Compensation for Evaporation Loss in Laser Welding of Magnesium Alloys

Magnesium alloys, with their low density and high specific strength, are increasingly specified in aerospace, automotive, and electronics applications [6†L33-L36]. However, laser welding of magnesium alloys presents a unique challenge: a low boiling point (approximately 1090 °C) that is only marginally higher than its melting point (approximately 650 °C) [4†L13-L14][6†L13-L16]. This narrow processing window leads to rapid and often severe evaporation of alloying elements, primarily magnesium itself and volatile constituents such as zinc and aluminum, during the welding process [0†L7-L8].

Evaporation loss directly alters weld pool chemistry, promotes porosity, reduces mechanical properties, and compromises corrosion resistance [0†L7-L8][1†L19-L20][9†L44-L45]. This article analyzes the physical mechanisms driving evaporation in magnesium alloys, quantifies the relationship between process parameters and material loss, and evaluates three practical compensation strategies: filler wire addition, process parameter optimization, and the use of rare-earth alloying elements.

---

The Physical Mechanism of Evaporation in Magnesium Alloys

1 Thermal Properties of Magnesium and Its Alloying Elements

The vapor pressure of a metal determines its tendency to evaporate at a given temperature. For magnesium, the vapor pressure at its melting point (650 °C) is already substantial, and it increases exponentially as the temperature rises toward the boiling point (1090 °C). During laser welding, the keyhole temperature can exceed 2000 °C, creating vapor pressures that approach one atmosphere. Under these conditions, evaporation becomes the dominant loss mechanism for volatile elements.

The vapor pressure $P_{vap}$Pvap of pure magnesium as a function of temperature follows the Clausius–Clapeyron relation:$${\log }_{10}{P}_{vap}(\text{kPa})=A-\frac{B}{T}$$log10Pvap(kPa)=A−TB

where $A\approx 4.7$A≈4.7 and $B\approx 6800$B≈6800 for magnesium. At 1100 °C (1373 K), $P_{vap}$Pvap exceeds 200 kPa — sufficient to drive rapid evaporation from the weld pool surface and the keyhole walls.

For common alloying elements in magnesium alloys, vapor pressures at typical welding temperatures are:

Alloying Element	Melting point (°C)	Boiling point (°C)	Vapor pressure at 1500°C (kPa)	Evaporation risk
Mg (matrix)	650	1090	≈200	Very high
Zn (common)	419	907	***	Very high (lower boiling point than Mg)
Al (common)	660	2470	≈100	Moderate
Mn (common)	1246	2061	≈50	Low
Rare earths	800–1400	2500–3500	<10	Low

Zinc, with a boiling point of 907 °C — well below the keyhole temperature — is particularly prone to selective vaporization. The preferential loss of zinc from the weld pool can change the alloy composition from the specified grade (e.g., AZ31 contains approximately 1% Zn), altering the solidification path and potentially forming brittle intermetallic phases.

2 The Evaporation Threshold and Its Effect on Absorption

Research has established that the relationship between magnesium evaporation and laser absorption is not linear — evaporation triggers a positive feedback loop. In pulsed laser welding of aluminum alloys (which contain magnesium as a key alloying element), a threshold exists for significant penetration enhancement. The critical Mg evaporation rate is $200\times {10}^{-6}{\text{g/cm}}^2$200×10−6 g/cm2. Once this threshold is exceeded, Mg vapor in the plasma plume increases the effective laser absorption coefficient, allowing more energy to couple into the workpiece [14†L29-L34].

However, increased laser absorption does not necessarily translate to improved weld quality. Beyond a certain point, excessive evaporation leads to keyhole instability, spatter, and porosity formation. The challenge is to operate within the evaporation regime where absorption is enhanced, but above the threshold where quality degrades.

---

Evaporation Loss in Detailed Practice: Quantitative Data

1 Correlation Between Pulse Frequency and Mg Loss

Controlled experiments on Nd: YAG pulsed laser welding of aluminum alloys (which contain 4–5% Mg) have quantified the relationship between pulse frequency and magnesium loss. At a fixed average laser power, increasing the pulse frequency from 15 Hz to 40 Hz decreases the average temperature of the weld pool, which reduces Mg loss in terms of mass per unit area from $230\times {10}^{-5}$230×10−5 to $120\times {10}^{-5}{\text{kg/m}}^2$120×10−5 kg/m2 [15†L26-L29].

However, the effective Mg concentration loss (mass per unit volume) can actually increase with higher frequency because the weld penetration depth and weld volume decrease. The study demonstrated that effective Mg loss increased from $110\times {10}^{-1}$110×10−1 to $260\times {10}^{-1}{\text{kg/m}}^3$260×10−1 kg/m3 as frequency increased [15†L29-L31]. This counterintuitive result underscores a critical point: reducing evaporation in absolute terms does not guarantee preservation of weld metal chemistry if the weld volume changes simultaneously.

2 Compositional Segregation in Filler-Free Laser Welding

When laser welding magnesium alloys without filler wire, compositional segregation is both measurable and consequential. Analytical studies on AZ31B-H24 magnesium alloy joints welded without filler wire have detected significant elemental segregation in the weld zone. Energy-dispersive X‑ray spectroscopy (EDS) mapping reveals clear segregation of aluminum, manganese, and zinc in the weld metal, with the formation of precipitates rich in Al–Mn and Al–Zn phases [9†L19-L23].

The precipitates show a large decrease in Mg content, with minimum values ranging from 10 to 60 wt.% in the precipitates themselves. This level of Mg depletion within the precipitate phase indicates that the surrounding matrix has lost alloying elements to the formation of these secondary phases — a direct consequence of evaporation and re‑solidification dynamics [9†L22-L23].

Porosity is an immediate consequence. Experimental work has quantified that porosity volume in filler‑free joints can reach 0.63 mm³ for a single weld joint, with corresponding tensile strength as low as 154 MPa — approximately 50% below the base metal strength [9†L30-L31].

3 Interaction with Other Alloying Elements

The loss of magnesium is not a simple function of temperature alone. The activity coefficient of magnesium in the weld pool — which determines its tendency to evaporate — is influenced by interactions with other alloying elements. To accurately model Mg evaporation, the activity coefficient must be modified based on alloy composition. This means that evaporation behavior varies significantly across different magnesium alloy grades, and generalizing from one alloy to another without calibration is unsafe [15†L31-L34].

---

Filler Wire Addition as a Compensation Strategy

The addition of filler wire during laser welding serves multiple functions. It directly compensates for elemental loss by supplying fresh alloying elements to the weld pool. It modifies the weld pool geometry and fluid flow, which affects the thermal history and the extent of evaporation. And it can bridge fit‑up gaps that would otherwise cause underfill or lack of penetration.

1 AZ61 Filler for AZ31B Substrate

A systematic study of AZ31B-H24 magnesium alloy (400 mm × 500 mm × 1.3 mm sheets) welded with AZ61 filler wire established optimal conditions: laser power of 1000 W, welding speed of 3 m/min, and a defocusing length of 2 mm [7†L9-L10]. Compared to filler‑free welding, the joints produced with filler wire exhibited significantly improved tensile strength and elongation at room temperature [7†L10-L14].

The improvement at room temperature was attributed to three mechanisms: reduction of porosity, elimination of under‑filled bead formation, and precipitation hardening from Al–Mn and Mg–Al–Zn precipitates. At elevated temperatures (200 °C to 350 °C), the fracture location of tensile specimens shifted from the weld metal to the base metal, indicating that the weld metal exhibited less thermal softening than the base material — a strong indicator that the filler wire successfully maintained the weld chemistry and microstructure under service conditions [7†L15-L16][7†L25-L28].

2 Filler Wire Feed Rate: A First‑Order Control Variable

The filler wire feed rate is not merely a parameter for adding material — it directly influences the formation of porosity. In a controlled study using a 1.2 mm diameter AZ31 filler wire and an Nd: YAG laser, decreasing the filler wire feed rate led to an increase in weld metal porosity [9†L16-L18]. The lowest porosity volume (0.06 mm³) was measured at higher feed rates, while the highest (0.63 mm³) corresponded to the lowest feed rate [9†L25-L26].

The explanation for this observation is rooted in keyhole dynamics. A higher filler wire feed rate supplies more material to the trailing edge of the keyhole, stabilizing the molten pool and preventing the collapse of keyhole walls. This stabilization reduces the entrapment of vapor cavities that would otherwise become porosity. Conversely, welding without filler wire not only suffers from elemental loss but also experiences the most severe porosity formation [9†L18-L19][9†L46-L47].

3 Filler Wire Composition: Matching and Overmatching

The composition of the filler wire relative to the base metal determines whether compensation is successful. Studies comparing different filler wires in aluminum alloys (with varying Mg content) have shown that increasing the Mg content of the filler wire increases the recoil pressure within the keyhole and decreases the surface tension of the keyhole wall. This promotes more efficient transfer of molten filler into the keyhole [1†L21-L23].

For magnesium alloys, two strategies are employed:

• Matching filler (same composition as base metal): Replenishes lost elements but does not alter solidification or grain structure.
• Overmatching filler (higher alloy content or additional elements): Actively modifies the weld microstructure to compensate for the effects of evaporation even after the loss has occurred.

The AZ61 filler used with AZ31B base metal is a mild overmatch — it contains higher aluminum content (6% vs. 3%) and similar zinc content (approximately 1%). This extra aluminum compensates for Al loss through evaporation and provides additional precipitation hardening [7†L13-L14].

---

Process Parameter Optimization

Compensation through filler addition is most effective when combined with optimized laser parameters that minimize the driving force for evaporation.

1 Heat Input Control

Heat input $E$E (energy per unit length) is a primary control variable. For AZ31 magnesium alloy, research has established that:

• When $E=36\text{ J/mm}$E=36 J/mm, weld strength reaches the base metal level.
• When $E>66\text{ J/mm}$E>66 J/mm, the grain coarsening coefficient exceeds 3.5×, and evaporation losses become significant [11†L14-L15].

For a 5 mm thick plate, the optimal heat input range is 80–100 J/mm. Exceeding this range causes grain boundary precipitates to coarsen by 40%, which accelerates evaporative loss from the interdendritic regions [11†L15-L16].

The relationship between laser power $P$P and welding speed $v$v is captured by the ratio $P\mathrm{/}v$P/v. An increase in $P\mathrm{/}v$P/v of 10% drives a 17% increase in grain boundary migration, which can exacerbate the segregation of alloying elements and potentially increase local evaporation at grain boundaries [11†L15].

2 Pulsed vs. Continuous Wave Operation

Pulsed laser operation offers advantages for magnesium alloys because it reduces the time during which the weld pool is at peak temperature. At a fixed average power, increasing pulse frequency reduces the average weld pool temperature, which lowers mass loss per unit area. However, as previously noted, this must be balanced against the reduced weld volume that can result in higher effective concentration loss [15†L21-L29]. The optimum is typically found at moderate frequencies (20–35 Hz) where both peak temperature and weld penetration are controlled.

3 Beam Oscillation (Wobble)

Oscillating the laser beam during welding widens the melt pool without proportionally increasing the peak temperature at the keyhole. This technique has been shown to:

• Increase the stability of the keyhole by distributing the energy over a larger area.
• Reduce the temperature gradient at the keyhole wall, lowering the evaporation rate.
• Improve the mixing of filler wire with the base metal, aiding compositional homogeneity.

The wobble amplitude and frequency interact with filler wire feed rate; optimal results are achieved when the wobble pattern overlaps the filler wire entry point, ensuring uniform distribution of compensating material.

---

Rare-Earth Alloying: A Material-Based Compensation Strategy

Rare-earth (RE) elements — particularly Gd, Y, Ce, and Nd — modify the weld pool behavior, solidification sequence, and final microstructure in ways that counteract evaporation damage.

1 Grain Refinement as a Compensation Mechanism

Evaporation loss removes alloying elements that would otherwise form grain‑refining precipitates. Adding RE elements creates new, thermally stable precipitates that serve the same function. For example, RE elements (Gd, Y, Ce) form high‑melting‑point precipitates such as Al–Ce and Mg–Gd–Y phases that pin grain boundaries and limit grain growth during solidification. The incorporation of zirconium (Zr) further refines grain size to as low as 13 μm through the Zener pinning effect [11†L16-L18].

The effectiveness of grain refinement as a compensation mechanism is demonstrated by the Mg-3Nd-1.5Gd-0.5Zr-0.3Zn filler developed for AZ31 magnesium alloy joints. This novel filler produces refined grain structures in the weld zone through enhanced dynamic recrystallization (DRX) and Zener pinning from fine RE-rich precipitates. The resulting weldments exhibit 3–9% higher tensile strength and superior microhardness compared to joints made with conventional AZ31 and AZ61 fillers — despite the fact that the base metal lost some elements through evaporation [10†L40-L46].

2 Porosity and Crack Suppression

Cerium (Ce) acts as a hydrogen trap in the weld pool, with an adsorption capacity of 14,500 mL per 100g, reducing hydrogen‑induced porosity by 82% [11†L19]. This is a chemical compensation that directly addresses a consequence of evaporation and moisture absorption.

Precipitates containing Ce that form at grain boundaries suppress the formation of liquid films at high temperatures, reducing crack sensitivity by 60% [11†L19-L20]. Gadolinium (Gd) stabilizes the keyhole by reducing oscillation frequency by 40% and increasing melt pool surface tension by 25% [11†L20]. This stabilization helps maintain a stable keyhole even when the evaporative loss of matrix elements would otherwise cause collapse.

3 High-Temperature Performance Retention

One of the major concerns with evaporation loss is the resulting softening of the weld at elevated service temperatures. RE elements create thermally stable precipitates — Mg–Nd phases that remain intact at 300 °C, maintaining creep strength at 185 MPa [11†L21]. Furthermore, RE elements reduce thermal stress concentration factors from 3.2 to 1.8 and increase crack propagation resistance by 2.5× [11†L21-L22]. These improvements directly offset the loss of Mg and Zn from the weld matrix.

A 2025 study on rare-earth-enriched filler for GTAW of AZ31 (which has direct relevance to laser welding) confirmed that RE elements produce grain refinement and reduced porosity, leading to improved mechanical properties and a transition from brittle to ductile fracture behavior [10†L47-L49].

---

Activated Fluxes and Surface Modification

When filler wire addition is impractical — for thin sheets, complex geometries, or micro‑welds — activated fluxes provide a non‑consumable method to influence evaporation and penetration.

1 SiO₂ Flux for AZ31

Research on activating fluxes in pulsed laser beam welding of AZ31 magnesium alloy has demonstrated that SiO₂ flux is particularly effective in improving weld penetration when the heat input is at a critical value: $E=53.42\text{ J/mm}$E=53.42 J/mm [12†L12-L14]. The mechanism involves the flux modifying the surface tension of the melt pool, changing the Marangoni flow direction from outward to inward. This inward flow concentrates heat at the center of the keyhole, increasing penetration without proportionally increasing evaporation.

2 Surface Micro‑Alloying with RE Powders

A patented method for surface micro‑alloying of magnesium alloy weld joints uses a mixed powder containing Al, Ti, Cr, Mn, Ni, and RE elements (Sc, Nd). The powder is applied to the weld surface and melted using a laser in a second pass. This creates a surface protective layer on the weld — a “skin” that seals the weld metal against further evaporation loss in service and provides a barrier that reduces the need for bulk filler addition [2†L16-L18].

This method is particularly useful for applications where adding filler wire would require tooling changes or where the geometry prevents consistent wire feeding.

---

Integration: Multi‑Strategy Compensation Protocol

No single compensation method addresses all aspects of magnesium evaporation loss. A production protocol combining multiple strategies is required for consistent results:

1. Baseline parameter optimization: Set heat input to the optimal range for the alloy and thickness (e.g., 36–66 J/mm for AZ31). Use beam oscillation (wobble) to stabilize the keyhole.
2. Filler wire selection: For sheet thickness ≥1.5 mm, use overmatch filler (AZ61 with AZ31 base) to compensate for element loss. Set filler wire feed rate sufficiently high to minimize porosity and avoid underfill.
3. In‑process monitoring: Monitor vapor plume characteristics (color, intensity, and stability). Sudden changes in plume behavior indicate loss of keyhole stability and the onset of excessive evaporation.
4. Post‑weld verification: Conduct cross‑sectional analysis of weld metal chemistry for critical applications. Acceptable Mg loss is ≤5% of nominal. If loss exceeds this, adjust filler composition or increase filler feed rate.
5. For thin sheet (<1.0 mm): Use activated flux (SiO₂) with optimized heat input in place of filler wire. Follow with surface micro‑alloying using RE‑containing powder if corrosion resistance is critical.

---

Conclusion

Magnesium evaporation during laser welding is not merely a defect mechanism — it is a physical inevitability driven by the narrow gap between melting and boiling points of magnesium and its volatile alloying elements. Compensation is not optional for high‑integrity applications; it is mandatory.

Three practical compensation strategies have been validated:

• Filler wire addition: Overmatch compositions (AZ61 with AZ31) compensate for elemental loss, reduce porosity through keyhole stabilization, and improve mechanical properties. Feed rate is a first‑order control parameter — low feed rates increase porosity, counterintuitively.
• Process parameter optimization: Heat input control (36–66 J/mm for AZ31) and moderate pulse frequencies reduce evaporation without sacrificing penetration. Beam oscillation widens the stable processing window.
• Rare‑earth alloying: RE elements (Gd, Y, Ce, Nd) create high‑temperature precipitates that refine grains, suppress porosity, stabilize the keyhole, and retain mechanical properties at service temperatures — effectively offsetting the loss of volatile elements.

For production welding of magnesium alloys above 1.5 mm thickness, filler wire addition combined with optimized parameters is the baseline standard. For thin sheet or micro‑welding, activated fluxes and surface micro‑alloying provide viable alternatives. The integration of multiple strategies yields the most robust process.

Read More： Distinction Between Laser Brazing and Laser Fusion Welding

Preventing Zinc Vaporization Pitfalls in Laser Welding of Galvanized Steel

---

Frequently Asked Questions

Q: How do I know if my weld has suffered significant evaporation loss without destructive testing?

A: Monitor the vapor plume: a large, bright plume that extends more than 5 mm above the workpiece indicates excessive evaporation. Also, observe the weld surface: excessive spatter, underfill, or surface discoloration (dark gray to black oxides) suggests that the alloying elements have been depleted and the weld chemistry has changed.

Q: Can I use the same filler wire for all magnesium alloys?

A: No. The filler composition must be matched or slightly overmatched to the base metal. For AZ31 (3% Al, 1% Zn), AZ61 (6% Al, 1% Zn) is an appropriate overmatch. For higher‑aluminum alloys such as AZ91 (9% Al), an AZ92 filler (9% Al, 2% Zn) is recommended. Using an incompatible filler can produce brittle intermetallic phases.

Q: Why does lowering the filler wire feed rate increase porosity?

A: A lower feed rate reduces the volume of molten filler entering the keyhole, making the keyhole walls less stable. This instability causes the keyhole to collapse intermittently, trapping vapor bubbles that become porosity. Higher feed rates provide a more consistent flow of material that stabilizes the keyhole and allows trapped vapor to escape.

Q: Are there any magnesium alloys that do not require evaporation compensation?

A: No. All magnesium alloys contain magnesium as the primary element, and magnesium has a high vapor pressure at welding temperatures. However, alloys with higher aluminum content (such as AZ91) experience less relative matrix loss because a smaller proportion of the weld pool is elemental Mg. But compensation is still recommended for consistent mechanical properties.

Q: Does adding RE elements to the filler wire change its handling or feeding characteristics?

A: RE elements make the filler wire slightly harder and more brittle, which can increase the tendency to break during feeding. Use a larger diameter wire (1.2 mm instead of 1.0 mm) and adjust the wire feeding rollers to apply less pressure. For applications where wire breakage is a concern, powder‑fed laser cladding with RE additions is an alternative approach.

---

References

1. Yihai Laser Cutting Machine Operation Manual. Section 2.6: Nitrogen safety; Section 5.1.4: Cutting auxiliary gas — gas purity and effects. (Company internal document)
2. Haferkamp, H., Bach, Fr.-W., & Burmester, I. (2001). Nd: YAG Laser Beam Welding of Magnesium Constructions. (Cited in [1†L14-L15])
3. Malekshahi Beiranvand, Z., Malek Ghaini, F., Naffakh Moosavy, H., et al. (2020). The relation between magnesium evaporation and laser absorption and weld penetration in pulsed laser welding of aluminum alloys: Experimental and numerical investigations. Optics & Laser Technology, 128, 106170. [14†L6-L14]
4. Ryu, C.-S., Bang, K.-S., Lee, M.-Y., & Chang, W.-S. (2008). *Laser Welding of AZ31B-H24 Mg Alloy with AZ61 Filler Wire*. Journal of KWJS, 26(6), 29-35. [7†L3-L14][8†L2-L16]
5. Slovak University of Technology. (2023). Effect of Filler Wire Feed Rate on the Formation of Porosity in Laser Welded Joints of Magnesium Alloy AZ31B–H24. Metals, 13(3), 460. [9†L4-L11]
6. Beiranvand, Z. M., et al. (2018). Magnesium Loss in Nd: YAG Pulsed Laser Welding of Aluminum Alloys. Metallurgical and Materials Transactions B, 49(5), 2896-2905. [15†L4-L6]
7. Shen, J., et al. (2012). Effects of activating fluxes on pulsed laser beam-welded AZ31 magnesium alloy. Science and Technology of Welding and Joining, 17, 665-671. [12†L3-L8]
8. (2025). Enhanced microstructure and mechanical properties of AZ31 magnesium alloy joints using rare-earth-enriched filler produced by GTAW. Results in Materials, 100910. [10†L5-L8]
9. (2025). 综述：稀土（RE）元素对镁合金激光焊接接头性能的影响：综述. Journal of Luminescence. [11†L2-L8]
10. (2025). Laser Welding Techniques of Magnesium Alloys — “A Brief Review”. National Academy of Sciences Letters, Springer Nature. [16†L2-L5]
11. Cao, X., Jahazi, M., & Immarigeon, J.P. (2006). A review of laser welding techniques for Mg alloys. Journal of Materials Processing Technology, 171, 188-204. [8†L33-L35]