By Dr. Mohammed Naeem
Laser welding of dissimilar materials is a dynamic process and its time has arrived in many industries. Where dissimilar metallic joining of components is required in electronics, medical devices, consumer goods, vehicular and aerospace applications, fiber laser welding stands out as a superior process because it reduces manufacturing costs and offers design flexibility.
Prima Power Laserdyne has an in-depth understanding of the challenges of welding dissimilar materials and has developed new technical and process controls which provide effective new solutions.
Differences Of Physical And Chemical Properties Greatly Effect Outcome
In theory, a laser can weld any material which 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 expansion, problems can occur making the resulting joint unacceptable.
Illustrated in Table 1 is the weldability of metal pairs. In the welding of 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 is obtained and often involves the formation of brittle intermetallics.
Nd:YAG Lasers Ideal Systems For Welding Dissimilar Metals
In the past, most dissimilar welding projects were done 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: Example of ramp-up pulse shape|
Prima Power Laserdyne has developed a range of pulse shapes to improve weld quality by reducing weld cracking and porosity. The 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, electronic, and aerospace. A variety of pulse shapes were generated using the new LASERDYNE® 811 system with the S94P controller which includes a full 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 highlights two examples where pulse shaping was used to improve the weld quality during the laser welding of dissimilar materials.
Example 1: Automotive welding application
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. One part of an automotive component requires joining 304 stainless steel to grey cast iron in a partial overlap weld configuration. Currently, this part is being welded with electron beam welding (EBW) to reduce the formation of excessive porosity and eliminate interface cracking. The end user is 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 a 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 which were capable of producing same quality or better welds compared to EBW, i.e. no porosity and no interface cracking. The laser parameter development work, including pulse shape, was carried out with a 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).
|Figure 2: 2 mm thick 304 SS + 4.5 mm thick grey cast-iron; overlap joint; N2 shield gas.||Figure 3: 2 mm thick 304 SS + 4.5 mm thick grey cast-iron; overlap joint; N2 shield gas; with pulse shaping.|
Example 2: Aerospace welding application
Welding and joining techniques play an important role in the aerospace industry for both manufacturing new parts and repairs of aerospace structures and components. A majority of aeroengine components are made from nickel based super alloys. The majority of these aeroengine materials are susceptible to porosity or 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 component in the aeroengine requires 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 bearing in mind that both of these alloys are prone to cracking when welded individually.
Figure 4 shows a dissimilar weld joints of two nickel based alloys welded using 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 micro cracking but no porosity, whereas welds made with argon shield gas had no cracking but excessive porosity. Reduced porosity with nitrogen shielding gas is due to reduced surface tension of the molten pool and hence the bubbles are more easily able to escape the weld pool.
|Figure 4: Haynes 230 + Waspaloy; overlap joint; CW output.|
Further tests were performed with pulse shaping to improve the weld quality. These operations were performed with nitrogen shield gas only. The results show in Figure 5a and Figure 5b 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.
|Figure 5a: Haynes 230 + Waspaloy; overlap joint with pulse shaping; N2 shield gas; no interface cracking or porosity at the root of the weld.|
|Figure 5b: Magnified photos of the material and welded material.|
Prima Power Laserdyne’s developmental work with laser pulse shaping is key to achieving high quality welds of dissimilar materials. These new processes integrated into the LASERDYNE S94P controller significantly improve weld quality by resolving microcracking and the formation of micro and macro porosity challenges.