Microstructure feature of friction stir butt-welded ferritic ductile iron
Introduction
Friction stir welding (FSW) is a novel solid-state welding technology that adopts the heat generated by the friction that results from a stir rod rotating at high speed and penetrating a base material, subsequently achieving the joining effect during the feeding process through the metal plastic flow phenomenon created by stirring [1], [2], [3], [4], [5], [6], [7], [8]. During the welding process, the temperature remains below the melting point of the welding material. In contrast to traditional welding methods, the absence of melting renders FSW applicable to materials that are difficult to weld such as age-hardened aluminum alloys [9], [10], [11]. Several recent studies have employed FSW to join carbon steel and stainless steel [4], [5], [6], [7], [8], [12], [13], [14], [15], [16], [17], [18], [19], [20]. The rotating tools must tolerate high temperatures (1000 °C), which are required to create plastic flow when welding iron-based alloys. The complex phase transformation of steel materials varies substantially with the carbon content of the base material. Furthermore, temperature changes that occur during welding and the cooling rate after welding significantly influence the microstructure of the welding regions, altering the mechanical properties of weldments [4], [5], [16], [17], [18], [19], [20]. Fujii et al. performed FSW on 0.12% carbon steel by using welding parameters of 400 rpm and 100 mm/min, and observed relatively small ferrite grains (3 μm) that resulted from dynamic recrystallization at the bottom of the weld [16]. Furthermore, Fujii et al. extensively investigated how welding temperature affects structural changes in various weld regions. By using 0.85% carbon tool steel for FSW, Chung et al. reported that martensite structures formed in the weld when the welding temperature exceeded that at the eutectoid transformation point (A1) [17]. A welding temperature lower than that of the A1 transformation temperature prevented martensite formation in the ferrite matrix and simultaneously induced grain refinement; the weldment also demonstrated superior toughness and ductility. Because Chung et al. exercised joint material which is a ferrite matrix with globular cementite, the structure characteristic obtained proper plastic flow in lower temperature by FSW. Consequently, the base material was joined by friction stir effect, and without any phase transformation produced in the joint region. Sun et al. indicated that performing FSW using a 3.2-mm thick, 0.45% carbon steel at a rotational speed of 600 rpm and a traveling speed greater than 300 mm/min produces martensite and bainite structures in the weld; however, a traveling speed lower than 300 mm/min inhibits the formation of martensite and bainite [5]. Lakshminarayanan et al. identified very fine ferrite and martensite structures in the weld when performing FSW at 1000 rpm and 50 mm/min and using ferrite-based stainless steel, which contains 0.026% carbon, 11.4% chromium, and 0.4% nickel [6]. According to the aforementioned studies, the chemical composition of iron-based alloys and changes in welding temperature extensively alter the microstructure of welds, affecting the mechanical properties of weldments.
Compared to traditional fusion welding technologies, FSW provides a feasible method for joining ductile irons. Previously, welding methods were rarely adopted for joining ductile irons because of various problems that could not be overcome during the welding process. The carbon content of ductile iron is considerably higher than that of carbon steel, causing poor weldability; thus, during high-temperature fusion welding, carbon in the graphite dissolves into the surrounding melted region and diffuses across the unmelted regions that contain austenite phase. Consequently, the hard and brittle carbide and martensite developed during the cooling and solidifying processes reduce the mechanical properties of the weldments [2], [21], [22]. To resolve this problem, several studies have employed FSW to join ductile irons. By conducting dissimilar welding using ductile iron and low-carbon steel, our previous studies observed martensite and pearlite structures in the weld, and found that proper heat treatment removes martensite and improves the tensile properties of the weldments [2], [3]. Fujii et al. and Imagawa et al. conducted friction stir processing with ferritic ductile iron, pearlitic ductile iron and flake graphite cast iron for surface hardening processing that rendered the hardening effect by forming martensite transformation in the stir zone. Their studies indicated that the carbon atom is more difficult to diffuse in the ferritic matrix of ductile iron, so that the optimal condition of friction stir processing was narrower than that of other two materials [23], [24], [25]. Furthermore, Cheng et al. applied friction stir surface hardening to ductile iron by using a stir rod without a pin and rotating and traveling speeds of 2200 rpm and 60 mm/min, respectively. Mixtures of structures containing ferrite, bainite, martensite, and retained austenite were found in the stirred region, and this hardened layer improved the resistance of ductile iron to erosion [26]. Based on previous studies, martensite forms in the joined region of ductile iron after FSW; however, the mechanism related to this formation process has not been investigated. Therefore, this study employs FSW to join ductile iron by using the butt welding technique, investigating the morphological and microstructural changes in the graphite surrounding the weld.
Section snippets
Experimental procedure
The base material in the current experiment comprised a 3-mm thick ferritic ductile iron plates that contained 2.0% carbon, 2.5% silicon, 0.09% manganese, 0.006% sulfur, 0.034% phosphorous, 0.039% magnesium, and iron. Based on the experimental requirements, the ductile iron plate was processed to the desired dimensions (95 mm × 40 mm × 3 mm). Before welding, the oxidized layer on the ductile iron surface was sanded, washed with acetone, and blow-dried. Fig. 1 presents a schematic of the relative
Microstructural characterization
Fig. 2 shows the microstructure of ductile iron used as the FSW base material. The graphite nodules (approximately 20–25 μm in diameter) were evenly distributed within the ferrite matrix, which were in the state of equiaxed crystals approximately 30–40 μm in diameter. Fig. 3 presents a cross-sectional schematic of the weld regions. The sub-surface region of the weld is the area where the stir rod shoulder acts on the base material and is approximately the width of the stir rod shoulder. The
Conclusions
The following findings were obtained based on the experimental results:
- (1)
Welding ductile iron by employing FSW at a rotational speed of 982 rpm and traveling speed of 72 mm/min produces a weld that displays a smooth surface and no defects.
- (2)
The weld microstructure is composed of deformed graphite, martensite, and dynamically recrystallized ferrite. High temperatures and the friction resulting from rotation caused the carbon atoms to diffuse from the graphite into matrix, transforming certain regions
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