구조용 합금재료의 고효율 레이저-아크 하이브리드 용접특성

Abstract

Driven by the ever aggravating economic conditions and the increasing needs for improvement of atmospheric environment worldwide, environment-friendly shipbuilding is seeing ever growing demand to cut carbon emission and energy consumption. To withstand huge external forces and deformations, and to maintain high strength and toughness with reduced weight, AH36 steel or aluminum alloy a major environment-friendly material is widely used as ship structural material.

Meanwhile the welding technique using arc heat source still remains as the key technique in shipbuilding, although it began to be applied in earnest in the shipbuilding process some 60 years ago. Arc welding techniques used in shipbuilding include SMAW, FCAW, SMAW, SAW, EGW, GTAW, etc.. The structure materials used in shipbuilding are ones that improved strength and toughness, but they are mostly used as thick plates 8 mm or thicker. Application of arc welding process to such thick plates produces several problems. Specifically HAZ is widened, and material is deformed seriously due to high heat input. Besides multi-pass welding, which is done to obtain penetration weld zone, prolongs work time, consumes more materials, and after all raises economic cost. Therefore survival in the ever intensifyingcompetition in world shipbuilding market requires development of welding techniques that can ensure high efficiency, high quality, sound weld zone.

As such high efficiency, high quality welding techniques, laser-arc hybrid welding method is enjoying high attentions that combined laser welding technique that boasts of such advantages as deep weld penetration, narrow HAZ and fast welding speed, and arc welding technique that offers advantages like wide gap tolerance, affordable investment cost and easy workability, etc.. However laser-arc hybrid welding technique, since it uses two different heat sources at once, involves various and complicated interactions during welding. Therefore use of this technique requires survey of the process parameters each for the two heat sources and optimization of the surveyed process parameters.

In this paper, laser-arc hybrid welding device was set up using fiber laser and high power disc laser with the most widely used MIG arc, flexible automation and high beam quality. To identify the welding characteristics of the process parameters of welding processes, relevant process parameters were reviewed by welding 8 mm or thicker structural steel material and aluminum alloy material in one pass.

First, to identify the optimal process parameters of laser-arc hybrid welding process for SS400 and AH36 the structural steel materials, the process parameters of MIG welding and those of CW disc laser welding were reviewed. The steel materials could obtain deep penetration with ease owing to high rate of absorbing laser beam energy. Shield gas, the distance between laser and arc, laser power, welding speed, and the effects of arc current and current wave etc the major process parameters were surveyed. In hybrid welding, the distance between laser and arc at which maximum welding penetration depth is obtained with increase in welding current tended to increase, and also maximum penetration depth could be obtained at a greater minus non-focal distance with increase in the concave depth of molten pool by arc power. Also in restricting or removing welding defects like undercut, defect management could be optimized by controlling pulse correction instead of general spherical current wave.

Second, based on the reviewed contents of process parameters for steel materials, the characteristics of laser-arc hybrid welding of A5083 and A6061 the aluminum alloys were surveyed. Aluminum alloy materials showed low energy absorption rate, low melting temperature and selective evaporation of alloy ingredients due to high rate of laser beam reflection, and as a result they decreased in strength with pores easily generated. Therefore as a way to overcome such defects, aluminum alloys need to remain low in heat input amount. To that end, the process parameters in MIG pulse arc welding method, low heat input CMT welding method and CMT+pulse arc welding method were surveyed, and the process conditions to implement complete penetration welding in high power laser-arc hybrid welding were sought to be optimized. As a result, in laser-CMT+pulse arc hybrid welding, piercing welding was accomplished in 8 mm thick aluminum alloy material, and one pass welding was implemented at faster welding speed.

Third, the hardness characteristics and microscopic structure characteristics of SS400, AH36 steels and A5083, A6061 aluminum alloys were reviewed based on microhardness test, tensile strength test, observation of optical microscopy, and qualitative analysis by EDX was done. SS400 and AH36 steels showed weld zone strength equivalent to base metal, but decreased in toughness to some degree. AH36 steel was high in hardness values in HAZ due to increase in martensite fraction by Mn effect. A5083 material produced strength of 90% over the tensile strength of base metal, and A6061 material strength of 75% over base metal, demonstrating slight softening phenomenon. It is judged that this phenomenon occurred as aluminum alloy strengthening mechanism is collapsed by the selective evaporation of Mg whichis low in boiling point, and by formation of many pores in the molten metal during welding. Also it was found that laser-arc hybrid welding technique can be applied to the field with outstanding advantage allowing 1.2 mm maximum gap size in butt welding.

In conclusion, in this study, complete penetration welding conditions that barely accompany deformations in lower laser power were obtained with ease by applying laser-arc hybrid welding to various structural materials and alloy materials. Also it was found that laser-arc hybrid welding technique can be applied widely to offshore plants where relatively thick plates are used from the shipbuilding stage for large cruise liners by maximizing the gap-bridge effect of butt faces by arc and thus allowing wider scope of weldable gap. It is anticipated that such hybridization among heat sources will improve economic effects by implementing sound welding quality with no deformation by low heat input.