Nd:YAG 레이저 용접을 이용한 이종소재 CFRP와 Steel의 용접 메카니즘에 대한 연구

Abstract

ABSTRACT

Study on welding mechanism for dissimilar materials of CFRP/steel by using Nd:YAG laser welding

Choi Woo-Seong Advisor : Prof. Bang, Hee-Seon, Ph.D. Department of Welding and Joining Science Engineering, Graduate School of Chosun University

Recently, the world is faced with the environmental problem such as greenhouse gases which causes global warming. Automobile industries are trying to solve the problem by reducing the exhaust gas and improving fuel-efficiency. Various solutions have been being suggested by the main countries in Europe, Asia, Northern America. Among many suggested solutions, the most practical and efficient one is to reduce the weight of the automobile. The lightweight materials applied to automobile is HSS, AHSS, Magnesium, Aluminium, composite materials, etc. When the weight is reduced 10%, the fuel efficiency increases 8% and the amount of exhaust gases reduce about 4~6%.

The existing way of joining CFRP and Steel are adhesive bond and mechanical joining. Those two ways are used widely in automobile industries, however additional cost for bonds and rivets is occurred and they make the products weight incraese. To remove those weaknesses many researches about joining CFRP/Steel by laser welding have been progressed through the world. This study is focused on welding CFRP and Steel by laser heat source. For the experiment, PA66, thermalplastic resin has been used for CFRP and SGAFC 590DP and SPFC 590DP have been used for steel. BOP welding was processed to select proper laser power. To get proper condition of welding speed, the experiment was progressed on lapped steel. After observe Macro surface of the specimen by Optical microscope, dissimilar experiment was processed.

This study intends to apply laser welding by way of heat conduction welding as a bonding technique for dissimilar material CFRP – Steel. The materials used for the experiment are 1.5mm thick of CFRP and SPFC and 0.9mm thick of SGAFC. In this study, the weldability and welding mechanism was observed by Tensile – Shear strength test and also observed the heat flow through the specimen by numerical analysis.

The parameters that used for experiment are as follow. The common conditions for each CFRP/SGAFC and CFRP/SPFC are beam shape : circle and Tiling angle : 5ﾟ. Ar 98%, O2 2% mixed gas was used as shielding for the experiment and the amount of flow is 20L/min for SGAFC, 30L/min for SPFC. Other conditions such as laser power, welding speed and defocusing were changed because of different thickness of steel that used for the experiment.

The experiment for CFRP/SGAFC was processed under the conditions that follows. Laser power : 600W, Welding speed : 3mm/s, Shielding gas : 20L/min, Defocusing : 0, 5, 10mm, and 1.5 ~ 2.0㎛ of surface roughness was applied to compare the weldability. For CFRP/SPFC, the experiment conditions were P : 1000W, V : 5 – 8mm/s, Defocusing : 0, 5, 15mm, Shielding gas : 30L/min.

As a result of Tensile – Shear test for CFRP/SGAFC, the strength was different about 25% in accordance with surface roughness. Also the change of defocusing determined joined or fractured. The highest strength was 0.648kN in the condition of P : 600W, V : 3mm/s, Defocusing : 10mm, roughness : 1.5㎛ which is 0.028kN higher than 5mm of defocusing which is almost the same value of strength and 0.148kN higher than without surface roughness.

The result of Tensile – Shear test for CFRP/SPFC shows that the highest strength was 1.518kN in the condition of P : 1000W, V : 5mm/s, Defocusing : 15mm. Nevertheless, the highest strength occurred, lots of surface defects were occurred on bead characteristics due to excessive heat input. Because of the defects, the strength were not steady for welding speed of 5 and 6mm/s. In the condition of 7mm/s, the strength was 1.215kN which is 0.303kN lower than 5mm/s, but the value was steady and no defects occurred on bead.

In this study, the welding characteristic of dissimilar materials CFRP/Steel by laser heat source was observed. The temperature distribution was observed by numerical analysis and optical inspection was processed to get proper condition for the experiment. The weldability was confirmed by tensile – shear strength test and compared the values among various conditions.