109 Chapter 5 MACROSTRUCTURE, MICROSTRUCTURE AND MICROHARDNESS ANALYSIS 5.1 INTRODUCTION The microstructural studies of friction welding helps in understanding microstructural changes occurred during friction welding process. High temperature and strain during friction welding process changes the microstructure of the parent material. The microstructure of friction welded specimen for Al alloys to SS 304 and copper gives very important information about formation of intermetallic compounds, heat affected zone, variation of grain size, weld defects such as voids, flaws, oxides, foreign inclusions, incomplete weld etc. The microstructure analysis helps in improving the weld strength and weld interface properties. The microstructural changes have great effect on mechanical properties of weld joint. The variation of grain size can influence the tensile strength of weld joint and the grain boundaries can determine the way in which the friction welded joint can fracture. Fine grain size generally exhibits greater yield strength than course grain size materials at low temperature where as at high temperature grain boundaries becomes weak and sliding occurs.
110 The information about formation of intermetallic compounds and its thickness helps in understanding the failure mechanism of weld joint. The formation of intermetallic layer leads to brittle failure of the weld components and generally these weld specimens fail at weld joint. 5.2 MACRO STRUCTURE ANALYSIS The appearance of friction welded joints of regular and new joint geometry for Al 6061 SS 304 and Al 6061- Copper are shown in Figure 5.1. In regular joint geometry flash of Al 6061 alloy was uniformly formed around the entire weld circumference where as in new joint geometry Al 6061 burrs are also formed along with flash around the entire weld circumference. The formation of burrs at the weld interface of new joint geometry facilitates in destruction and removal of oxides and other contamination at the weld interface. The amount of flash formed in regular joint geometry is more than new joint geometry. (a) Regular joint geometry (b) New joint geometry (Al 6061- SS 304) (Al 6061- SS 304)
111 (a) Regular joint geometry (Al 6061- Copper) (b) New joint geometry (Al 6061- Copper) 5.1 Appearance of flash for Al alloys SS 304 and Al alloys - Copper Similar flash pattern was observed in other weld combinations such as Al 5052 SS 304 and Pure Al SS 304. 5.3 INFLUENCE OF UNSYMMETRICAL PLASTIC DEFORMATION ON WELD INTEGRITY OF REGULAR AND NEW JOINT GEOMETRY Friction welding of Al alloys to SS 304 and Al alloys to copper leads to unsymmetrical deformation with respect to the plane of the joint interface as shown in Figure 5.2. The unsymmetrical deformation is due to differences in thermal, physical and chemical properties. The properties such as melting point, hardness, heat conductivity have greater influence on welding of dissimilar materials. In friction welding of Al and its alloys to SS 304, the SS 304 specimen has relatively no deformation due to low thermal conductivity, heat capacity, high hardness and higher melting point where as aluminum has large deformation due to high thermal conductivity, lower hardness and lower melting point. The insufficient plastic deformation of SS 304 can lead to incomplete destruction and removal of oxides films and other
112 contaminations from the weld interface and it results in lowering the strength of weld joint. The problem is very severe in regular joint geometry as oxides and other contaminations are entrapped at the central region of the weld interface. This entrapment of oxides and other contaminations is due to insufficient heat generation at the central region (inner region) and also due to progression of weld from outer region to inner region. The insufficient heat generation at the inner region is due to difference in rotational speed. The rotational speed at the periphery is relatively greater than that of inner region. Therefore heat generation in the outer region is greater than inner region. Similar problem exists in friction welding of Al alloys to copper. The new joint geometry helps in improvement of weld quality as the shape of new joint geometry facilitates in removal of oxides and other impurities from the weld interface including from the inner region. SS 304 Al 6061 (a) Unsymmetrical plastic deformation of Al 6061 SS 304
113 SS 304 Al 5052 (b) Unsymmetrical plastic deformation of Al 5052 SS 304 SS 304 Pure Al (c) Unsymmetrical plastic deformation of pure Al- SS304 Copper Pure Al (d) Unsymmetrical plastic deformation of Al 6061 Copper Figure 5.2 Unsymmetrical Plastic Deformation during friction welding of Al alloys to SS 304 and Al alloys to copper.
114 5.4 MICROSTRUCTURAL ANALYSIS The combination of Al 5052 SS 304, Al 6061 SS 304, Al 6061 Copper, pure Al SS 304, pure Al Copper, are friction welded with weld parameters as given in Chapter 4 for regular and new joint geometry. The microstructure specimens are prepared as described in section 3.5.6 of chapter 3. The microstructure of new joint geometry and regular joint geometry weld specimens are observed using scanning electron microscope (SEM) and optical microscope attached with image analyzer. The above weld combinations are susceptible to intermetallic compounds formations. The intermetallic thickness is not constant across the weld interface. The variations of these intermetallic compounds are discussed in detail in section 5.5 of this chapter. The maximum intermetallic thickness and corresponding microstructures are shown in this chapter. In new joint geometry excellent contact is obtain along the weld interface due to following: (a) Removal of oxides and other unwanted impurities from the weld interface of new joint geometry as weld progresses from inner region to outer region thus avoids entrapment of oxides and other impurities. (b) No unbound region is formed in new joint geometry. (c) In new joint geometry, temperature distribution is more uniform across the weld interface.
115 In new joint geometry, it was observed that the heat affected zone (HAZ) at the weld interface was less than regular joint geometry 5.4.1 MICROSTRUCTURAL ANALYSIS OF Al 5052 SS 304 The microstructure of new joint geometry and regular joint geometry of weld interface are shown in Figure 5.3 (a) and Figure 5.3 (b) respectively. The grain size of Al 5052 is very fine due to mechanical crushing of grains during the course of wearing of the surface layers of the metal resulting from friction and plastic deformation. The grain size decreases as the distance from the weld line decreases. There is negligible change in microstructure of SS 304 due to high melting point, high hardness and low thermal conductivity. The thickness of intermetallic layer of new joint geometry and regular joint geometry were examined. There is no intermetallic compounds formed in new joint geometry where as in regular joint geometry, intermetallic thickness in the range of 4µm to 9 µm were formed. The formation of intermetallic compounds in regular joint geometry was due to high weld temperature attained during friction welding and they are responsible for brittle failure of weld joints. The intermetallic compounds formed during friction welding process are identified by X ray diffraction technique are discussed in section 5.6
116 Figure 5.3 (a) Microstructure of Al 5052 SS 304 weld interface for new joint geometry 400X (outer region)) Figure 5.3 (b) Microstructure of Al 5052 SS 304 weld interface for regular joint geometry (400X) The microstructure of base material of Al 5052 and SS 304 are shown in Figure 5.3 (c) and 5.3 (d) respectively.
117 Figure 5.3 (c) Microstructure of Al 5052 Base Material Figure 5.3 (d) Microstructure of SS 304 Base Material 5.4.2 MICROSTRUCTURAL ANALYSIS OF Al 6061 SS 304 The microstructural analysis of friction weld combination of Al 6061-SS 304 for new joint and regular joint geometry are shown in Figures 5.4 (a) and Figure 5.4 (b) respectively.
118 Figure 5.4 (a) Microstructure of Al 6061 and SS 304 weld interface for new joint geometry Figure 5.4 (b) Microstructure of Al 6061 and SS 304 weld interface for regular joint geometry
119 A very fine grain size is observed in Al 6061 near the weld joint and grain size decreases as the distance from the weld line decreases. There is negligible change in microstructure of SS 304. The intermetallic thickness of new joint geometry was negligible (less than 1 µm) where as the intermetallic thickness of regular joint geometry was in the range of 5µm to 7 µm. The thickness of intermetallic layer of new joint geometry was less than regular joint geometry because the weld temperature attained during welding of new joint geometry was less than regular joint geometry Figure 5.4 (c) Microstructure of Al 6061 Base Material 400X The microstructure of Al 6061 base material is shown in Figure 5.4 (c) and the microstructure of SS 304 is shown in Figure 5.3 (d)
120 5.4.3 MICROSTRUCTURAL ANALYSIS OF Al 6061 Copper The microstructure of Al 6061 Copper friction welded specimens for new and regular joint geometry are shown in Figure 5.5 (a) and Figure 5.5 (b) respectively. The grain size of Al 6061 decreases as distance from weld line decreases. The intermetallic thickness of new joint geometry is less than regular joint geometry. The intermetallic thickness of new joint geometry was less than 1 µm whereas the intermetallic thickness of regular joint geometry was in the range of 3 µm to 7 µm. The microstructure of base material of copper is shown in Figure 5.5 (c). Figure 5.5 (a) Microstructure of Al 6061- Copper 400X weld interface for new joint geometry
121 Figure 5.5 (b) Microstructure of Al 6061 and Copper weld interface for regular joint geometry Figure 5.5 (c) Microstructure of Al Copper Base Material
122 5.4.4 MICROSTRUCTURAL ANALYSIS OF PURE Al SS 304 The microstructure of friction weld combination for Pure Al SS 304 for new and regular joint geometry is shown in Figure 5.6 (a) and Figure 5.6 (b) respectively. The intermetallic layer thickness of new joint geometry is less than 1 µm where as the intermetallic thickness of regular joint geometry is in the range of 2 µm to 4 µm. The microstructure of base material of Pure Al is shown in Figure 5.6 (c). Figure 5.6 (a) Microstructure of Pure Al SS 304 weld interface for new joint geometry
123 Figure 5.6 (b) Microstructure of Pure Al SS 304 weld interface for regular joint geometry Figure 5.6 (c) Microstructure of Pure Al Base Material
124 5.4.5 MICROSTRUCTURAL ANALYSIS OF PURE Al COPPER The microstructure of friction welded Pure Al Copper for new joint geometry and regular joint geometry are shown in Figure 5.7 (a) and 5.7 (b) respectively. Figure 5.7 (a) Microstructure of Pure Al Copper weld interface for new joint geometry Figure 5.7 (b) Microstructure of Pure Al Copper weld interface for regular joint geometry Entrainment of copper is observed in regular joint geometry.
125 5.4.6 MICROSTRUCTURAL ANALYSIS OF EN 8 SS 316L The microstructural analysis of friction welded En 8 SS 316L for new joint geometry and regular joint geometry are investigated to understand the microstructural changes occur during friction welding. The combination of En8 SS 316L is not susceptible to intermetallic compounds formation. The location where microstructure of friction weld combination is observed at En 8 and SS 316L welded part is marked on Figure 5.9 En 8 Base Material ----- Refer Figure 5.9 (a) En 8 HAZ -------Refer Figure 5.9 (c) En8 Weld Zone ------ Refer Figure 5.9 (e) SS 316L Weld Zone----- Figure 5.9 (f) SS 316L HAZ ------ Refer Figure 5.9 (d) SS 316L Base Material ---- -Figure 5.9 (b) Figure 5.8 Location of microstructure observed for En 8 SS 316L
126 The microstructure of base of En8 and SS 316L is shown in Figure 5.9 (a) and Figure 5.9 (b) respectively. The microstructure of heat affected zone of En8 and SS 316 are shown in Figure 5.9 (c) and Figure 5.9 (d) respectively. The microstructure of weld specimen etched on En8 side and the microstructure of weld specimen etched on 316L side are shown in Figure 5.9 (e) and Figure 5.9 (f) respectively The grains near the weld line are very fine and elongated parallel to bond line due to temperature and upset pressure. The grain size decreases as distance from weld zone decreases. The decrease in grain size is due to mechanical crushing of grains during the course of wearing of the surface layers of the metal resulting from friction and plastic deformation The width of heat affected zone of new joint geometry is approximately 25 % less than regular joint geometry. Figure 5.9 (a) Microstructure of En8 base material Figure 5.9 (b) Microstructure of SS 316L base material
127 Figure 5.9 (c) Microstructure of En8 HAZ Figure 5.9 (d) Microstructure of SS 316L HAZ Figure 5.9 (e) Microstructure of En8 HAZ (Weld Zone) Figure 5.9 (f) Microstructure of SS 316L HAZ (Weld Zone) 5.5 VARIATION OF INTERMETALLIC LAYER THICKNESS The formation of intermetallic compounds and its thickness is strongly depends on weld interface temperature. The variation of intermetallic thickness across the weld interface depends on heat generation pattern.
128 In regular joint geometry the heat generation is least at the center and it increases radially as distance from the center of the weld increases as illustrated in Figure 5.10. This variation of heat generation is due to difference in relative speed. The speed at the center of the weld is least and increases radially as radial distance increases. The formation of intermetallic compounds and its thickness follows the above heat generation pattern. The thickness of intermetallic layer is measured by image analyzer attached with microscope. The thickness of intermetallic compounds is least at the center and it increases radially as radial distance increases. But, at periphery the intermetallic thickness again decreases due to expulsion of flash. In new joint geometry, either no intermetallic compounds are formed or very thin and (almost uniform thickness) intermetallic compounds are formed and variation of intermetallic thickness across the weld interface is shown in Figure 5.11. Similar pattern was also observed in other weld combinations of Al alloys to SS 304 and Al alloys to copper.
129 Figure 5.10 Illustration of heat generation pattern in regular joint geometry (Heat generation increases radially as distance from centre increases) Figure 5.11 Variation of intermetallic layer with distance from centre of the weld interface
130 5.6 IDENTIFICATION OF INTERMETALLIC COMPOUNDS AT WELD INTERFACE BY X- RAY DIFFRACTION TECHNIQUE. To determine the types of intermetallic compounds formed in the weld interface, the fracture surface of specimens (sustaining fracture at the weld interface) were examined by X-ray diffraction. Figure 5.12 shows X-ray pattern obtained from tensile surface of Al 6061-Cu. It shows diffraction lines corresponding to three types of intermetallic compounds. These compounds are identified as Al2Cu3, AlCu4 and CuAl2. Figure 5.12 X ray Diffraction pattern on tensile fractured surface of Al 6061-Cu
131 Figure 5.13 shows x-ray pattern obtained from tensile surface of Al 5052-304. It shows diffraction lines corresponding to two types of intermetallic compounds. These compounds are identified as FeAl2 and AlFe. Figure 5.13 X ray Diffraction pattern on tensile fractured surface of Al 5052-SS 304 5.7 COMPARISION OF EVOLUTION OF FRICTION WELDING PROCESS AND INTERMETALLIC COMPOUND FOR REGULAR JOINT GEOMETRY AND NEW JOINT GEOMETRY Figure 5.14 and Figure 5.15 illustrates the evolution of friction welding process and intermetallic compound formation for regular and new joint geometry.
132 SS 304 Al 6061 SS 304 Al 6061 Figure (a) Friction time = 0 Second Figure (a) Friction time = 0 Second SS 304 Al 6061 SS 304 Al 6061 Figure (b) Friction time = 0.2 Second Figure (b) Friction time = 0.2 Second SS 304 Al 6061 SS 304 Al 6061 Figure (c) Friction time = 0.5 Second Figure (c) Friction time = 0.5 Second SS 304 Al 6061 SS 304 Al 6061 Figure (d) Friction time = 1 Second Figure (d) Friction time = 1 Second Figure 5.14 Illustration of friction welding process between Al 6061 and SS 304 for regular joint geometry Figure 5.15 Illustration of friction welding process between Al 6061 and SS 304 for new joint geometry
133 The comparison of evolution of friction welding process, intermetallic compounds formation for regular and new joint geometry are given below 1) Figure 5.14 (a) shows the regular joint geometry where flat faces of Al 6061 is in contact with flat face of SS 304 at the start of friction welding process at friction time ft = 0 seconds. Figure 5.15 (a) shows the new joint geometry where inner region of Al 6061 is in contact with flat face of SS 304 at the start of friction welding process at friction time ft= 0 seconds. 2) In regular joint geometry, at friction time ft = 0.2 seconds only outer region is welded and there exists a unwelded zone at the central portion of the components, this is due to relatively lower rotational speed at the central portion of the components. As a result heat generated at the central portion is relatively lower therefore the tensile strength of full size and half size welded specimens are very less. At this stage no intermetallic compounds are formed as shown in Figure 5.14 (b). In new joint geometry, at friction time ft = 0.2 seconds heating is started only at inner region and the initial taper cylindrical portion of Al 6061 was gradually consumed as shown in Figure 5.15
134 (b) in the form of burrs. (The burrs are not shown in figure for clarity) The oxides and other impurities at the interface surfaces are removed from the inner portion of weld specimens along with the burrs. At this stage no intermetallic compounds are formed. 3) In regular joint geometry, as friction time increase to ft = 0.5 seconds the temperature at the interface increases, the size of unwelded region at the central portion is reduced but not completely eliminated and a small intermetallic reaction layer is formed in the outer region as shown in Figure 5.14 (c). At this stage the strength of standard specimen is good but the strength of half size specimen is weak. In new joint geometry, as friction time increase to ft = 0.5 seconds, the taper cylindrical portion is further consumed and the oxides and other contaminations at the contact surfaces is further removed. The contact area between the Al 6061 and SS 304 is increased as shown in Figure 5.15 (c). No intermetallic compounds are formed at contact region as temperature evolved in this region is relatively lower and also no intermetallic layer was formed at the outer region as this area was gradually coming in contact with SS 304. 4) In regular joint geometry, as friction time further increases to ft = 1 second the entire weld region is welded but the intermetallic
135 compounds increases further in the outer region as shown in Figure 5.14 (d). Good weld strength is obtained for full size but lower strength is achieved for half size welded specimens when compared with new joint geometry In new joint geometry, as friction time further increases to ft = 1 second the entire taper cylindrical portion is consumed in the form of burrs and the complete interface surfaces are welded without formation of intermetallic layer as illustrated in Figure 5.15(d) and weld strength (for both full size and half size welded specimens) was better than regular joint geometry. 5.8 MICROHARDNESS ANALYSIS The large the strains and the temperature, the weld interface causes change in the hardness. Increase in hardness was observed near the weld and it is due to strain hardening and grain size reduction. Hardness values can give information about the metallurgical changes caused by welding. Hardness values are usually sensitive to heat input, friction pressure and upset pressure. The micro-harness for weld combinations of Al 6061 SS 304 is shown in Figure 5.16
136 Figure 5.16 Variation of hardness in weld combination of Al 6061 SS 304