OF FRICTION STIR WELDED HDPE WITH ADDITIONS OF SILICON. CARBIDE, SILICA, NANO-ALUMINA, AND GRAPHITE. Abdul Shaafi Shaikh, Muhammad ...
Materials Science and Technology (MS&T) 2012 October 7-11, 2012, Pittsburgh, Pennsylvania Copyright © 2012 MS&T'12® Joining of Advanced and Specialty Materials (JASM XIV)
EXPERIMENTAL INVESTIGATION OF MECHANICAL PROPERTIES OF FRICTION STIR WELDED HDPE WITH ADDITIONS OF SILICON CARBIDE, SILICA, NANO-ALUMINA, AND GRAPHITE
Abdul Shaafi Shaikh, Muhammad Shamir Tahir, Muhammad Kashan Akhtar Qureshi, Muhammad Zain-ul-abdein, Fazal Ahmad Khalid
Faculty of Materials Science and Engineering, GIK Institute of Engineering Sciences and Technology, Topi, Pakistan
Keywords: Friction Stir Welding; HDPE; Composite; Polymer Joining
Abstract Polymeric materials are rapidly replacing metallic materials in many applications, and as a consequence joining techniques for polymers are continually improving. Friction Stir Welding (FSW) is one such technique that has shown great potential in the joining of thermoplastic polymers, especially high-density-polyethylene (HDPE). In this paper, welding was performed at a tool revolution speed of 1800 rpm and an advancing speed of 16 mm/min to produce joints of HDPE. In order to determine the strain rate sensitivity of the joint strength, tensile testing of these joints was conducted at different displacement rates ranging from 0.50 to 5.00 mm/min. A maximum of 85% of base material strength was observed. Composite joints were formed through the additions of SiC, SiO2, nano-alumina, and graphite powders during welding at the selected parameters. The relative effects of these reinforcements on the joint strength were evaluated by tensile testing. Micro-hardness across the welds was also measured to assess the role of different reinforcements on the weld joints. The highest strength among the reinforced joints was shown by silicon carbide reinforced HDPE, and the same composite also showed an increase in hardness across the weld zone.
Introduction Compared to metallic materials, polymers and polymer matrix composites (PMCs) are fast gaining importance in the manufacturing industries. Consequently, the joining of polymeric materials is an area of special significance. Established techniques of adhesive bonding and
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various welding processes have proved inadequate in one way or another [1, 2], and more economical and robust methods are required. For thermoplastic polymers such as polyethylene or polypropylene, friction stir welding (FSW) is an efficient process for producing continuous joints quickly and efficiently [3, 4, 5]. Pioneered for Aluminium in 1991, FSW has been transferred to thermoplastics in the early 2000s [6], and is now being applied to a wide variety of high performance materials [7, 8]. Due to their low melting point and inherent low-hardness, polymeric materials are easier to join through FSW where tool material does not need to be as hard as for metals, tool wear is minimal, and the machine power requirement is relatively low. Previously, particulate reinforcements have been added into polymers to form composites, which have shown remarkable improvements in mechanical properties over simple polymers [9, 10]. Most particulate reinforced polymer composites are formed by melt compounding, though the possibility of production through Friction Stir Processing exists. This avenue is also explored in the present work. This work was aimed at investigating the mechanical properties of the FSW joint of high density polyethylene (HDPE) with and without the additions of SiC, SiO2, nano-alumina, and graphite powders.
Experimental Work Friction stir welds were produced on commercial 6 mm thick HDPE sheet. A rigid and immobile fixture was used to counteract the considerable amount of force being exerted by the weld tool on the sheet (see Figure 2). The tool for welding was fabricated from stainless steel. As illustrated in Figure 1, the important features of the tool include a 15° tapered pin, having a length of 5.2 mm. A pair of grooves was machined into the pin, 1 mm deep and 1.2 mm apart. The shoulder diameter was 20 mm with a concavity of 6°. A vertical milling machine was used to produce the welds, as shown in Figure 2. The tool was traversed along the joint line at 16 mm/min with a rotation speed of 1800 rpm. The weldments produced were 350 mm long by 150 mm wide. In the case of composite welds slots were produced along the faying surfaces of the weld panels using a drill bit of diameter 2 mm. The dimensions of the slots were 12mm x 2mm x 3mm. The distance between each slot was 25 mm. These slots were then filled with powders of SiC, SiO2 (as received and reduced), nano-alumina, and graphite of particle sizes shown in Table 1. The slots were then sealed by applying candle wax to prevent powder particles from being ejected during welding.
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Figure 1: Tool used for welding. All dimensions in mm Powder Silicon Carbide Silica (As Received)
Avg. Particle Size (μm) 45 50
Silica (Reduced)*
5
Graphite Nano-Alumina
50 0.05
*Reduced from 40-50 μm by ball milling for 10 hrs at 150 rpm with WC balls and jars
Table 1: Powders used for reinforcement Face-milling of the weldment was done on both the crown and root sides. The purpose of face milling was to remove any cracks, voids or any irregularities induced from fusion at the bottom side of the weld and to remove surface porosity and weld beads produced on the top. Dog-bone shaped tensile samples (see Figure 3) were cut from welded sheets using an end-mill cutter of diameter 2 mm. Tensile testing was carried out on an Instron 8501 series 100 KN tensile testing machine. Base material and plain HDPE welded samples were tested at 0.50, 1.63, 2.75, 3.88, and 5.00 mm/min displacement rates. Composite weld samples were only tested at 5.00 mm/min.
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Samples for hardness testing were ground and polished to a finish of approximately 5 microns. Micro-hardness testing using a Vickers indenter was conducted by applying a 50 g load for a dwell time of 15 seconds, with a 1 mm gap between indents.
Figure 2: Friction stir welding on a vertical milling machine with fixture and back plates
Figure 3: (a) Schematic of tensile test samples used. Not drawn to scale. All dimensions in mm. (b) Tensile test sample of composite weld showing region of graphite reinforcement
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Results and Discussion Figure 4 shows the trend of ultimate tensile strength (UTS) with displacement rate, for both the base material and the welded joint. In the base material, there is a gradual increase in UTS as the displacement rate increases. This trend is exactly replicated in the weld joints. The UTS values of welded joints were found to be less than those of base material by an amount of approximately 10%.
Figure 4: Effect of displacement rate on base material and welded joints The overall highest UTS of 20.7 MPa was achieved in welded HDPE, at 5.00 mm/min displacement rate. The base material UTS at the same displacement rate was 24.3 MPa – hence the weld had a strength of 85% of the base material. The ductility of the welded sample however was drastically reduced. Whereas the base material achieved an elongation of 150%, the welded joint fractured at only 60% (see Figure 5). In order to have comparable results the tensile testing of all composite welds was conducted at 5.00 mm/min. Figure 5 shows that there was a drop in the UTS for all composite welds. The highest UTS of 17.7 MPa was achieved with SiC additions, and lowest with silica additions (both reduced and as-received particle sizes). The UTSs of nano-alumina and Graphite powder composites was intermediate at around 16 MPa. There was a significant increase in the ductility of graphite reinforced HDPE welds, however, with an elongation of 100%.
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Figure 5: Comparison of stress-strain curves of the base material and plain and composite welds
Hardness profiles of the various welds are shown in Figure 6. Compared to plain welded HDPE, the hardness of graphite and as received silica composites is lower within the weld zone. The hardness of silicon carbide composites is higher than the rest of the welds. In the case of nanoalumina, the hardness profile closely mirrors that of the plain welded HDPE. The increase in hardness of silicon carbide reinforced welds can be deemed a result of the combination of its inherent hardness and its particle size. An even distribution of silicon carbide particles within the weld zone would have the effect of increasing resistance to indentation of the composite.
Conclusions 85% of base material ultimate tensile strength was achieved in plain HDPE welds. Increasing the loading rate resulted in a gradual increase in strength. Composite welds were successfully produced with no excess loss in strength, and increase in ductility in the case of graphite addition. Hardness across the weld was increased in the case of silicon carbide reinforced HDPE welds.
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Figure 6: Hardness profiles of the various welds
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