마찰교반용접을 이용한 Al6061-T6 코너용접시 툴 핀 길이에 따른 용접 특성

Abstract

ABSTRACT

Effect of Tool Pin Length on Weldability of Al6061-T6 Friction Stir Corner Welded Joints

Son Yeong-Bin Advisor : Prof. Bang, Hee-Seon, Ph.D. Department of Welding and Joining Science Engineering, Graduate School of Chosun University

Recently, the development of technologies to reduce energy and protect the global environment of automobiles, aircraft, rail vehicles and ships has been actively underway. Lightweight technologies include efficient engine development, application of lightweight materials and structural rationalization. As part of reduce lightweight, it is used for ultra-high strength steel plates and steel materials is replaced by Al and Mg alloys.

Compared to steel materials, Al alloys have about four times the thermal conductivity, two times the specific heat and 1.5 times the melting latent heat. And the low melting point and difficult to concentrate energy in heat source. Also, The surface is easily oxidized and the formed aluminum oxide film has a high hardness and high melting point, so that it is difficult to remove. Therefore, application of friction stir welding rather than melting welding is more advantageous to reduce defects. Friction stir welding (FSW), one of the solid-state welding methods was invented at TWI. 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 Magnesium alloys, Aluminum alloys because of its low heat input.

In this study, we applied the friction stir welding (FSW) process as a method for Al6061-T6. For this purpose, the reliability of the joints is secured by evaluating the weldability and the metallic and mechanical characteristics according to the parameters of each process.

Aluminum alloy 6061-T6 with 6 mm (skin) and 15 mm (stringer) thickness are adopted as base metal in this study. Tool rotation speed, travel speed, plunge depth and different pin length were adopted as welding parameter. In terms of pin length, 4.7 mm (about 80% of the material) and 5.4 mm (about 90% of the material) were used in this study. The tool rotation speed was from 400 to 600 rpm, the traveling speed was 0.6 to 0.8 mm/s. Moreover, when the pin length was 4.7 mm, the pin was plunged 0.7 to 1.0 mm, and 0.2 to 0.4 mm when the pin length was 5.4 mm. Tensile strength and hardness characteristics of the joint were investigated to figure the mechanical characteristics of the corner joint. Plus, thermal distribution characteristics of the Al6061-T6 corner joints were investigated by numerical analysis. To validate the numerical simulation result, the experimental result, carried out with thermocouple, was compared.

As a result of Vickers Hardness test, the hardness value of BM, TMAZ, HAZ, and SZ were 99 Hv, 71 Hv, 53 Hv, 69 Hv respectively. The hardness value of SZ and TMAZ decreased to 70% of the base material’s value, because the welding process gave annealing effect to the base material, which was already heat treated to increase its mechanical strength. When using the pin length 4.7 mm tool (about 80 % of the material), a maximum tensile strength of 186 MPa which is aprroximately 63 %. And When using the pin length 5.4 mm tool (about 80 % of the material), a maximum tensile strength of 196 MPa which is aprroximately 67 %. When the 5.4 mm tool was used, the effect of improving the tensile strength value of about 4 % compared to the conventional 4.7 mm was obtained.