A study on the microstructure of weld nugget and mechanical properties of ductile iron jointed by friction stir welding

• Main Authors: Hung-Tu Chang, 張宏圖
• Other Authors: Chaur-Jeng Wang
• Format: Others
• Language: zh-TW
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/994ee6

博士 === 國立臺灣科技大學 === 機械工程系 === 102 === Abstract Friction stir welding (FSW) is a novel solid-state welding technology, which features a non-consumable rotating tool acting on metal plates. Because the spindle rotation and the feeding process cause severe plastic deformation in the stir zone of the metal plate, welding is achieved through thermo-mechanical reactions. During the entire welding process, the welding temperature remains lower than the melting point; thus, melting in the weld area does not occur, resulting in the development of weldments with low-deformability, high-quality, and low-cost characteristics. Ductile iron is widely applied in the field of mechanical engineering because, in addition to its affordability, it possesses outstanding formability, appropriate mechanical properties, and superior damping capacity. The objective of this study is to demonstrate the feasibility of friction stir butt-welding for joining of ferritic ductile iron by different welding parameter. According to the experimental results, the welding region was composed of deformed graphite, martensite phase, and dynamically recrystallized ferrite structures. In the surface region and on the advancing side (AS), the graphite displayed a striped configuration and the ferritic matrix transformed into martensite. On the retreating side (RS), the graphite surrounded by martensite remained as individual granules and the matrix primarily comprised dynamically recrystallized ferrite. A micro Vickers hardness test showed that the maximum hardness value of the martensite structures in the weld was approximately 800 HV. After welding, diffusion increased the carbon content of the austenite around the deformed graphite nodules, which transformed into martensite during the subsequent cooling process. To resolve the poor weldability of ductile irons, this study employed AISI-1008 low-carbon steel as the top plate and ferritic ductile iron as the bottom plate and varied the rotational and traveling speeds to conduct a friction stir lap welding test. After welding, the weldments underwent microstructure analysis and hardness testing followed by a tensile shear test to evaluate the joint strength. At a high rotational speed above 1000 rpm combined with traveling speeds that ranged between 40 and 70 mm/min indicated the following: An excellent joining effect was achieved; the interfacial regions of the carbon steel and cast iron primarily comprised pearlite, although the vicinity of the retreating side and the stir zone matrices of ductile irons were composed of martensite structures; individual graphite granules were present; the tensile shear strength of the weldments was high; and the fracture portion was located in the low-carbon steel base metal. In order to explore the weldability of austenitic ductile iron, this study employed AISI-1008 low-carbon steel as the top plate and austenitic ductile iron as the bottom plate and varied the rotational speed to conduct a friction stir lap welding test. According to the experimental results of tensile shear stress test, at the welding condition of 1100 rpm-50 mm/min indicated the tensile shear stress value was about 900 N. In addition to conducing post-welding friction stir processing that combined traveling speed 50 mm/min with rotational speed between 1200 and 2100 rpm. Consequently a well joining effect elevated tensile shear stress value above 6000 N.