Theoretical Background
4.1 Friction stir welding
4.1.1 Experimental set-up
The experimental setup used in this work is a vertical milling machine with 7.5 HP motor capacity. Butt welding of commercial grade aluminum alloy was performed in the initial trial experimental where the dimension of test samples was 200 mm × 100 mm × 6 mm.
The edges of the test samples are machined to obtain a perfect square butt. They are clamped on the horizontal machine bed without root gap by the help of fixture. The developed fixture has four main components, namely top plate, clamps, backing plate and support plates. Each part has been fabricated individually with all appropriate features required for holding workpiece rigidly against high welding forces. The clamping of the test pieces is done in such a way that the movement of the plates is totally restricted against the welding forces. Several tests were carried out by varying tool rotational speed and feed rate. It was found that the fixture is highly useful for carrying out FSW operations in vertical milling machine. The FSW setup used in this work is shown in Fig. 4.1.
Figure 4.1. Experimental setup for FSW.
Agilent 34970A Data Acquisition system is used in the present work. It is used with multiplexer to acquire temperature data at different thermocouple point with the help of K- type thermocouples having two-core with wire protraction grill outside. The operating temperature is between -200°C to 1260°C for single exposure. The limits of error of thermocouple is conforms to ASTM E230. However, a special fixture has developed for
Chapter 4 clamping system and plunging force measurement. It is an instrumented setup for a vertical milling machine for friction stir welding operations and measuring the process forces as shown in Fig. 4.1. It is an adjustable fixture to hold workpiece during welding and reduce the chances of gap formation due to lateral and transverse movement of the workpiece. For force measurement, a strain gauge based force dynamometer have been designed, developed, and fabricated. The strain gauges are imbibed on the specially designed four hexagonal supporting members to support the welding plates. During welding, the applied force by tool is transferred to the four hexagonal members and it induces strain in the member. This strain is measured by strain gauges in terms of voltage using a Wheatstone bridge circuit. This voltage is measured by a data acquisition system with amplifier. This voltage signal is calibrated with forces measured by standard dynamometer. Transient temperature is recorded on the top surface at eight different locations using 36 gauge K-type thermocouples. All the temperature measurements are made in four on the advancing side and four on the retreating side of the welds. The location of thermocouple is shown in Fig. 4.2. The material used in this work is commercial grade aluminium alloy rolled plate. The chemical composition and mechanical properties of the base metal are presented in Table 4.1 and Table 4.2, respectively.
Figure 4.2. Layout of thermocouples for temperature measurement.
Experimental Investigation
Table 4.1: Chemical composition of aluminum alloy (AA1100).
Alloy Fe Cu Si Ti Mg Mn Ni Zn Al
Weight percentage 0.57 0.12 0.13 0.03 0.02 0.013 0.017 0.01 Bal.
Table 4.2: Mechanical properties of aluminum alloys.
Property Value
Tensile strength (MPa) 150.78 Yield strength (MPa) 118.62 Elastic modulus (GPa) 66.94 Percentage of elongation (%) 13.75 Micro hardness (HV) 48.51
Density (g/cm3 ) 2710
Once the welding is over, test samples for different mechanical testing are prepared to evaluate quality of weld joints. Transverse tensile test, bending test and hardness test are performed on the specimens. The tensile specimen is cut from the center of the welded joint and prepared as per the dimension provided by ASTM E08 standard [276-277] which is shown in Fig 4.3. Tensile test is carried out on a 100 kN capacity electro-mechanical controlled Universal Testing Machine (model: INSTRON-8801) (Fig. 4.5b). All the samples are tested at a constant speed of 1.5 mm/min. After the testing it was observed that most of the samples failed inside the welded zone. The 0.2 % offset yield strength, ultimate tensile strength and percentage of elongation are evaluated from test data. The bending sample is prepared along with the tensile sample as shown in Fig 4.4. Bending test is carried out using three-point bending test setup attached with the same Universal Testing Machine (Fig. 4.5a). The specimen is tested at constant speed of 2.5 mm/min as per ASTM specifications, so that bending specimen undergoes bending load [277]. The bending angle and bending strength are evaluated for each welded joint.
Figure 4.3. Tensile test sample.
Chapter 4
Figure 4.4. Bending test sample.
Figure 4.5. (a) Three-point bending test setup (b) Tensile test setup with extensometer.
The hardness distribution in the weld zone is measured by using an Akashi AAV-500 Vickers indenter as shown in Fig 4.6 (a) with a load of 100 gf and dwell time of 10 s, according to the ASTM: E384-11 [278]. Hardness is measured both in across the welding direction and in through the thickness direction from top to bottom at the center of weld zone. Micro- hardness is measured in three lines at 1, 3, and 5 mm from top surface with 1 mm of space between two indents as shown in Fig 4.6 (b).
Experimental Investigation
Figure 4.6. (a) Vickers indenter for vickers micro hardness test (b) Schematic illustration of three lines in cross-section of different weld zone for hardness at 30 points in 1 mm of spacing.
Microstructural examination is carried out using an optical microscope (MEJI, Japan;
model MIL-7100) incorporating image analyzing software (Aixo Vision 4.2) as shown in Fig 4.7(a). The specimens are ground with silicon carbide papers of 240, 400, 800, 1000, and 1200 grades followed by polishing on a rotating wheel with 1 and 0.3 μm alumina suspension.
Furthermore, the optical microscope is used for microstructural study of polished specimens.
All polished specimens are etched with a solution comprising 25 ml methanol, 25 ml HCl, 25 ml HNO3, and HF of one drop to expose grain boundary [279]. A detailed microstructural observation is conducted for each welded specimen using optical microscopy to determine the variation of grain size and presence of any weld defect. After obtaining the grain structure the same sample is used to obtain the macrograph using the optical microscope (Leica S6D) at two different resolutions 1.0X and 1.25X as shown in Fig. 4.7(b).
In stirred zone, two different materials are plasticized and mixed together that produce intermetallic compound within the matrix of two base materials. Therefore, it is quite difficult of get clear microstructure of stirred zone due to presence of three different materials (Cu, Al and IMC). There is no common etchant available to reveal clear microstructure in stirred zone after polishing of the sample. Modified Poulton’s reagent is used to reveal the microstructures of copper while Killer’s etchant is used for aluminum. However, Poulton’s reagent is more reactive for aluminum and Keller’s etchant is less reactive for copper. It is followed in sequence i.e. first Keller’s etchant is applied and then Poulton’s reagent. As a result, the desired image from stirred zone is obtained.
Chapter 4
Figure 4.7. (a) An optical microscope (MEJI, Japan; model MIL-7100) for microstructure (b) An optical microscope (Leica S6D) for macrograph.