CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Heat Input on Joint Microstructure in Magnesium Alloy TIG Welding
Literature Overview
This 2009 publication by Wang Chun and Liang Guoli of the Department of Mechanical and Electrical Engineering at Tangshan College was published in the journal Hot Working Technology. The research was supported by the Tangshan Science and Technology Development Plan Project (Grant No. 07160203B-1). The paper investigates the effect of welding heat input on the microstructure of magnesium alloy joints produced by TIG welding. Magnesium alloys are of growing interest in lightweight structural applications due to their low density and good specific strength, but their welding presents unique challenges due to their high reactivity, low melting point, and susceptibility to hot cracking and oxidation.
Magnesium Alloy Material Characteristics
Magnesium alloys are the lightest structural metals, with a density of approximately 1.7-1.9 g/cm³, which is about 33% of the density of aluminum and 75% less than steel. The most common welding magnesium alloys include AZ31, AZ91, and AZ61, which contain aluminum and zinc as the primary alloying elements. These alloys offer excellent specific strength and stiffness, making them attractive for automotive, aerospace, and industrial applications where weight reduction is critical.
The following table summarizes the key material properties and welding characteristics of common magnesium alloys:
| Property | AZ31 | AZ91 | AZ61 | Welding Implication |
|---|---|---|---|---|
| Density (g/cm³) | 1.81 | 1.81 | 1.82 | Lightweight, good specific strength |
| Melting point (°C) | 450 | 450 | 460 | Low melting point, high fluidity |
| Thermal conductivity (W/m·K) | 75 | 50 | 55 | Moderate heat input required |
| Coefficient of thermal expansion (μm/m·K) | 26 | 25 | 25 | High distortion risk |
| Hot cracking susceptibility | Low | High | Medium | Depends on alloy composition |
| Oxidation rate | High | High | High | Requires excellent shielding |
| Hydrogen solubility | Low | Low | Low | Hydrogen embrittlement risk |
The high reactivity of magnesium with oxygen and nitrogen at elevated temperatures is a major challenge for welding. The oxide layer (MgO) has a very high melting point (2800°C) and is difficult to dissolve in the weld pool, leading to oxide inclusions and porosity. The shielding gas coverage must be excellent to prevent oxidation of the weld pool and heat-affected zone. Additionally, magnesium has a low solubility for hydrogen, which can lead to hydrogen embrittlement and delayed cracking if hydrogen is introduced during welding.
Effect of Heat Input on Microstructure
The welding heat input is a critical parameter that determines the microstructure and mechanical properties of magnesium alloy welds. The heat input (Q) is defined as Q = (V × I × η) / v, where V is the arc voltage, I is the welding current, η is the efficiency factor (typically 0.7-0.8 for TIG), and v is the travel speed. Higher heat input results in a larger weld pool, slower cooling rate, and different solidification microstructure.
The following table presents the expected microstructural changes with increasing heat input:
| Heat Input (J/mm) | Weld Pool Size | Cooling Rate | Solidification Microstructure | Grain Size | Mechanical Properties |
|---|---|---|---|---|---|
| Low (<15) | Small | Fast | Fine dendrites, equiaxed grains | Fine | High strength, lower toughness |
| Medium (15-30) | Medium | Moderate | Dendritic, mixed structure | Medium | Balanced strength and toughness |
| High (>30) | Large | Slow | Coarse dendrites, columnar grains | Coarse | Lower strength, higher toughness |
The solidification microstructure of magnesium alloy welds is primarily dendritic, with the primary phase being the α-Mg solid solution. The secondary phases include Mg17Al12 (β phase) and MgZn2 (η phase), which form at the dendrite boundaries. The morphology and distribution of these secondary phases are strongly influenced by the cooling rate, which is directly related to the heat input.
Welding Process Parameters and Microstructure Control
The TIG welding of magnesium alloys requires careful parameter selection to achieve the desired microstructure and mechanical properties. The following table summarizes the typical TIG welding parameters for magnesium alloys:
| Parameter | Typical Range | Influence on Microstructure |
|---|---|---|
| Current (A) | 80-250 | Higher current increases heat input, coarsens grains |
| Voltage (V) | 10-18 | Affects arc energy and weld pool shape |
| Travel speed (mm/min) | 100-300 | Higher speed reduces heat input, refines grains |
| Shielding gas flow (L/min) | 15-25 | Prevents oxidation, critical for Mg alloys |
| Wire feed rate (mm/min) | 150-400 | Affects deposition rate and dilution |
| Preheat temperature (°C) | 100-200 | Reduces cracking, controls cooling rate |
| Tungsten electrode diameter (mm) | 2.4-4.0 | Affects current density and arc characteristics |
The shielding gas selection is critical for magnesium alloy welding. Pure argon is the standard shielding gas, with flow rates of 15-25 L/min to ensure complete protection of the weld pool. A trailing shield or back purge is often used to protect the hot weld metal from oxidation on the back side of the joint.