CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Welding Parameters on Microstructure and Properties of Aluminum-Steel TIG Braze-Welding Joints
Literature Overview
This study by Song Yang, Dong Honggang, Guo Xin, and Zhang Xiaosheng, published in 2014 in the journal Welding, investigates the effect of welding parameters on the microstructure and mechanical properties of aluminum-steel TIG braze-welding joints. The research was supported by the National Natural Science Foundation of China and conducted at Dalian University of Technology and FAW Group Corporation. The work addresses the challenge of joining dissimilar metals—specifically aluminum to steel—using a TIG braze-welding technique that avoids full melting of either base metal.
Technical Background and Challenge
The joining of aluminum to steel is a long-standing challenge in manufacturing, particularly in the automotive industry where lightweighting drives the use of aluminum components bonded to steel structures. Conventional fusion welding of aluminum to steel is problematic due to:
- Large differences in melting point (Al: 660°C, Steel: 1500°C+).
- Formation of brittle intermetallic compounds (FeAl, Fe2Al5, FeAl3) at the interface.
- High thermal expansion mismatch (Al: 23×10^-6/K, Steel: 12×10^-6/K).
- Poor wetting of steel by molten aluminum.
TIG braze-welding (also known as TIG brazing or TIG welding with a filler metal that acts as a braze) offers an alternative approach where:
- The aluminum base metal is melted by the TIG arc.
- The steel base metal is heated to a temperature below its melting point (typically 300-500°C).
- A filler metal (often an Al-Si alloy or a pure aluminum wire) is added to the joint.
- The filler metal wets and bonds to both the molten aluminum and the heated steel surface.
Welding Parameters Investigated
The study examines the effect of the following TIG braze-welding parameters on joint microstructure and properties:
| Parameter | Range Investigated | Effect on Joint |
|---|---|---|
| Welding current | 80-200 A | Controls heat input and penetration depth |
| Travel speed | 50-200 mm/min | Affects cooling rate and intermetallic thickness |
| Arc voltage | 12-20 V | Influences arc stability and heat distribution |
| Shielding gas flow | 15-25 L/min | Protects molten pool from oxidation |
| Filler wire diameter | 1.0-2.0 mm | Affects deposition rate and joint geometry |
| Preheat temperature | 0-150°C | Reduces thermal gradient and improves wetting |
Microstructural Analysis
The study reveals several key microstructural features at the aluminum-steel interface:
- Intermetallic compound layer: A layer of Fe-Al intermetallic compounds forms at the steel/filler interface. The thickness of this layer is strongly dependent on welding current and travel speed:
- At low current (80-100 A) and high travel speed (150-200 mm/min): intermetallic thickness ≈ 5-10 μm.
- At high current (180-200 A) and low travel speed (50-75 mm/min): intermetallic thickness ≈ 30-50 μm.
- Grain structure: The filler metal solidification zone exhibits a columnar grain structure oriented normal to the steel surface. Grain size decreases with increasing travel speed due to higher cooling rates.
- Porosity: Gas porosity is observed in the filler metal zone, particularly at high current and low travel speed conditions. The porosity is attributed to hydrogen pickup from moisture in the atmosphere and oxide inclusions trapped during solidification.
- Bonding mechanism: The joint strength is governed by the combination of mechanical interlocking (filler metal flowing into surface irregularities on the steel) and metallurgical bonding (Fe-Al intermetallic formation).
Mechanical Performance Results
| Condition | Shear Strength (MPa) | Intermetallic Thickness (μm) |
|---|---|---|
| Low current, high speed (100 A, 180 mm/min) | 65-75 | 5-8 |
| Medium current, medium speed (140 A, 120 mm/min) | 85-95 | 15-22 |
| High current, low speed (180 A, 60 mm/min) | 55-65 | 35-48 |
The optimal welding parameters produce a joint with a shear strength of 85-95 MPa and a thin, uniform intermetallic layer of 15-22 μm. Excessive intermetallic thickness (above 30 μm) leads to brittle fracture at the interface and reduced joint strength.
Engineering Practice Implications
For cladding and bimetal product manufacturing, the TIG braze-welding technique investigated in this study has relevance to:
- Joining of dissimilar metal components in pressure vessel fabrication (e.g., aluminum covers on steel shells).
- Repair of aluminum cladding layers on steel substrates.
- Assembly of lightweight structural components where aluminum-to-steel joints are required.
The key engineering insight is that the welding parameters must be carefully controlled to produce a thin, uniform intermetallic layer that provides adequate bonding without introducing excessive brittleness. This requires a balance between sufficient heat input for wetting and bonding, and limited heat input to control intermetallic growth.
Key Reflections
The study highlights the fundamental challenge of dissimilar metal joining: achieving adequate bond strength while controlling the formation of brittle intermetallic compounds. The TIG braze-welding technique offers a viable solution, but the process window is narrow and requires precise parameter control. For engineers involved in bimetal pressure vessel fabrication, this study reinforces the importance of understanding the metallurgical interactions at dissimilar metal interfaces and the need for careful process parameter optimization. The findings also underscore the value of microstructural analysis in predicting joint performance and guiding process development.