Impact behaviour and fractography of 6061 alloy with trace addition of Sn

A BSTRACT . The impact behaviour of 6061 alloy with trace amount of 0, 0.04, and 0.08 wt.% Sn was studied in the as-cast (AC), as-roll (AR) and peak-age roll (PAR) processing state. Additionally, the fracture mechanism was also studied in the AC and PAR state. The experimental investigation revealed that at all processing states, trace addition of Sn improves the impact strength of the 6061 alloy. Compared to the other processing states, the PAR condition contribute most to the impact strength. Fractography analyses showed that the fracture in the alloys occurred primarily by the crack propagation of Al(Fe, Mn)Si particles. The fractures in the AC alloys took place by mixed ductile and brittle mode by larger ductile dimples, cracks and cleavages, while in the PAR alloys was primarily by ductile mode by the smaller dimple fractures.

INTRODUCTION 000 series Al alloys have higher resistance to corrosion, good strength and are light in weight. Hence, these alloy series are widely used in the aerospace and defence applications. The properties of 6000 alloys rely on the quantity, composition and morphology of the Mg2Si and Fe-rich intermetallic particles. Mg2Si particle strengthen the 6000 alloys [1], whereas, Fe-rich particles contributes to the hardness, but decrease the ductility [2,3]. Trace addition is an art of introducing a minor amount of alloying elements (0.1 wt.% or less) in aluminium for the improvement in the properties. Alloying elements like Mg and Si in 6000 alloys reduce the dislocation motion through formation of hardening Mg 2 Si precipitates, thereby increasing the mechanical strength [4]. Sn in 6000 Al alloys is a potential trace element [5][6][7][8][9][10]. Sn introduction slows down the natural ageing process of 6000 Al alloys, thus helping in keeping the alloy stable during the transportation period from one place to another [5]. It was also observed that Zinc 6 addition eliminates the negative effect of Sn addition in the properties of high magnesium content 6000 Al alloy [6]. Sn addition in 6000 alloys had been also shown as a potential accelerating artificial age hardening agent [7][8][9]. He et. al. [10] also reported a positive effect in the tensile strength of 6000 alloy at lower Sn content of 0.03 wt.%. The previous studies of 6000 Al alloys with trace Sn are limited to the hardness [5][6][7][8][9][10][11] and tensile behaviour [6,[9][10]. It is worth mentioning that the studies on the impact behaviour and fractography of 6000 series aluminium alloys with trace Sn are limited. Hence, in this experimental investigation, the impact behaviour of 6061 Al alloy with trace addition 0, 0.04 and 0.08 wt.% Sn was studied systematically in as-cast (AC), as-roll (AR) and peak-age roll (PAR) states. Moreover, the fractography study of the alloys was also carried out in this experimental investigation.

METHODOLOGY
Casting vacuum die melting and casting furnace (Make: SwamEquip) was used for casting the 6061 alloys with 0 (Alloy A), 0.04 (Alloy B) and 0.08 (Alloy C) wt.% Sn. The casting setup is shown in Fig. 1. The 6061 alloy bars and tin (Sn) powder purity 99.9% (Make: Loba), was used for preparation of the investigated alloys. Rectangular shaped mild steel die of dimensions 300×100×15 mm was used for casting the alloys. The casting temperature and vacuum pressure used are 750 ℃ and 800 mbar [11]. The chemical compositions of the alloys were determined by an optical emission spectrometer (   A

Processing and heat treatments
After die-casting, the alloys were homogenised for 4 hrs at 530 ℃. The homogenised alloy specimens were then hot rolled at 460 ℃ in a laboratory grade rolling mill (Make: Bharat; Fig. 2). Hot rolling was accomplished in four passes with a total of 28.5% reduction in thickness. The hot rolling temperature of the investigated alloys was based on the data obtained from the literature of Tu et al. [7]. All the hot rolled alloy plates were then peak-aged at 190 ℃ after solution heat treatment for 1 hour at 530 ℃ and quenching in water. The age hardness of the studied alloys was determined by micro-Vickers hardness tester (MVH auto).

Impact test
For toughness measurement, Charpy impact tests were conducted on A, B and C alloys in the AC, AR and PAR state, using an impact testing machine (Make: MCS; Model: MIT-30). Impact testing specimens were prepared from as-cast (AC), as-roll (AR) and peak-age roll (PAR) alloys. The Charpy test specimen (ASTM E23) specifications is shown in Fig.  3.  Fractography Fig. 4 shows the fracture specimen after the impact test. The impact fracture study of the alloys was carried out in AC and PAR states. Fractography study was carried out using FE-SEM and EDS (Zeiss; Sigma 300).

RESULTS AND DISCUSSIONS
Microstructure ig. 5 shows the FE-SEM micrographs of the three alloys in the AC state. The micrograph in Fig. 5 reveals three phases, viz., white (phase A), dark-black (phase B) and light-grey (phase C) in the alloys. From EDS analyses, phase C (light-grey) is identified as Al, phase A (white) is identified as Al(Fe, Mn)Si and phase B (dark-black) is confirmed as Mg 2 Si. Detail microstructural studies of the investigated alloys were reported elsewhere [11].

Ageing
Tab. 2 shows the peak age hardness of the PAR alloys. The peak age hardness of the PAR alloys A, B and C was found as 103, 91 and 114 HV after 12, 6 and 8 hours, respectively. It was found that in comparison to alloy A, the hardness of alloy B decreases and alloy C increases. The decrease and increase in hardness of the alloys with Sn additions is attributed to the increase and decrease in the amount and size of Mg 2 Si particles in the matrix and intermetallic phase morphologies [6][7][8][9][10][11]

Impact strength
The impact behaviour of the alloys was investigated in AC, AR and PAR state. Fig. 6 shows the impact strengths of the alloys in AC, AR and PAR state. The impact strengths of the A, B and C alloys in AC state were determined as 12, 16 and 22 J, respectively. After hot rolling, the impact strengths of the alloys were increased to 29, 36 and 32 J, and after peakageing, impact strengths were further increased to 48, 54 and 53 J, respectively. An increasing trend in the toughness of the alloys was observed from AC state to PAR state. The impact strengths of the AC alloys were increased by 2.4, 2.3 and 1.5 times after hot rolling, and by 4, 3.4 and 2.4 times after peak-ageing treatment, in A, B and C alloys, respectively. It was observed that peak-ageing treatment was more effective in increasing the toughness of the alloys. The marked improvement in the toughness of the alloys after peak-aging can be attributed to the improvement in the ductility [11], as indicated by the finer ductile dimple features on the fracture surface of the alloys in PAR state than AC state (cf. Fig. 7). Moreover, it was found that Sn addition had resulted in the increase in impact strength of the 6061 alloys. In the AC state, the highest strength was observed in the alloy C. Whereas, in AR and PAR states, the highest toughness was observed in the alloy B. At maximum strength (i.e., PAR state), the impact strengths 6061 alloy were improved by about 12.5 and 10.4 % by the addition of 0.04 and 0.08 wt.% Sn, respectively.  ] also revealed cracks, facets, and dimple features. It is observed that dimples are smaller in PAR alloy B than in PAR alloys A and C. A significant difference was also seen in the fracture surfaces of the respective alloys at the AC and the PAR states. It can be seen from Figs. 7(d-f) that relative to AC alloys, the dimples are smaller and denser in PAR alloys. This observation is in line with Xu et al. [13]. The impact fracture surface of the PAR alloys were mostly dominated by smaller dimples, indicating the fracture mechanism is primarily by ductile mode. Fig. 8 shows the magnified view of the fracture surfaces of the PAR alloys. The presence of second-phase particles in the alloy had resulted in crack propagation, and a larger number of crack particles were observed on the fracture surface. These crack particles are more prominent in the dimple regions. Moreover, coarse dimples could be seen in A and C alloys [cf. Figs. 8(a, c)] compared to the fine deep dimples seen in B alloy [cf. Fig. 8(b)], which is an indication of better ductility of alloy B [14]. Hence, a higher impact strength can be observed in alloy B in rolled condition. Additionally, from the elemental concentration profile (cf. Fig. 8), it could be seen that the concentration of aluminium increases in the tear ridges as compared to the other regions. With the decrease in the intensity of the Al concentration, an increase in the intensity of Mg, Si and Fe concentrations was observed and vice versa. The intensities of the concentration of Si and Fe particles were higher in the fracture surfaces with crack particles than in the fracture surfaces in the absence of the crack particles. The EDS spectrum of the crack particle [see spot 1 in Fig. 8(c)] is also recorded to determine the elemental compositions. The EDS spectrum detects Al, Fe, Mn, Si elements in the crack particles [cf. Figs. 8(d)]. This indicates that Al(Fe, Mn)Si elements were accountable for fracture of the alloys [15].