Microstructure Analysis of TA15 Titanium Alloy and 18-8 Stainless Steel Dissimilar Joint by Filler Wire TIG Welding

Literature Overview

This paper, published in Welding Journal (Vol. 38, No. 2, 2017, pp. 57-60) by Liu Kun, Li Yajiang, Wang Juan, and Lan Yazhou from Shandong University's Key Laboratory of Liquid-Solid Structural Evolution and Processing (Ministry of Education), investigates the microstructure of a dissimilar joint between TA15 titanium alloy and 18-8 stainless steel welded using copper filler wire in TIG welding. Funded by the National Natural Science Foundation of China (51575316) and Shandong Provincial Natural Science Foundation (ZE2015EM040), this work addresses a challenging welding problem with significant aerospace and marine engineering applications.

Core Technical Findings

Weld Zone Microstructure

The copper filler wire creates a unique metallurgical environment in the weld zone:

• Weld metal: Copper solid solution dendrites form as the primary microstructure, indicating that the weld metal composition is dominated by the copper filler rather than the base metals.
• TA15 titanium alloy side fusion zone: A mixed microstructure of α-Ti and brittle phosphide phase Ti₃P, along with Ti(Cu,Fe) intermetallic compounds, forms in the melted but unmixed region.
• 18-8 stainless steel side fusion zone: Fe-P eutectic microstructure develops, with phosphorus segregation promoting eutectic formation.

Phase Distribution and Hardness

The EPMA analysis reveals critical element distribution patterns:

Phosphorus Segregation Behavior

The paper identifies phosphorus as a critical element driving the formation of brittle phases:

1. In the TA15 side: Phosphorus combines with titanium to form Ti₃P, a hard and brittle intermetallic phase that significantly increases local hardness.
2. In the 18-8 side: Phosphorus segregates to the fusion boundary, promoting Fe-P eutectic formation and creating a network of brittle phases along grain boundaries.
3. Unreacted copper: Copper that does not participate in eutectic reactions forms globular aggregation zones, potentially serving as crack initiation sites.

Metallurgical Mechanism Analysis

Dissimilar Joint Welding Challenges

The TA15/18-8 dissimilar joint presents several fundamental metallurgical challenges:

1. Large melting point difference: TA15 (melting point ~1660°C) and 18-8 stainless steel (melting point ~1420°C) have a significant melting point differential, leading to asymmetric melting and dilution.
2. Immiscibility: Titanium and iron are essentially immiscible in the liquid state, leading to segregation and intermetallic compound formation at the interface.
3. Thermal conductivity mismatch: Titanium's high thermal conductivity (~22 W/m·K) compared to stainless steel (~15 W/m·K) creates asymmetric heat flow patterns.
4. Thermal expansion coefficient difference: TA15 (8.6 × 10⁻⁶ /K) and 18-8 SS (17.3 × 10⁻⁶ /K) have different thermal expansion behaviors, leading to residual stress development.

Role of Copper Filler Wire

The selection of copper filler wire is a strategic choice that addresses the immiscibility problem:

• Copper acts as a "buffer" material between titanium and steel, reducing direct Ti-Fe contact.
• Copper forms solid solution dendrites rather than brittle intermetallics with either base metal.
• The dilution ratio controls the composition of the fusion zones, with higher copper dilution potentially reducing intermetallic formation.

However, the presence of phosphorus (from the base materials or filler) creates Ti₃P and Fe-P phases that compromise joint integrity. The maximum hardness of 720 HV0.5 in the TA15 fusion zone indicates extremely hard, brittle conditions that are susceptible to cracking.

Engineering Practice Implications

Application Areas

TA15/18-8 dissimilar joints find applications in:

1. Aerospace: Titanium-to-steel transitions in aircraft structures, such as landing gear components or engine mounts.
2. Marine engineering: Titanium propeller shafts connected to steel hulls, requiring corrosion-resistant dissimilar joints.
3. Chemical processing: Titanium heat exchangers with stainless steel connections.
4. Nuclear industry: Titanium containment structures with stainless steel penetrations.

Quality Control Considerations

For production welding of such dissimilar joints, the following quality control measures are essential:

1. Non-destructive testing: UT or TOFD is recommended for detecting intermetallic phases and porosity at the fusion boundary. MT or PT can identify surface-breaking cracks along the brittle phase networks.
2. Microhardness mapping: Vickers hardness traverse across the joint is critical for identifying brittle zones and ensuring hardness gradients are acceptable.
3. Metallographic examination: Cross-sectional examination with appropriate etchants is necessary to identify phase distributions and potential cracking.
4. Mechanical testing: Tensile and bend tests should be performed on production coupons to verify joint integrity.

Process Optimization Recommendations

Based on the findings, the following process modifications could improve joint quality:

1. Filler wire composition control: Using low-phosphorus copper filler (P < 0.02%) would reduce Ti₃P and Fe-P formation.
2. Welding parameter adjustment: Lower heat input and higher travel speed would reduce base metal dilution and limit the extent of the fusion zones.
3. Preheating strategy: Selective preheating of the titanium side could promote more symmetric melting and reduce asymmetric fusion zone formation.
4. Post-weld heat treatment: A controlled solution treatment and aging sequence might dissolve some intermetallic phases, though this requires careful temperature control to avoid adverse effects on the titanium side.

Study Insights and Outlook

This research provides valuable fundamental understanding of the metallurgical behavior of TA15/18-8 dissimilar joints, but also highlights significant challenges for practical application. The formation of brittle Ti₃P and Fe-P phases at both fusion boundaries creates potential crack initiation sites that could compromise joint integrity under cyclic or impact loading.

The maximum hardness of 720 HV0.5 in the TA15 fusion zone is particularly concerning. For comparison, typical acceptance criteria for welded joints limit hardness to 350 HV or less (per API 5L or ASME B31.3). The extreme hardness indicates that the joint would likely fail in brittle fracture mode if subjected to service loading.

For engineers working with dissimilar metal welds in piping systems (per ASME B31.3 Appendix A), this research underscores the importance of:

1. Careful filler metal selection to minimize intermetallic formation.
2. Strict control of welding parameters to limit fusion zone extent.
3. Comprehensive non-destructive examination to detect potential defects.
4. Consideration of post-weld heat treatment to relieve residual stresses and modify microstructure.

The work also demonstrates the value of advanced characterization techniques (EPMA, SEM, OM) in understanding complex weld microstructures. For qualification purposes, these techniques should be employed during procedure qualification and production verification to ensure consistent joint quality.