive grain coarsening.

Thermal Process Analysis

The thermal simulation revealed that the pulse TIG arc AM process produces a complex thermal history characterized by repeated heating and cooling cycles. Each deposition pass subjects the previously deposited material to a reheating event, which can cause grain coarsening and phase transformation in the heat-affected zone of the previous layer. The researchers used thermocouples embedded at various locations within the built component to measure the temperature profiles and validate the simulation results.

The peak temperature at the arc contact point reached approximately 1,800-2,200°C, while the maximum temperature in the deposited material was limited to 900-1,100°C due to the pulse welding mode. The cooling rate was found to be significantly higher than in conventional welding, with values of 50-200°C/s in the weld metal, which promotes the formation of fine microstructures and high-strength phases. The thermal gradient was oriented primarily in the vertical direction, with the highest gradients at the interface between the deposited layer and the previously deposited material.

Microstructure and Mechanical Properties

The microstructure of the 960 MPa high-strength steel produced by pulse TIG arc AM consisted primarily of acicular ferrite, bainite, and martensite phases, with the relative proportions varying with the local cooling rate and thermal history. The fine grain structure, with grain sizes of 5-20 μm, was attributed to the rapid cooling rates achieved in the AM process. The presence of carbide particles, primarily of the M23C6 and M7C3 types, contributed to the strength through precipitation hardening.

The mechanical properties of the AM-fabricated component were found to be comparable to the base plate, with the weld metal achieving yield strengths of 900-950 MPa and tensile strengths of 960-1010 MPa. The elongation values were slightly lower than the base plate due to the presence of some martensite phases, but still within acceptable limits for structural applications. The researchers also conducted impact testing at -40°C and found that the Charpy V-notch impact energy was 35-50 J, indicating good low-temperature toughness.

Residual Stress and Distortion Analysis

One of the critical challenges in additive manufacturing of high-strength steels is the management of residual stresses and distortion. The researchers used the X-ray diffraction method to measure the residual stress distribution within the built component. The maximum residual stress was found to be approximately 400-500 MPa in the tensile direction, primarily located near the surface and at the interfaces between deposited layers. This level of residual stress is significant for high-strength steels, as it can contribute to fatigue crack initiation and stress corrosion cracking.

To mitigate the residual stresses, the researchers evaluated the effectiveness of post-weld stress relief heat treatment at 600°C for 2 hours. The stress relief treatment reduced the residual stresses by 60-70%, but also caused some softening of the weld metal, reducing the yield strength by approximately 50-80 MPa. The trade-off between residual stress reduction and strength retention must be carefully considered for each specific application.

Engineering Practice and Quality Assurance

For the application of pulse TIG arc AM-fabricated 960 MPa high-strength steel components in pressure vessel or structural applications, several quality assurance measures are essential:

• Welding procedure qualification in accordance with ASME Section IX or NB/T 47014, including mechanical property testing and microstructural evaluation.
• Non-destructive testing using ultrasonic testing (UT) or phased array ultrasonic testing (PAUT) to detect internal defects such as porosity, lack of fusion, and cracks.
• Residual stress measurement using X-ray diffraction or hole-drilling methods to verify that the residual stresses are within acceptable limits.
• Stress relief heat treatment when required by the design specification or when the residual stresses exceed the allowable limits.
• Dimensional inspection to verify that the component meets the required tolerances and that distortion is within acceptable limits.

The researchers also emphasized the importance of material traceability and process monitoring. The chemical composition of the filler wire must be verified before use, and the welding parameters must be monitored in real-time to ensure consistency throughout the fabrication process. Any deviation from the qualified parameters should be documented and evaluated for its potential impact on weld quality.

Study Insights and Reflections

This research represents a significant advancement in the application of additive manufacturing to high-strength steel fabrication. The pulse TIG arc AM process offers a practical and cost-effective alternative to laser-based AM systems, particularly for large-scale components where the build rate and equipment cost of laser AM are prohibitive. The ability to achieve 960 MPa yield strength in the deposited material, with mechanical properties comparable to the base plate, demonstrates the technical maturity of the process. However, engineers must remain vigilant about the challenges of residual stress management, distortion control, and quality assurance when applying this technology to critical applications. The integration of thermal simulation with experimental validation provides a powerful tool for process optimization and quality prediction, and this approach should be adopted as a standard practice in the development of AM welding procedures for high-strength steels. The future of this technology lies in the further refinement of process parameters, the development of advanced monitoring systems, and the establishment of comprehensive qualification standards that address the unique challenges of additive manufacturing.