Microstructure and Microhardness of Laser-TIG Double-Sided Composite Welding

Literature Overview

The paper by Li Rongbing from Xuzhou Institute of Technology, published in Manufacturing Automation in 2014 under Xuzhou Science and Technology Development Fund Project XZZD1103, investigates the microstructure and microhardness distribution of a novel laser-TIG double-sided composite welding process. This hybrid welding technique combines laser welding (front side) with TIG welding (back side) in a single pass, offering the advantages of both processes while minimizing their individual limitations.

Core Technical Content

Process Configuration and Parameters

The laser-TIG double-sided composite welding process employs:

Parameter	Laser Side (Front)	TIG Side (Back)
Power source	Fiber laser	TIG (DCEN)
Power	1.5–4.0 kW	100–250 A
Focal position	On surface	—
Spot diameter	0.2–0.5 mm	—
Travel speed	500–2000 mm/min	500–2000 mm/min
Shielding gas	Ar (front)	Ar (back)
Gas flow	10–15 L/min	8–12 L/min
Filler wire	Optional (TIG side)	ER308L or matching

Synergistic Effects of Hybrid Process

The combination of laser and TIG arc produces several synergistic effects:

Reduced porosity: The TIG arc on the back side provides additional shielding and helps collapse keyhole porosity formed during laser welding
Improved penetration profile: The laser provides deep, narrow penetration while the TIG arc adds heat to the back side, creating a more uniform weld profile
Enhanced microstructure: The dual heat source creates a complex thermal cycle that promotes grain refinement and phase transformation
Reduced residual stress: The TIG arc on the back side acts as a local stress relief mechanism, reducing拘束 stress

Microstructural Characteristics

The microstructure of laser-TIG composite welds exhibits distinct zones:

Zone	Microstructure	Hardness (HV)
Laser weld center	Fine equiaxed grains, martensite (if applicable)	350–500
Laser weld HAZ	Fine-grained, possibly martensitic	300–400
TIG weld zone	Coarse equiaxed grains, austenite + ferrite	200–280
Overlap zone	Mixed microstructure, refined grains	250–350
Base metal	Original microstructure	180–250

Microhardness Distribution

The microhardness profile across the weld cross-section reveals:

Laser weld zone: Higher hardness due to rapid cooling rates (1000–5000 K/s) and fine microstructure
TIG weld zone: Lower hardness due to slower cooling rates (10–100 K/s) and coarser microstructure
Transition zone: Gradual hardness change from laser side to TIG side, with the steepest gradient occurring in the overlap region
Overall: The composite weld exhibits a more uniform hardness distribution compared to single-process welds, reducing the risk of cracking at hardness mismatches

Process-Standard Analysis

Comparison with Single-Process Welding

Parameter	Laser Welding Only	TIG Welding Only	Laser-TIG Composite
Penetration	Deep, narrow	Moderate, wider	Deep, uniform
Porosity	High (keyhole)	Low	Low (reduced)
Dilution	Very low	Moderate	Low to moderate
Productivity	Very high	Low	High
Cost	High (laser)	Low	Moderate
Equipment complexity	Moderate	Low	High
Applicable thickness	Thin (≤ 6 mm)	All	Medium (3–20 mm)

Standards and Qualification

For pressure vessel applications, laser-TIG composite welding must be qualified per:

ASME IX: Hybrid processes require qualification of both the laser and arc components, with essential variables for each
ASME VIII Div.1 Appendix 12: For stainless steel weld overlay, the procedure must demonstrate adequate corrosion resistance
NB/T 47014: Chinese qualification standards require specific documentation of hybrid process parameters
ISO 15614-1/-6: International qualification standards for hybrid welding processes

Key Questions and Reflections

The laser-TIG composite welding process offers compelling advantages for specific applications:

Thick-section welding: The hybrid process can achieve full penetration in plates up to 20 mm in a single pass, which is impractical with laser welding alone
Dissimilar metal welding: The ability to control dilution on both sides makes this process suitable for bimetallic joints (e.g., carbon steel to stainless steel)
Repair welding: The process can be used for repairing defects in thick-section components where conventional processes would require multiple passes

However, several challenges remain:

Equipment cost: The dual-source setup is significantly more expensive than single-process equipment
Process control: Synchronization of laser and TIG parameters requires sophisticated control systems
Operator skill: The process demands higher operator skill to manage the dual heat sources
Qualification complexity: Code qualification of hybrid processes is more complex than single-process qualification

Study Insights and Implications

This research demonstrates the potential of laser-TIG composite welding for applications requiring both deep penetration and high-quality weld profiles. For bimetal pressure vessel fabrication, the process could be employed for welding dissimilar metal joints where controlling dilution on both sides is critical. The reduced porosity and improved microstructure make it particularly attractive for high-integrity applications governed by ASME VIII Div.2 or equivalent standards.

The microhardness distribution analysis highlights the importance of hardness matching in welded joints. Excessive hardness gradients can lead to stress concentration and cracking during service. The laser-TIG composite process, by creating a more gradual transition between the laser weld and TIG weld zones, reduces this risk compared to single-process welds where sharp hardness boundaries exist.