알루미늄 합금(Al5052) FSW T-joints 용접시 툴 형상이 용접성에 미치는 영향

Abstract

Currently, all countries of the world concentrate effort to conservate energy and improve environment for promoting the welfare of all mankind and the living standard. Especially, in terms of saving energy, the development of technology for welding and bonding of aluminum alloy is intensively invested for energy saving of the car and environmental protection. The use of lightweight aluminum alloy reduces the vehicle’s weight. When reduces weight 10%, the vehicle’s fuel efficiency is to be 8%.

This study is focused on welding of aluminum alloys T-joints which is very important in several fields of the transportation, especially in aerospace, automobile and shipbuilding, etc. T-joints are applied to be fabricated by traditional fusion welding such as MIG or Laser welding. However, fusion welding is diccicult for T-joints to avoid porosities, cracks, high residual stresses and distortions.

In this case, friction stir welding(FSW), one of the solid-state welding methods was invented at TWI(The Welding Institute), Cambridge, involves the joining of metals without fusion materials or filler materials; in other flank, it joins materials by using friction heat. This welding method is energy efficient, environment friendly, and versatile. It used to weld nonferrous light materials of Aluminum alloys, Magnesium alloys because of its low heat input.

Aluminum alloy 5052 with 3mm(stringer) and 5mm(skins) thickness is used as base metal for this study. The optimum welding parameters were shape of tool, tool rotation speeds, tool traveling speeds. The detailed parameters are as follows. Shape of tool : cylinder type, frustum type, tool rotation speeds : 400~500rpm, tool traveling speeds : 60~300mm/min.

The weldability was evaluated in accordance with mechanical and metallurgical characteristic with ratio of rotation and travel speed(tool rotation speed/tool travel speed). When using the cylinder type, a maximum tensile strength of 164MPa which is approximately 85% of base metal. When using the frustum type, a maximum tensile strength of 149.6MPa which is aprroximately 78% of base metal. the tensile strength of cylinder type is higer than those of frustum type. The fracture occurred in the round of skins (3mm). It indicates that the main factor affecting the tensile properties along the skin directly is not tunnel defects but kissing bond defects. The Vickers hardness values in nugget zone(NZ) of 55~60 hv are lower than those in a base metal(BM), which materials in NZ undergo softening by friction heat. EDS analysis results of kissing bond defect shows chemical compositions of the kissing bond defect in the T-joints. The results show that the percentage of oxygen and aluminum element reaches up to 54.84% and 45.16%, respectively, which is caused by the oxidation film generated by friction heat during FSW process. Also zigzag line can be found in all the T-joints in the present experiments. The zigzag line is the remnant oxide layer due to the insufficient heat input and stirring, but more effective metallurgical bond compared with kissing bond defects.