The weld specimens were sectioned and metallographically pol

The weld specimens were sectioned and metallographically polished. The specimens were etched with Keller\'s reagent for AA6061 side and with Nital solution for AISI 4340. Tensile testing was performed employing standard specimen configuration confirming to ASTM standard E8-04, having gauge length of 25 mm with weld interface located at the centre of the specimen. Tensile test specimen used for testing is shown along with its dimensions in Fig. 2. Tensile tests were performed on an Instron 1185 universal testing machine. The cross head speed during the tensile test was maintained at 1 mm/min. Three tensile test specimens were tested and the average of the tensile test results are reported. The failed tensile specimens were subjected to fractographic examination using LEO scanning electron microscope. Quantitative analysis and X-ray mapping was carried out to know the elemental distribution across the weld interface by scanning electron probe microanalysis (SEPMA). Quantitative analysis was carried out at an interval of 2 μm. The welds were subjected to X-ray diffraction by employing Philips PHL 3020 X-ray diffracto-meter using copper K radiation for the identification of various phases. Micro Vickers hardness was measured across the interface of the weld using 100 gm load to determine nature of interface.
Results
Discussion The friction welding process can be used to produce a metallurgical bond through the interaction of frictional heating and simultaneous deformation along the interface separating the material to be joined. Heat generated along the interface flows either axially away from the interface or radially along the interface as material is upset from the joint forming the characteristic flash. The different thermal and physical properties of the materials welded in dissimilar metal welding, including heat capacity, thermal conductivity, relation between hardness and temperature, generally results in tsa inhibitor deformation. The formation of flash on aluminium side and no flash on low alloy steel (Fig. 3) can be attributed to lower thermal conductivity and higher hardness of low alloy steel at elevated temperatures compared to aluminium alloys. The same phenomenon has been reported during friction welding of dissimilar welds namely Al to Cu [20], Al to stainless steel [21] and titanium to steel [22]. From the metallographic study it is observed that direct welding of AISI 4340 to AA6061 is not feasible due to the presence of continuous intermetallic layer across the weld width (Fig. 4). A correlation of strength data of welds with microstructure at the interface suggests that the direct welding of AISI 4340 to AA6061 aluminium alloy results in the formation of continuous intermetallic zone, and therefore exhibits very poor strength and almost nil ductility (Table 1). X-ray diffraction data of fractured tensile samples of direct weld contains highly brittle intermetallics such as Fe2Al5 and FeAl3 (Fig. 10). Incorporation of silver interlayer in the form of electroplating has been observed to be a solution to realize the welding of low alloy steel to aluminium alloy. Silver interlayer is found to be most useful as it implants good ductility as well as strength. Silver as an interlayer results in the formation of Ag3Fe2, Ag2Al and Ag3Al in addition to Fe2Al5 and FeAl3 (Fig. 10). Silver, presenting in the form of interlayer, acts as a barrier for the direct interaction between aluminium and steel, resulting in partial replacement of Fe–Al based intermetallic compound with Al–Ag based intermetallic compound. Mg and Si are the major alloying elements in AA6061. Mg is reported to be favourable for increasing the width of Fe–Al based intermetallic compound layer [23], particularly Fe2Al5. Uniform distribution of Mg at weld interface towards Al base metal side of the joint can be observed in the joint without silver interlayer (Fig. 5), which is a favourable condition for the formation of Fe–Al based intermetallics. However the joints with silver interlayer show that Mg is intermittently replaced by Si at the weld interface (Fig. 7). The absence of Mg near interface for joint with silver interlayer results in less favourable condition for the formation of Fe2Al5, and the situation is more favorable for the silver/aluminium intermetallic formation at the interface. This intermetallic compound is reported to be significantly softer than that formed between iron and aluminium and hence much greater thickness can be tolerated in the attainment of good quality welds between aluminium and its alloy to low alloy steel [24].