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Laser Pulse Shaping for Joining Dissimilar Materials

May 17, 2023

Laser welding of dissimilar materials is a dynamic process and its time has arrived.

Where such welding is required—in electronics, medical devices, consumer goods, automotive and aerospace applications—fiber laser welding stands out as a superior process. It reduces manufacturing costs while offering design flexibility.

In theory, a laser can weld any material that can be joined by conventional processes. However, because of their differences in physical and chemical properties—such as melting and boiling points, thermal conductivity, density and coefficient of expansion—problems can occur when welding dissimilar materials, making the resulting joint unacceptable.

Illustrated in Table 1 is the weldability of metal pairs. When welding dissimilar metals, good solid solubility is essential for sound weld properties. This is achieved only with metals having compatible melting temperature ranges. If the melting temperature of one material is near the vaporization temperature of the other, poor weldability occurs and often means the formation of brittle intermetallic structures.

In the past, most dissimilar welding projects were performed with pulsed lamp Nd:YAG lasers. Lamp-pumped lasers are capable of producing long, multi-millisecond pulses with peak powers many times above the rated average power of the laser, provided that the duty cycle is sufficiently low. High peak power pulsed, lamp-pumped Nd:YAG lasers, coupled with pulse shaping capabilities, make these lasers ideal for welding dissimilar materials. Weld depth that is too deep—which can lead to defective joints and also insufficient weld depths—can be avoided by adjusting the starting power and the correct ending power to the joint geometry and the material properties (Figure 1)

At Prima Power Laserdyne, welding experts have developed a range of pulse shapes to improve weld quality by reducing weld cracking and porosity. Their focus has been to provide dissimilar material welding solutions in product applications prone to welding defects such as cracks, porosity or a combination of both. The industries most commonly affected include automotive, medical, electronics and aerospace. A variety of pulse shapes were generated using the new LASERDYNE 811 system with a S94P controller, which includes a complement of hardware and software features designed for pulse shaping. These projects were accomplished with both continuous wave (CW) and quasi-continuous wave (QCW) fiber lasers.

The following are two examples where pulse shaping was used to improve weld quality during the laser welding of dissimilar materials.

Grey cast iron is widely used in the automotive industry. A major limitation is the weldability of a dissimilar material onto cast iron due to hot cracking and the formation of porosity as a result of the lack of ductility from the graphite and casting process. In the first example, one part of an automotive component required joining 304 stainless steel to grey cast iron in a partial overlap weld configuration. In the previous process, the part was welded with electron beam welding (EBW) to reduce the formation of excessive porosity and eliminate interface cracking. The end user was keen to replace the EBW with laser beam welding (LBW) to reduce the cost per weld and the weld preparation. The biggest difference is that EBW is performed in vacuum, whereas laser welding is carried out in an ambient air pressure environment and the danger of X-rays is eliminated from the process. Developmental work was carried out to design laser parameters that were capable of producing the same or better quality welds compared to EBW, i.e. no porosity or interface cracking. The laser parameter development work including pulse shape was carried out with the CW fiber laser.

Microscopic examinations of the weld metal made with a standard CW laser output exhibited severe porosity in the cast iron portion of the weld (Figure 2). There was no sign of any microcracking at joint interface. The weld made with the LASERDYNE S94P controller and pulse shaping produced porosity free welds (Figure 3).

The second example focuses on the aerospace industry. Welding and joining techniques play an important role in aerospace, both for manufacturing new parts and for repairs of aerospace structures and components. A majority of aero-engine components are made from nickel-based superalloys. The majority of these aero-engine materials are susceptible to porosity, cracking or both during laser welding. The risk of weld cracking and the formation of porosity depends on the welding conditions. To a large extent, these welding defects can be avoided by changing the welding process, i.e. optimizing the laser and processing parameter.

One aerospace component required laser welding of Haynes 230 (solution-treated nickel-chromium-tungsten-molybdenum alloy) to Waspaloy (age-hardenable, nickel-chromium-cobalt superalloy) in an overlap weld configuration. The weld quality requirements were no cracking or porosity in the fusion zone, considering that both of these alloys are prone to cracking when welded individually.

Figure 4 shows a dissimilar weld joint between two nickel-based alloys welded with CW output. The weld was made with two different shield gases, i.e. nitrogen and argon respectively. Laser welding with nitrogen shield gas resulted in interface microcracking but no porosity, whereas welds made with argon shield gas had no cracking but excessive porosity. The reduced porosity with nitrogen shielding gas was due to reduced surface tension of the molten pool, allowing the bubbles to more easily escape the weld pool.

Further tests were performed with pulse shaping to improve the weld quality. These operations were performed with nitrogen shield gas only. The results shown in Figure 5a and Figure 5b show that there was no sign of any microcracking at the joint interface. The weld penetration and interface width are slightly different compared to welds made with the CW output; however, the weld shape can be controlled by adjusting the average power and weld speed without changing the pulse shape configuration.

Prima Power Laserdyne's developmental work with laser pulse shaping has achieved high quality welds of dissimilar materials. These new processes and the capabilities of the LASERDYNE SP94 controller improve weld quality by resolving microcracking and formation of micro and macro porosity. The company has developed additional pulse shape processes to evaluate and improve many other applications for fiber laser welding.

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Mohammed Naeem