The pure titanium whose corrosion resistance and formability are excellent, can maximize the heat conduction efficiency because it is processed into a thin plates. When plate-type heat exchanger is manufactured, the pure titanium with superior properties is being used as the material of heat exchanger plates in different sizes and shapes. Since plate-type heat exchanger was developed at the end of 19th century, it has been widely used in many industries such as refrigeration, air-conditioning system, food processing sector, dairy production process, chemical, petroleum, and shipbuilding.
The welded heat exchanger which is one of the plate-type heat exchanger can reduce the risk of leakage more than gasket-type heat exchanger. welding area in the welded type is divided into three parts ; straight, curved and overlap. However because the pure titanium has a strong chemical affinity with hydrogen, oxygen and nitrogen in the high temperature, it reacts with these elements during welding and then oxide and nitride emerge. As a results, brittle region is formed. Thus the weld must be protected from the atmosphere by using appropriate shielding conditions. In addition, controlling the overlap weld is important because it is easy to occur the welding defect such as crack, humping bead and porosity in the area overlapped welding start and end.
Welding heat source used in this paper is a continuous wave fiber laser with a maximum output of 6.3 kW having a wavelength of 1,070 nm and the material used in this study was 0.5 mmt pure titanium. The parameters for fundamental welding parameters were optimized through the bead welding and the parameters for the defect suppression in the lap welding were controlled.
First of all, experiments were performed by changing defocused distance, tilting angle, type and flow rate of shielding gases, variables of side shield nozzle and coaxial shield nozzle in the bead welding. It determined welding conditions obtaining the sound bead with the deepest penetration by changing defocused distance and tilting angle. Furthermore, it could be seen than various colors of bead were obtained by changing the shielding conditions at the same laser power and welding speed. The welds with bead colors of silvery white, gold, brown and blue were sound and not brominated while in case of purple, yellow and gray bead, the welds were brominated and the hardness was increased. The more oxidation and nitrification occurred, the darker color in microstructure became and a thin layer was observed in the surface of the weld. As a result of EPMA analysis, it was estimated that this thin layer was the oxide layer and as oxidation proceeded more, thickness of this layer was increased. Also, when the weld didn''t protected properly form the atmosphere, the content of oxygen and nitrogen tended to be increased in bead surface and overall weld.
Second, crater and humping bead were suppressed by applying the wave shape control in the overlap welding. The experiments were conducted by changing slope up & down time and tailing power. After then the bead shape was observed and the maximum depth and the area of crater were measured. If slope down time was increased, it tended to reduce the maximum depth and the area of crater because molten metal could fill the crater as solidification rate become slow. Moreover by deriving the appropriate length of the overlap weld, it is possible to suppress the humping bead caused by the lack of penetration.
Third, the hardness and tensile-shear test were carried out to evaluate the mechanical properties of overlap weld according to applying the wave shape control. As a result of hardness test, if wave shape control didn''t applied to the overlap weld, the hardness of weld increased due to rapid solidification rate. but, when wave shape was controlled during overlap welding, it could be possible to prevent the increasing of hardness through decrasing the solidification rate. After the tensile-shear test, if wave shape control didn''t applied to the overlap weld, the fracture occurred in the weld whereas if wave shape control was applied to the overlap weld, the fracture occurred in the base metal and the properties of tensile-shear in the overlap weld was more excellent.
As mentioned above, the oxidation of the weld was prevent by depriving optimal shielding condition and the crater and the humping bead in the overlap weld were suppressed by applying the wave shape control of laser power. Thus, it is expected that the results of this study can be utilized not only the production of welded plate heat exchangers but also important data on laser welding application of titanium.