Magnesium is the lightest structural material and has the best strength-to-weight ratio among metals that are commercially available. Its density is 36 percent lower than aluminum and 78 percent less than steel, making magnesium a seemingly ideal choice for lightweight construction. However, welding magnesium is — to put it mildly — challenging.
Pores and cracks are the two main problems. The issue of cracking has been largely resolved thanks to improvements in alloy production, but the pore formation problem remains. Pores in the weld bead affect the mechanical properties of the weld and in particular its tensile strength.To tackle this problem, it is important to understand how pores form and how they can be mitigated during the laser welding process.
The oxide layer is a key factor
A number of different mechanisms and factors can cause pore formation, including hydrogen pores, an unstable keyhole, pre-existing pores, surface coatings, gas entrapment, and alloys with a low vaporization point. Magnesium reacts easily with oxygen, causing an oxide layer to form on its surface. Furthermore, newer generations of magnesium alloy coatings — such as Keronite, Magoxid-Coat and Tagnite — all use oxide-based coatings.
The porous oxide layer on magnesium easily absorbs moisture from the atmosphere. This moisture leads to the formation of magnesium hydroxide (Mg(OH)₂). Magnesium hydroxide has a low decomposition temperature of about 200 degrees Celsius, at which point it releases the molecular water to the atmosphere (Mg(OH)₂ ⇌ MgO + H₂O).
The problem is that the water vapor thus released cannot escape through the two superimposed sheets. Instead, it seeks a way to escape through the molten pool, resulting in pore formation inside the weld bead.
Three ways of achieving the same goal with laser technology
One technique used to stop the oxide layer from triggering pore formation is preheating with a plasma arc. Our research group did this by introducing a plasma arc torch in front of the laser welding head. We used this method to weld the overlapping magnesium alloys and then compared the results with those obtained from welding without preheating. The preheating parameters were optimized based on the temperature at the faying surface.
Our results showed that using a plasma arc — in advance of the laser head and preheating the metal — can improve the quality of the weld by reducing the pore ratio. To verify the results, we carried out a separate experiment in which we heated the samples in a furnace prior to welding. We discovered that a preheating temperature higher than 200 degrees Celsius effectively mitigates pore formation.
Dual beam welding can effectively improve weld quality, yielding pore-free samples.
The second technique is dual beam welding, which involves welding magnesium alloys using two laser beams, one behind the other, with different beam ratios. A comparison of the beam ratios ( 42.5 : 57.5 | 35 : 65 | 27.5 : 72.5 und 20 : 80) in our experiments revealed that a ratio of 20 percent for the leading beam and 80 percent for the lagging beam can effectively improve weld quality, yielding pore-free samples.
When the lead beam taps 20 percent of the laser power for preheating, 80 percent of the laser power is left for welding, which is enough to form a stable keyhole. The keyhole acts as a chimney, exhausting the hydrogen gas from the decomposed magnesium hydroxide.
When we tested the other beam ratios, we found that the leading beam provided more than enough energy to preheat the sample, but the weaker lagging beam was not able to form a stable keyhole. This increased the likelihood of hydrogen gas being trapped in the solidified weld pool. So you need both mechanisms to happen simultaneously during laser welding: the leading beam preheating the faying surface to decompose the magnesium hydroxide and the lagging beam forming a stable keyhole to vent the hydrogen gas.
Another alternative is to use a two-pass laser welding procedure. In the first pass we directed a defocused laser beam across the top of the two overlapped sheets to preheat the faying surface prior to laser welding. The second pass was then used to melt and weld the samples. We optimized the laser power and focal distance for the preheating pass so as to obtain enough heat at the interface of the two overlapped sheets to produce higher quality welds.
Real time inspection
One way to detect pore formation non-destructively in real time is to use a spectrometer to measure the intensity of the laser-induced plasma plume. We know that the stability of the laser welding process affects the plasma plume, so we can use this knowledge to predict unstable conditions and pore formation in the weld.
We tested with two different situations on the surface of the sheet, one with an oxide layer at the faying surface and the other without this layer. Then we calculated the electron temperature from the spectrum intensity using the Boltzmann plot method and correlated this with the formation of pores. We found a good correlation between the calculated electron temperature detected at the laser-induced plasma and the presence of defects in the lasered welding beads.
A promising future for magnesium
Between 1995 and 2007 magnesium production jumped by 390 percent. This increase indicates the growing demand for magnesium in various industries including, but not limited to, the auto industry. There are still plenty of challenges in using magnesium alloys — in terms of both costs and joining techniques, but it’s clear that these alloys will be playing a more significant role in the future.
The author Masoud Harooni studied mechanical engineering at Isfahan University of Technology in Iran. He went on to earn his PhD from Southern Methodist University in Dallas, Texas, with a project on joining lightweight alloys that he carried out in collaboration with General Motors. He now works as a laser welding specialist at Keihin North America.