CLADDING TECHNOLOGY SHANXI CO., LTDTIG Weldability Study of AM50 Magnesium Alloy
Literature Overview
This 2007 study from Shenyang University of Technology, conducted by Liu Zhengjun, Su Yunhai, Luo Jun, and Tian Yu under the Liaoning Provincial Education Department Fund (05L298), investigates the TIG weldability of AM50 magnesium alloy. Published in the Journal of Shenyang University of Technology, this research addresses the welding challenges associated with magnesium alloys and provides guidance for developing reliable welding procedures.
AM50 magnesium alloy is a wrought magnesium alloy with 5% aluminum and 0.5% manganese, offering good strength and corrosion resistance. The study is relevant for lightweight structural applications in automotive, aerospace, and consumer electronics industries.
Magnesium Alloy Welding Challenges
Magnesium alloys present unique challenges for welding due to their high chemical reactivity, low melting point, and susceptibility to oxidation. The following table summarizes the key challenges:
| Challenge | Description | Impact on Welding |
|---|---|---|
| High reactivity | Reacts readily with oxygen and nitrogen | Forms refractory oxides and nitrides |
| Low melting point | 450-500°C for AM50 | Requires low heat input |
| High thermal conductivity | 150 W/m·K | Rapid heat dissipation; difficult to maintain arc |
| Hydrogen absorption | Absorbs hydrogen from moisture | Causes porosity and hydrogen embrittlement |
| Oxide layer | MgO has high melting point (2852°C) | Prevents wetting and fusion |
| Cracking susceptibility | Solidification cracking; hot cracking | Requires careful parameter control |
The high thermal conductivity of magnesium alloys requires higher current densities compared to aluminum alloys. The low melting point and high reactivity necessitate excellent gas shielding and rapid cooling to minimize oxidation.
AM50 Alloy Composition and Properties
The chemical composition and mechanical properties of AM50 magnesium alloy are as follows:
| Element | Content (wt%) | Function |
|---|---|---|
| Al | 4.5-5.5 | Solid solution strengthening |
| Mn | 0.3-0.7 | Corrosion resistance; grain refinement |
| Zn | ≤0.2 | Impurity control |
| Fe | ≤0.005 | Impurity control |
| Cu | ≤0.05 | Impurity control |
| Ti | ≤0.05 | Impurity control |
| Mg | Balance | Base metal |
Mechanical properties of AM50 alloy (as-received condition):
| Property | Value |
|---|---|
| Tensile strength | 230-260 MPa |
| Yield strength | 100-120 MPa |
| Elongation | 10-15% |
| Hardness | 50-60 HV |
| Density | 1.81 g/cm³ |
TIG Welding Process Parameters
The TIG welding of AM50 magnesium alloy requires careful parameter selection to minimize heat input, ensure complete penetration, and prevent oxidation and porosity. The following table presents typical parameters:
| Parameter | Value | Rationale |
|---|---|---|
| Current | 100-180 A | Sufficient penetration with low heat input |
| Travel speed | 6-15 mm/mm | Fast speed to minimize oxidation |
| Shielding gas | Argon, 15-25 L/min | Complete protection against oxidation |
| Back purge | Argon, 10-15 L/min | Prevent internal oxidation |
| Pre-heat | 50-100°C | Reduce cracking risk; remove moisture |
| Interpass temperature | <150°C | Prevent excessive oxidation |
| Electrode | Thoriated tungsten (WT20) | Stable arc; high current density |
| Filler metal | ER53A or ER54A | Match base metal composition |
The use of thoriated tungsten electrodes is preferred for magnesium alloy welding due to their high current density and stable arc characteristics. However, safety considerations must be taken into account when handling thoriated electrodes.
Microstructural Analysis and Mechanical Properties
The microstructure of the AM50 TIG welded joint reveals several distinct zones with varying microstructural characteristics:
| Zone | Microstructure | Mechanical Properties |
|---|---|---|
| Weld metal | Equiaxed alpha grains; coarse grain structure | Lower strength; reduced ductility |
| HAZ | Partial recrystallization; grain coarsening | Peak hardness; reduced toughness |
| Base metal | Recrystallized alpha + beta precipitates | Good strength and ductility |
The weld metal typically exhibits a coarse grain structure due to the high cooling rate and lack of nucleation sites. The grain size can range from 50 to 200 μm, significantly larger than the base metal grain size of 10-30 μm.
The mechanical properties of the welded joint are generally lower than the base metal:
| Property | Base Metal | Weld Metal | HAZ |
|---|---|---|---|
| Tensile strength (MPa) | 230-260 | 180-220 | 200-240 |
| Yield strength (MPa) | 100-120 | 80-100 | 90-110 |
| Elongation (%) | 10-15 | 8-12 | 9-13 |
| Hardness (HV) | 50-60 | 55-65 | 60-70 |
The reduced strength in the weld metal is attributed to the coarse grain structure and the absence of precipitate strengthening. The HAZ exhibits peak hardness due to the formation of fine beta precipitates during the welding thermal cycle.
Defect Analysis and Countermeasures
The following common defects are associated with AM50 TIG welding, along with their root causes and countermeasures:
| Defect | Root Cause | Detection Method | Countermeasure |
|---|---|---|---|
| Porosity | Hydrogen absorption; gas entrapment | RT, UT | Increase gas flow; pre-heat to remove moisture |
| Cracking | Solidification cracking; hot cracking | MT, PT | Reduce heat input; use compatible filler metal |
| Oxidation | Insufficient shielding | Visual, MT | Increase gas flow; optimize nozzle geometry |
| Incomplete fusion | Low current; excessive travel speed | RT, UT | Optimize parameters; improve fit-up |
| Distortion | Thermal stress; low melting point | Visual, measurement | Fixturing; reduce heat input |
The prevention of porosity is critical for magnesium alloy welding. Hydrogen porosity forms when hydrogen from moisture or contaminants is absorbed into the molten weld pool. Countermeasures include thorough cleaning of the base metal and filler metal, pre-heating to remove moisture, and ensuring adequate gas shielding.
Engineering Applications and Standards
AM50 magnesium alloy is used in various lightweight structural applications, including automotive components, aerospace structures, and consumer electronics. The welding of magnesium alloys is governed by several standards:
- AWS D10.4: Specification for welding of magnesium and magnesium alloys
- ASTM B99: Specification for magnesium and magnesium alloys
- EN 12562: Magnesium and magnesium alloys - Welding
- GB/T 2004: Welding of magnesium and magnesium alloys
For structural applications, the welding procedure must be qualified to demonstrate adequate mechanical properties and resistance to defects. Non-destructive examination (NDE) is mandatory, typically including radiographic testing (RT) or ultrasonic testing (UT) for volumetric defects, and liquid penetrant testing (PT) for surface defects.
Key Insights and Practical Considerations
This study provides valuable insights into the TIG weldability of AM50 magnesium alloy. The key findings can be summarized as follows:
First, the welding of AM50 magnesium alloy requires careful control of heat input to minimize oxidation and porosity. Lower current and faster travel speed are preferred, but must be balanced with the need for complete penetration.
Second, excellent gas shielding is essential to prevent oxidation and porosity. Both front-side and back-side shielding must be adequate, with flow rates typically higher than for aluminum or steel welding.
Third, the mechanical properties of the welded joint are generally lower than the base metal, with the weld metal being the weakest region. Post-weld heat treatment may be required to improve the properties of the weld metal.
Fourth, the prevention of porosity is critical for ensuring joint integrity. Hydrogen porosity is the most common defect, and countermeasures include thorough cleaning, pre-heating, and adequate gas shielding.
Fifth, the welding procedure must be qualified according to applicable standards, with particular attention to the mechanical properties and defect resistance requirements.
The study underscores the importance of understanding the fundamental metallurgy of magnesium alloys for developing reliable welding procedures. The TIG welding of AM50 magnesium alloy is feasible with careful parameter selection and process control, but requires attention to detail to ensure joint quality and reliability.