By Lorraine Blais, Novika Solutions
In the last years, several groups have worked on the development of hybrid laser/GMAW welding (HLAW), for various segments of the industry. Depending on the intended application, the objectives are significantly different, and the definition of quality itself can differ a lot. Nevertheless, new HLAW applications usually benefit from the ability of the process to produce deep welds in one pass without having to chamfer the parts. As shown on Figure 1, the HLAW welds are drastically smaller than GMAW welds, going from several passes (six on the picture on the left) with chamfer and back gouging to a single pass without chamfer.
Figure 1: GMAW weld vs HLAW weld on 12mm HSLA steel.
Recent tests showed that, when compared to manual GMAW, HLAW brings a 90% diminution in welding time and consumables (see table below).
Table 1: Consumables and time savings for 12mm HSLA steel.
However, to survive the transition from laboratory to production floor, the process must perform in the following ways:
- Produce welds showing good mechanical properties, as defined by the application
- Produce these quality welds repeatedly despite the variations inherent to the industrial environment
- Minimize operation costs.
Here below is a brief discussion on the points to consider in order to meet these results.
PARTS PREPARATION
Although HLAW is less demanding than autogenous laser welding when it comes to parts fit-up, it still often requires improvements in the quality of the joints, when compared to standard arc or resistance welding processes. HLAW is particularly interesting for the realization of long welds for which machining can be necessary to ensure a constant fit-up, thus bringing important preparation costs.
Working on the process to make it more tolerant to variations, it is possible to reduce the tolerance requirements so that laser-cut parts can be used directly, without having to machine the edges. However, the joint fit-up still needs to be sufficiently good to maintain the gap below a certain limit, e.g. 2mm maximum gap for 12mm thick steel parts.
These looser precision requirements combined to the ability of the HLAW to weld thick material in one pass, without chamfering, lead to huge savings in term of machining.
ADAPTIVE WELDING
To reduce the requirements related to machining and positioning, the preferred avenue is the use of adaptive welding. Unless one is able to tolerate low quality or justify investing heavily in the precision of the primary parts, making HLAW an adaptive process becomes necessary, in order to ensure the profitability of the process. This is achieved by determining the welding parameters in real-time, according to the actual configuration of the joints – and not their theoretical position.
A camera placed right in front of the welding head evaluates the relative position of the workpieces: the controller (PLC, PC) selects the welding parameters to match the actual gaps and applies them automatically. The welding parameters are stored in a database, allowing automatic selection of proper parameters for all acceptable configurations, i.e. all configurations for which it is possible to obtain welds that meet the quality requirements. This allows maintaining the quality of the welds in an economically beneficial way, using reasonable dimensional tolerances for the parts and their positioning.
Control of the Cooling Rate
Everyone wants to weld fast to achieve a high productivity. However, success rarely lies in excess… Very fast welding, as permitted by processes using lasers, leads to high cooling rates, which can cause the formation of fragile metallic phases.
In contrast, slow cooling rates can lead to a coarse lamellar grain structure, conducive to cracks propagation. The use of a CCT diagram may be essential to develop good HLAW welding parameters, producing welds having the desired mechanical characteristics.
Thus, a 12mm deep HLAW weld on HSLA steel (full penetration in one pass) led to the formation of coarse lamellar ferrite, responsible for bad performances in Charpy testing. By reducing the wire feed speed, the cooling rate decreased sufficiently so that the absorbed energy at -30°C went up from 40J to100J. Hardness remained at all times lower than that produced by GMAW (see Figure 2 below)
Figure 2: Micro-hardness measurements.
On the other hand, a 19mm deep HLAW weld on alloyed steel (partial penetration, one pass) experienced high cooling rates leading to the formation of very hard and brittle martensite. Preheating the parts allowed controlling the metallic phases formed during cooling and obtaining resilient sound welds.
Control of the Plume
High power laser welding causes the formation of a partially ionized cloud, having a high density of fine particles which remain suspended between the laser optics and the material to be welded. This laser-generated plume scatters part of the laser beam, reducing significantly the efficiency and the stability of HLAW.
To overcome this problem, it is necessary to get rid of suspended particles, either by blowing them off or by extracting them. Tests performed at Novika showed that at 15kW of laser power, the addition of a plume management system brought a 77% decrease in the average attenuation of the laser beam, helping to improve efficiency and stability of the HLAW process. While implementing such a system, care should be taken not to interfere with the GMAW shielding gas while being close enough to the torch and laser beam to extract the fume.
Figure 3: HLAW welding without (left) and with (right) plume management.
CONCLUSION
Development of a performing HLAW process requires particular care and knowledge, and its implementation can be quite costly. But once this process is in production, it can bring huge benefits in terms of productivity as well as savings in consumables and labor cost. And with the great performances and decreasing cost of the high power fiber and disk lasers, one can expect better and better ROI’s, and stronger presence of the HLAW process on the shop floors.