Thermal Process and Microstructure Evolution in Pulsed TIG Additive Manufacturing of 960 High-Strength Steel

Literature Overview

The study published in Materials Reports (2023) by Lu Wanquan, Qiao Jisen, Wang Lei, Liu Yongtao, Feng Rui, and Zhu Wei from Lanzhou University of Technology investigates the thermal behavior, microstructural development, and mechanical properties of 960 high-strength steel fabricated via pulsed TIG arc additive manufacturing. This research is supported by the National Natural Science Foundation of China (Grant No. 52063017) and the State Key Laboratory of Advanced Processing and Recycle of Non-ferrous Metals. The work addresses a critical gap in understanding how pulsed TIG parameters influence the thermal cycling history and subsequent microstructural refinement in high-strength steel additive manufacturing, which has direct relevance to overlay welding and cladding applications where layer-by-layer deposition is employed.

Core Technical Content

The authors systematically examined the thermal process characteristics of pulsed TIG additive manufacturing on 960-grade high-strength steel, focusing on how pulsed current parameters govern the heat input distribution, solidification rate, and cooling rate within each deposited layer. The thermal history of each layer is inherently complex due to the cumulative heat accumulation from successive layers, which creates a non-uniform thermal field that differs significantly from single-pass welding.

Key parameters investigated include:

Parameter	Typical Range	Influence on Process
Pulsed peak current	150–300 A	Controls penetration depth and dilution
Background current	50–100 A	Maintains arc stability between pulses
Pulse frequency	5–20 Hz	Determines solidification interval and grain refinement
Pulse duration ratio	0.2–0.6	Governs heat input per cycle
Travel speed	5–20 cm/min	Affects layer thickness and geometry
Inter-layer temperature	50–150 °C	Controls residual stress and microstructure

The thermal analysis reveals that pulsed TIG additive manufacturing produces a unique thermal cycling pattern where each pulse creates a local solidification event followed by partial remelting during the background current phase. This results in a grain refinement mechanism that is fundamentally different from continuous arc welding. The peak cooling rate observed in the deposited layers ranges from 5 to 25 °C/s, depending on the specific parameter combination, which is significantly higher than conventional welding due to the intermittent heat input.

Microstructural Analysis and Mechanical Properties

The microstructural evolution in 960 high-strength steel deposits shows a progressive refinement with increasing layer number, attributed to the self-annealing effect of subsequent layers acting as nucleation sites for epitaxial growth. The base material typically exhibits a tempered martensite structure with lath boundaries, while the deposited layers develop a finer martensitic microstructure with increased carbide precipitation.

Microstructural Feature	Base Material	First Layer	Middle Layers	Top Layer
Grain size	25–35 μm	15–25 μm	10–18 μm	12–20 μm
Martensite lath width	1.5–2.5 μm	0.8–1.5 μm	0.5–1.0 μm	0.6–1.2 μm
Carbide density	Low	Moderate	High	Moderate-high
Retained austenite (%)	2–5	5–12	8–15	6–12

The mechanical properties demonstrate that the deposited layers achieve yield strengths exceeding 960 MPa with tensile strengths reaching 1100–1250 MPa, maintaining ductility with elongation values of 12–18%. The hardness profile shows a gradient distribution, with the middle layers exhibiting the highest hardness (350–400 HV) due to the combined effects of rapid solidification and thermal cycling.

Engineering Implications for Cladding Applications

The findings have direct relevance to weld overlay and cladding operations on high-strength steels. The pulsed TIG approach offers several advantages for overlay applications:

Reduced dilution: The intermittent heat input limits the melting of the base material, preserving the metallurgical integrity of the substrate.
Controlled microstructure: The pulse parameters can be tuned to achieve specific microstructural targets, such as fine martensite for high hardness or tempered structures for toughness.
Lower residual stress: Compared to continuous arc methods, the pulsed approach produces lower peak temperatures and more uniform cooling, reducing residual stress accumulation.
Layer quality control: Each pulse cycle provides a natural inspection interval, enabling real-time monitoring of layer geometry and surface quality.

However, the thermal accumulation effect remains a challenge. In multi-layer cladding applications, the inter-layer temperature can reach 200–300 °C if not actively cooled, leading to grain coarsening and reduced mechanical properties in the upper layers. Active cooling strategies, such as water spray or forced air, are recommended to maintain inter-layer temperatures below 150 °C.

Study Insights and Reflections

This research contributes valuable data to the growing body of knowledge on additive manufacturing of high-strength steels. From a cladding engineering perspective, the thermal process insights can be directly transferred to pulsed TIG overlay welding of high-strength pressure vessel components. The key insight is that pulse frequency and duration ratio serve as primary control levers for balancing microstructural refinement against thermal accumulation. In practice, for overlay applications on 960-grade steels, a pulse frequency of 10–15 Hz with a duration ratio of 0.3–0.4 appears optimal for achieving fine microstructure while maintaining reasonable deposition rates.

The research also highlights the importance of thermal monitoring during multi-layer deposition. Without proper inter-layer temperature control, the benefits of pulsed welding can be negated by thermal softening of previously deposited layers. This has implications for quality assurance protocols in cladding fabrication, where thermocouple monitoring at critical locations should be standard practice.