디스크 레이저를 이용한 순 타이타늄의 비드용접시 비드색에 따른 용접특성
Weldability with bead colors during disk laser bead welding of a pure titanium
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본 연구에서는 최대 출력 3.3kW의 연속 출력 파형 디스크 레이저를 사용하여 순 타이타늄 용접을 실시하였다. 타이타늄은 용접시 고온에서 산소, 질소, 수소 및 탄소 등의 침입형 원소와의 반응성이 매우 높으므로 산화 및 질화에 대한 취성영역이 형성되어 용접부의 내식성과 기계적 성질을 저하시킬 수 있다. 따라서 최적의 실드가스를 선정하기 위하여 실드가스의 종류를 변화시켜 실험을 실시하였으며 그에 따른 용접성을 평가하였다. 비드색에 따른 용접부에 대하여 광학현미경 및 주사전자현미경을 이용하여 미세조직을 관찰하였다. 또한 경도측정을 통해 기계적 특성을 파악하였다. 용접부가 산화 또는 질화될 경우 비드표면에 덴드라이트 형상의 조직이 관찰되었으며 비드표면뿐만 아니라 용접부 내부까지 산소 및 질소량이 증가하여 경도값이 모재 대비 3배 이상 크게 증가하였다. 결과적으로 비드색에 따라 용접부의 미세조직, 기계적인 특성 등이 달라짐을 알 수 있었다.
Abstract
In this study, pure titanium welding was performed using a continuous output waveform disk laser with a maximum output of 3.3 kW. Titanium is highly reactive with interstitial elements such as oxygen, nitrogen, hydrogen and carbon at a high temperature during welding, and thus a brittle region is formed owing to oxidation and nitridation, which may deteriorate the corrosion resistance and mechanical properties of the welded portion. Therefore, in order to select the optimal shield gas, experiments were conducted by varying the type of shield gas and the weldability was evaluated. The changes in the microstructures of the welds with respect to the bead colors were examined using an optical microscope and a scanning electron microscope. The mechanical properties were also determined by performing a hardness measurement. When the welds were oxidized or nitrided, a dendritic structure was observed on the surface of the bead. The oxygen and nitrogen content increased not only on the bead surface but also inside the weld, resulting in an increase in the hardness value of more than 3 times that of the base metal. As a result, the microstructure and mechanical properties of the welds were found to vary depending on the bead color.
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