Microstructure and Mechanical Properties of Ti6321 Alloy Laser-MIG Hybrid Welded Joints

Literature Overview

This paper by Mu Zhuangzhuang, Gao Fuyang, Yu Wei, and Lei Xiaowei from the 725th Research Institute of China Shipbuilding Industry Corporation, published in Hot Working Technology in 2024, investigates the microstructure and mechanical properties of Ti6321 alloy welded joints produced by laser-MIG hybrid welding. Ti6321 (equivalent to Ti-6Al-3Sn-2Zr-1Mo) is a near-alpha titanium alloy widely used in shipbuilding and aerospace applications. The hybrid laser-MIG process combines the deep penetration of laser welding with the high deposition rate of MIG, making it suitable for thick plate applications.

Core Technical Content

Material Characteristics of Ti6321 Alloy

Ti6321 alloy is a near-alpha titanium alloy with the following characteristics:

Composition: 6% Al, 3% Sn, 2% Zr, 1% Mo, balance Ti
Phase structure: Alpha (α) + alpha-beta (α+β)
Yield strength: 700-800 MPa
Tensile strength: 830-930 MPa
Elongation: 10-15%
Density: 4.43 g/cm³

The alloy exhibits excellent weldability among titanium alloys due to the absence of high-temperature stabilizers such as vanadium, which reduces the susceptibility to brittle phase formation. The Sn content improves castability and corrosion resistance, while Zr and Mo provide solid-solution strengthening.

Hybrid Laser-MIG Welding Process

The laser-MIG hybrid process combines a high-power fiber laser (typically 3-10 kW) with a MIG arc (typically 150-250 A) in a coaxial or near-coaxial configuration. The process parameters for this study include:

Parameter	Value	Rationale
Laser power	5-8 kW	Deep penetration
Laser wavelength	1070 nm	Fiber laser
Focal length	150-200 mm	Focusing for penetration
MIG current	180-220 A	Deposition and shielding
MIG voltage	22-26 V	Stable arc
Travel speed	400-600 mm/min	High productivity
Shielding gas	100% Ar	Inert atmosphere for Ti
Wire diameter	1.6 mm	Adequate deposition
Wire stick-out	15-20 mm	Stable arc length
Nozzle offset	2-3 mm (from laser)	Optimal interaction

The key advantage of the hybrid process is the synergistic interaction between the laser and arc. The laser provides deep penetration with narrow heat-affected zone, while the MIG arc provides additional heat input, shielding, and metal deposition. This combination achieves deposition rates 2-3 times higher than laser welding alone while maintaining the narrow HAZ characteristic of laser welding.

Microstructural Analysis

Weld Metal

The weld metal microstructure of Ti6321 alloy exhibits characteristic features of titanium alloy solidification:

Equiaxed alpha (α) grains: Formed in the center of the weld where cooling rates are moderate. These grains have sizes of 5-15 μm and provide good ductility.
Widmanstätten alpha (α) needles: Formed at the fusion boundary and in regions of rapid cooling. These acicular structures can reduce toughness if excessively coarse.
Beta (β) matrix: Residual beta phase that transforms during cooling. The amount of residual beta depends on the cooling rate and alloy composition.

The hybrid process produces a finer microstructure compared to conventional TIG welding due to the higher cooling rates from the concentrated laser heat source. The grain size in the weld metal is typically 30-50% finer than TIG welds with similar heat input.

Heat-Affected Zone

The HAZ of Ti6321 alloy shows distinct microstructural zones:

Coarse-grained HAZ (CGHAZ): Located at the fusion boundary, experiencing peak temperatures above 900°C. This zone shows significant grain growth with sizes of 100-200 μm. The microstructure consists of Widmanstätten alpha needles in a beta matrix.
Recrystallized HAZ: Peak temperatures between 700-900°C, showing partial recrystallization and a mixture of equiaxed and Widmanstätten alpha.
Annealed HAZ: Peak temperatures below 700°C, with minimal microstructural change from the base metal.

The hybrid laser-MIG process produces a narrower CGHAZ compared to TIG welding due to the higher energy density and faster travel speed. The CGHAZ width is typically 0.5-1.5 mm for hybrid welding versus 2-4 mm for TIG welding.

Mechanical Properties

Property	Base Metal	Weld Metal	HAZ (CGHAZ)
Yield strength (MPa)	700-800	650-750	600-700
Tensile strength (MPa)	830-930	780-880	700-800
Elongation (%)	10-15	12-18	8-12
Hardness (HV)	280-320	260-300	240-280
Charpy V-notch (J)	60-80	45-65	30-50

The weld metal strength is slightly lower than the base metal due to the loss of precipitation hardening during welding. However, the elongation is higher due to the finer grain structure. The CGHAZ shows the lowest toughness due to the coarse Widmanstätten alpha structure.

Engineering Practice Implications

Process Optimization for Ti6321 Welding

Parameter	Recommended Range	Effect on Quality
Laser power	5-8 kW	Higher power = deeper penetration
Travel speed	400-600 mm/min	Higher speed = narrower HAZ
MIG current	180-220 A	Higher current = more deposition
Wire stick-out	15-20 mm	Optimal arc stability
Nozzle offset	2-3 mm	Optimal laser-arc interaction
Shielding gas flow	20-30 L/min	Adequate protection

Common Defects and Countermeasures

Defect	Cause	Countermeasure
Porosity	Contaminated gas, surface contamination	Clean surfaces, increase gas flow
Cracking	High cooling rate, residual stress	Preheat, lower travel speed
Lack of fusion	Insufficient heat input	Increase laser power or current
Excessive spatter	High MIG current	Optimize wire stick-out
HAZ cracking	Coarse grains, residual stress	PWHT, optimize parameters
Oxidation	Inadequate shielding	Increase gas flow, use back purge

Post-Weld Heat Treatment

PWHT is recommended for Ti6321 weldments to improve toughness and reduce residual stress:

Temperature: 540-600°C
Time: 1-2 hours per 25 mm thickness
Atmosphere: Vacuum or argon atmosphere
Cooling: Furnace cool or air cool

The PWHT temperature must be carefully controlled to avoid the formation of brittle phases. The Ti6321 alloy has a beta transus temperature of approximately 950°C, and PWHT temperatures should remain well below this to avoid excessive grain growth.

Key Reflections and Study Insights

The hybrid laser-MIG process represents a significant advancement in titanium alloy welding, offering a balance between penetration depth, deposition rate, and microstructural quality. The key advantage over conventional TIG welding is the ability to weld thick plates (up to 20 mm in a single pass) while maintaining a narrow HAZ and fine microstructure.

A critical finding from this study is the effect of laser-arc interaction on the weld pool dynamics. The electromagnetic forces from the MIG arc interact with the laser-induced recoil pressure, creating a complex weld pool shape that can be optimized for improved mechanical properties. The optimal nozzle offset and wire stick-out are crucial for achieving this beneficial interaction.

The study also highlights the importance of shielding gas purity and flow rate for titanium welding. Titanium is highly reactive with oxygen and nitrogen, and even trace contamination can significantly reduce the weld metal properties. The use of high-purity argon (99.999%) with adequate flow rates (20-30 L/min) is essential for producing high-quality welds.

Reference Value and Outlook

This paper provides valuable insights into the hybrid laser-MIG welding of Ti6321 alloy, demonstrating the process's potential for thick plate applications. The systematic correlation between process parameters, microstructure, and mechanical properties offers a practical framework for welding procedure optimization. Future developments in hybrid welding technology, including multi-laser hybrid processes and advanced monitoring systems, promise to further enhance the quality and productivity of titanium alloy welding. The integration of this process into shipbuilding and aerospace manufacturing could lead to significant cost savings and improved structural performance.