The concept of combining a laser welding system with a conventional arc welding system (GTAW) was first proposed in order to improve the stability of the laser welding system and to allow greater flexibility in part fit-up. Prior research stated that using a YAG laser at a power of 3 kW, one was able to hybrid laser weld a 4-mm-thick aluminum alloy at a speed of 4 n/min. For a mild steel plate, butt welding at 1 m/min with 5 kW of 6-mm-thick plate was realized. Just as significantly as the weld speed was the ability to hybrid laser weld with gaps up to 1.5 mm in a plate 6 mm thick. The glass metal arc welding (GMAW) laser hybrid process can increase the gap bridging ability, i.e., it appreciably broadens the range of tolerances with regard to edge preparation quality. The arc’s energy input in the hybrid welding process also permits control of the cooling conditions. Via the keyhole, the laser beam brings about easier ignition of the arc, stabilization of the arc welding process, and penetration of the energy deep into the material. The improvement of the energy input leads to a greater welding depth and speed being achieved with the hybrid process compared with individual processes on their own.

The hybrid laser process offers significant advantages in terms of welding speed with respect to the conventional GMAW or GTAW processes without the need for very close fit-up. This work compares an autogenous laser weld to a hybrid laser weld in a 6.3-mm-thick structural steel called HSLA-65. A thermal analysis was conducted for both welds based on a constrained optimization technique. The results of that technique allow extraction of cooling curves at any point in the weld. The cooling curves are then combined with the chemical composition in order to calculate the volume fractions of microstructures and their hardness. The hardness was mapped over the entire weld, and the experimental results were compared to the calculations.

The entire fusion line of the weld was used as input for the thermal model, which uses a constrained optimization process; input for the thermal model is effected via the specification of constraint conditions defined according to experimental information concerning solidification cross-sections. The thermal model allows extraction of the cooling curves at any point in the weld. These curves indicate that the cooling time from 800 to 500 ºC is independent of peak temperature and thus dependent only on the thermal properties of the steel.

An autogenous laser beam weld was fabricated in 6.3-mm-thick HSLA-65 steel at a Nd:YAG laser power of 4 kW and a welding speed of 0.63 m/min. The corresponding hybrid laser weld was fabricated at the same power and a welding speed of 0.89 m/min. The GMAW had a voltage setting of 26.8 volts, a current setting of 190 amps, and a wire speed of 17.3 m/min with a gas shielding of 90% Argon/10% CO2. The plates were butted against each other without any gap.

Based on the composition, the cooling curves, and a prior austenite grain size, the volume fractions of the daughter products of the austenite decomposition were determined. The results of this calculation indicate that the microstructure is 99% martensite for both welds. The microstructure of the autogenous weld has a calculated hardness of 348 HV, whereas the hardness of the hybrid laser weld has a calculated hardness of 365 HV. The hardness of both welds was mapped over the entire weld at incremental steps of 0.25 mm using an automated hardness indenter. The maximum hardness measured in the autogenous weld was 360 HV, whereas in the hybrid laser weld the maximum hardness was 388 HV.

The maps indicate significant areas in the fusion and HAZ where the hardness exceeds 350 HV. The addition of filler metal does not necessarily modify the cooling rate sufficiently to alleviate the high hardness associated with the formation of martensite. This is especially true when the carbon content of the consumable is greater than the carbon content of the base plate.

Laser hybrid welding permits gaps in the joint that are unacceptable with autogenous laser welding. The selection of the consumable must be done with great care since the hardness of the hybrid laser weld can be greater that that of the autogenous laser weld.

This work was done by E. A. Metzbower, D. W. Moon, C. R. Feng, and S. G. Lambrakos of the Naval Research Laboratory, and P. E. Denney of Edison Welding Institute. NRL-0055

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Thermal Analysis and Microhardness Mapping in Hybrid Laser Welds in a Structural Steel

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This article first appeared in the August, 2012 issue of Defense Tech Briefs Magazine.

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