Determination of strain rate in Friction Stir Welding by three-dimensional visualization of material flow using X-ray radiography

Abstract

Recrystallization, which is mainly caused by the induced strain, is one of the most important factors of Friction Stir Welding. In this study, strain and strain rate are directly obtained by the change in the material flow velocity which is observed by three-dimensional visualization of the material flow. The grain size of the pure aluminum in the stir zone estimated by the Zener-Hollomon parameter using the obtained strain rate shows good agreement with that observed by Electron Back-Scatter Diffraction mapping.Much attention has recently been paid to Friction Stir Welding (FSW) because of the low distortion and 20 excellent mechanical properties of the joint [1][2][3][4]. A rotating tool is inserted into the interface at the butt line of the metal plates and produces a highly plastically deformed zone. The metal plates are joined by the traveling of the rotating tool along the interface. Sever plastic deformation occurs and strain is induced in the stir zone (SZ). The interface of the two metal plates disappears by the material flow, and it is the main bonding mechanism of FSW. Generally, the SZ consists of recrystallized fine and equiaxed grains which are formed by the induced strain during the FSW 30 [5][6][7]. Therefore, the recrystallization is one of the most important factors of FSW. It is well known that the recrystallized grain size can be estimated by the strain rate [8,9]. However, it is difficult to measure the strain rate accurately during the FSW because the various changes in the SZ cannot be observed directly using conventional methods. Therefore, determination of the strain rate is very important for estimating the microstructure in the SZ.There are several reports related to the strain rate in the FSW. Chen et al. showed that the strain was $3.5 and the 40 strain rate was $85 s À1 before entering into the thread space during the FSW with a simple threaded pin at 740 rpm [10]. Arora et al. indicated that the computed strains and strain rates were in ranges of À10 to 5 and À9 to 9 s À1 , respectively [11]. However, the obtained results are computed values based on a three-dimensional coupled viscoplastic flow and heat transfer model. The strain rate of 34.8 ± 5.2 s À1 was estimated for magnesium alloy (AZ31) during FSW by the experimentally verified finite element model [12]. If it can be obtained these values by a direct 50 observation of the phenomenon, it should be very valuable to compare the other results.In this study, the strain rate was directly calculated by the change in the material flow velocity. The material flow during the FSW was observed by the three-dimensional visualization process [13][14][15]. The material flow has been studied using various approaches such as the tracer method [16][17][18][19][20], analysis of the crystallographic texture in a weld [21], measuring the eutectic Si distribution [22]. However, it is impossible to know the material flow velocity using 60 these approaches because the obtained results show only one part of the process. In this study, the material f...

Double Reinforcement of Al–Fe Intermetallic Composites Fabricated by Friction Stir Processing

“…The particles were subjected to severe plastic deformation during FSP, which resulted in an L/W of 12. However, the strain rate in the flow zone was low due to the constant angular velocity of the material flow [21]. So, spherical 34CrNiMo6 particles and slim particles both existed in the specimens.…”

Friction Stir Welding of Medium Carbon Steel with Laser-Preheating

“…Additionally, the effect of the laser-preheating on the material flow during the FSW was investigated by in-situ observations using X-ray radiography. [16][17][18]…”

“…In this study, the material flow during the conventional FSW and laser-preheating FSW was observed by two pairs of high brightness X-ray inspection apparatuses consisting of X-ray generators and high responsive image intensifiers. [16][17][18] The perspective images observed from two different direction were recorded by high speed cameras. The appearance and schematic illustration of high brightness X-ray inspection apparatuses were shown in Figs.…”

Influence of Welding Temperature on Material Flow During Friction Stir Welding of AZ31 Magnesium Alloy

“…[14] It is interesting to note that the material may experience several rotations around the tool before its final deposition in the stir zone. [15,16] The combination of the rotation and translation motions of the tool gives rise to a characteristic asymmetry of FSW: the advancing side (AS), where the direction of the tool rotation is the same as that of the tool translation, and the retreating side (RS), where they are opposite. The material flow on the AS is believed to be relatively stagnant and may result in a wormhole defect, [15,[17][18][19][20] but the nature of this undesirable effect is not completely clear.…”