IMPROVEMENT OF FRICTION SPOT WELDING PROCESS

Transcription

1 192 Chapter-9 IMPROVEMENT OF FRICTION SPOT WELDING PROCESS 9.1 INTRODUCTION The aerospace and automotive are continuously exploring opportunities to reduce the weight by replacing conventional materials with aluminum, magnesium, composites, etc. In automotive industry, the body structural weight can be reduced by replacing conventional steel with aluminum for the closure panels such as roof, deck lid, etc. It is very difficult to completely replace the steel with aluminum because of various strength requirements defined by regulatory requirements and standards such as Federal Motor Vehicle Safety Standards (FMVSS), ECE standards etc. Therefore combination of advanced high strength steel (AHSS) and aluminum are most suitable for weight reduction. But there are challenges when it comes to joining of these materials. Generally, automotive body is welded by using resistance spot welding process but, it is not suitable for welding aluminum to aluminum and aluminum to steel. Therefore friction stir spot welding process was used for welding. But it has some limitations as describe in section 9.2. The aerospace industry is involved in extensive research to replace the riveting process by spot welding process. The present

2 193 technology such as friction stir spot welding process and refill friction stir spot welding process are not reliable because of certain draw backs as discussed in section PROBLEMS ASSOCIATED WITH FRICTION STIR SPOT WELDING PROCESS AND REFILL FRICTION STIR SPOT WELDING PROCESS The problem associated with friction stir spot and refill friction stir spot welding process are as follows: 1. Formation of void in the middle of the weld zone after welding. 2. Very small contact zone 3. Ineffective removal of oxides and other contamination from the weld interface (as there is no relative motion between them) 4. Lower weld strength 5. Poor mixing of materials (sheets) used for lap joint welding (as there is no relative motion between them) and 6. Large heat effected zone, etc. To address the above issues, refill friction stir spot welding process was introduced but, it also has certain drawbacks, due to which they are not reliable for critical applications such as aerospace. follows: The drawbacks of refill friction stir spot welding process are as

3 194 The refill friction stir spot welding sometimes fails to refill the plasticized material in the cavity formed by the tool thus, resulting in cavity at the middle of the upper and lower sheet of lap joint as shown in Figure 9.1. Ultrasonic method and X-ray technique is required to detect such defects. The cavity formed is not visible from outside therefore it is more dangerous for critical applications as 100% inspection of such defects is not possible. Thus, reduces the reliability of this welding process. Sheet 1 Sheet 2 Cavity is formed in the middle of the weld Figure 9.1 Defects in refill friction stir spot welding process To overcome the above mentioned problems and new friction spot welding process is invented during this research and same is discussed in next section 9.3.

4 THE NEW FRICTION SPOT WELDING PROCESS The new friction spot welding is a process which utilizes a new joint design for joining two sheets by heat generated due to friction between rotating filler material rod and the sheets under compressive force. Weld is completed by application of upset force after cessation of relative motion. In the present research the following new spot welding processes are investigated and compared with conventional friction stir spot welding process. a) Friction spot welding process (with same material filler rod) b) Friction spot welding process (with steel filler rod) c) Friction spot welding process (with copper filler rod) 9.4 EXPERIMENTAL PROCEDURE In this research following material combinations are used for investigations. a) Al 6061 sheet to Al 6061sheet (with Al 6061 as filler material) b) Al 6061 sheet to Al 6061sheet (with copper as filler material) c) Al 6061 to steel (with steel as filler material)

5 196 The thicknesses of the sheets were considered as 1 mm and width and length of the sheet were considered as 25mm and 100 mm respectively. The diameter of filler material and pin were considered as 9 mm and 6 mm respectively. The pin is tapered at an angle of 5 degrees. The height of pin was considered as 1.4 mm. The operational steps involve in new friction spot welding process (with same material filler rod), new friction spot welding process (with steel or copper filler rod) and the conventional friction stir spot welding process are illustrated in the Figure 9.2, Figure 9.3 and Figure 9.5 respectively. Optimum weld parameters for welding were selected for each combination of weld. Friction welding was done using friction welding machine. Shear test was conducted to evaluate the weld strength. The weld specimens were cut perpendicular to top surface of upper sheet and passing through the centre of the weld. The weld specimens are prepared for microstructural study as per standard procedure. The microstructure is analyzed using optical microscope attached with image analyzer. The results of proposed new friction spot welding process with same material (Al 6061) filler rod and new friction spot welding process with steel and copper filler rod are compared with conventional friction stir spot welding process.

6 THE NEW FRICTION SPOT WELDING PROCESS WITH FILLER ROD OF SAME MATERIAL (AL 6061) This new process is designed for joining two sheets with a filler rod of same material as of sheets. In this case, Al 6061 sheet to Al 6061 sheet are welded with Al 6061 filler rod as described in Figure 9.2. Al 6061 Al 6061 Step 1: Both sheets are clamped together Al 6061 Step2: The joint design is prepared by machining as shown in figure 9.2(b). Step3: The weld cycle is started by rotating the cylindrical filler rod under the compressive force and heat is generated as Al 6061 filler rod comes in contact with the upper sheet and the material gets plasticized and facilitates in penetration of filler rod. Step4: Much more heat is generated as the shoulder of the filler rod touches the upper sheet which plasticizes the interface material and filler rod is pushed and stirred to form metallurgical bond and weld is completed by application of upset pressure Step5: Heat is generated by friction between filler rod and sheets and weld is completed by application of upset pressure. Figure 9.2 Illustration of operation steps for new friction spot welding process

7 THE NEW FRICTION SPOT WELDING PROCESS WITH FILLER ROD OF STEEL AND COPPER The new friction spot welding of Al 6061 sheet to Al 6061 sheet with copper filler rod and Al 6061 sheet to steel sheet with steel as filler rod is as described in Figure 9.3. Al 6061 Step 1: Both sheets are clamped together Steel Steel or Copper Step2: The weld cycle is started by rotating the cylindrical filler material, under the compressive force and heat is generated as with filler rod comes in contact with the upper sheet and the material gets plasticized which facilitates in penetration of filler rod. Step3: Much more heat is generated as the shoulder of the filler rod touches the upper sheet and interface material is softened. The filler rod is pushed and stirred to form metallurgical bond and weld is completed by application of upset pressure. Step 4: The extra filler rod can be removed by machining operation Figure 9.3 Illustration of operational steps for new friction spot welding process (with steal filler rod)

8 199 The photograph of new friction spot welded part of Al alloy sheet to Al alloy sheet with copper as filler rod is shown in Figure 9.4 Figure 9.4 New friction spot welded part (Two Al alloy sheets welded with copper filler rod) THE FRICTION STIR SPOT WELDING The conventional friction stir spot welding process is described in Figure 9.5. In this process heat is generated due to friction between non consumable tool made up of polycrystalline cubin boron nitrade (PCBN) and the sheets under the compressive force. The operational steps involved in the conventional friction stir spot welding process are illustrated in Figure 9.5 The photograph of friction stir spot welded specimen is shown in Figure 9.6.

9 200 Al 6061 Al 6061 Step 1: Both sheets are clamped together. Pin PCB Shoulder Step2: The weld cycle is started by rotating the non consumable tool under the compressive force. The upper sheet is plastisized when pin comes in contact. The softened material under the pin facilitates in penetration of tool. Step3: Much more heat is generated when tool shoulder comes in contact with the upper sheet. The material around the pin is pushed and stirred to form metallurgical bond. Step4: The tool is retracted after completion of welding. The void remains in the middle of the weld. Figure 9.5 Illustration of operational steps for friction stir spot welding process Figure 9.6 Friction stir spot welded specimen

10 RESULTS AND DISCUSSIONS The weld strength of friction spot joint is evaluated by shear strength testing machine by applying shear load on the weld. The load at which it shears is that of weld strength of the spot weld COMPARISON OF SHEAR STRENGTH FOR AL 6061 AL 6061 The shear strength of weld combination for Al 6061 sheet Al 6061 sheet is as shown in Table 9.1. The same is shown graphically in Figure 9.7. Shear Strength Results Filler material S.No. Welding Process Weld materials (rod) Friction Spot Welding Process Friction Spot Welding Process Friction Stir Spot Welding Process Maximum Load at which weld Failed (KN) Al Al 6061 Al Al Al 6061 Copper 3.21 Al Al6061 Not Applicable 2.85 Table 9.1 Comparison of shear strength for Al 6061 sheet Al 6061sheet

11 202 Figure 9.7 Comparison of shear strength for Al 6061 sheet to Al 6061 sheet. It is observed from Figure 9.7 that, higher shear strength is obtained by new friction spot welding with copper filler rod when compared to friction stir spot welding process. It is also observed that, in friction spot welding process, the copper filler rod gives better shear strength compared to Al 6061 filler rod. Another advantage of copper filler rod is no need for machining the sheets material prior to welding COMPARISON OF SHEAR STRENGTH FOR AL 6061 STEEL The comparison of shear strength for Al 6061 sheet Steel sheet is shown in Table 9.2 and corresponding graphical representation is shown in Figure 9.8.

12 203 Shear Strength Results S.No. Welding Process Weld materials Filler material 1 2 Friction Spot Welding Process Friction Stir Spot Welding Process Maximum Load at which weld Failed (KN) Al 6061 Steel Steel 3.5 Al 6061-Steel Not Applicable 2.47 Table 9.2 Comparison of shear strength for Al 6061 sheet Steel sheet Figure 9.8 Comparison of shear strength for Al 6061 sheet- Steel sheet It is observed from Figure 9.8 that high shear strength was obtained by new friction spot welding process with steel filler material rod as compared to shear strength obtained by conventional friction stir spot welding process.

13 204 The reason for low shear strength in conventional friction stir spot welding process is due to small contact zone as well as due to formation of void in the weld zone, ineffective removal of oxides and other contaminations. The weld strength of new friction spot welding of Al 6061 to steel was more compared to others welding processes due to following: The steel filler material is in direct contact to steel sheet and this combination has higher strength when compared to other weld combinations Interlocking of filler steel with the steel sheet and Al 6061 sheet Large weld area. No unbound region, as oxides and other contaminations are removed effectively at the weld interface due to direct contact of filler rod and the sheets No void is formation; therefore there is no thinning of lower sheet and upper sheet due to penetration of tool pin and tool shoulder. The aspects are shown in Figure 9.9.

14 205 Figure 9.9 New friction spot welding processes for Al 6061 to steel (With steel as filler material) The new friction spot welding process can be used for following applications: 1) The new friction spot welding process can be good replacement for friction stir welding process, refill friction stir welding process and riveting process. 2) Welding of two sheet which are incompatible (non weldable) can be welded together by a third filler material which is weldable with both the sheet metals.

15 MICROSTRUCTURAL ANALYSIS OF FRICTION STIR SPOT WELD The microstructure helps to study interface, lack of bond, inclusions and defects etc. In friction stir spot welding process of Al 6061 sheet Steel sheet void is formed at the centre of weld, as shown in the Figure Figure 9.10 Void and unbound zone in friction stir spot welded sheets of Al Steel Al 6061 Al 6061 Steel Steel (a) Welding Specimen (b) Microstructure Figure 9.11 Microstructure of friction stir spot welded sheets of Al 6061-Steel

16 207 Sometimes unbound zone are formed at the weld interface. This is due to presence of oxide layers as there is no relative motion between the interface surfaces. Therefore removal of oxide layer from the interfaces is not effective in conventional friction stir spot welding process. The weld contact zone in this type is very small and it is only at the inner periphery and at the adjacent interface surface as indicated by red color circle in the Figure 9.11 (a). The microstructure for friction stir spot welding is as shown in Figure 9.11 (b). The grains adjacent to the tool were very fine and these fine grain sizes 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 phenomenon is accompanied by hardening of the metal MICROSTRUCTURE OF NEW FRICTION SPOT WELD OF AL 6061-STEEL The location where microstructure of friction spot weld of Al 6061-Steel is observed is at contact of zone of Al 6061 upper sheet with steel filler material rod is shown in Figure The corresponding microstructure is shown in Figure Excellent contact is observed throughout the weld zone. No cavity is formed in the weld zone as external filler material is used. No unbound zone is formed at the weld zone as rotating filler material rod which was in

17 208 direct contact with the sheets during welding, which removes the oxides and other contamination effectively from the interface. No weld defects where observed at the interface. Refer Figure 9.13 for microstructure Figure 9.12 Friction spot weld (Locations where Microstructure is observed) The grains adjacent to the steel filler rod were very fine and these fine grain sizes 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 grain size decreases as distance from the weld line decreases. Figure 9.13 Microstructure of friction spot welding (Al 6061 steel interface)

18 MICROSTRUCTURE OF NEW FRICTION SPOT WELD OF AL 6061-AL 6061 The microstructure of friction spot weld Al 6061-Al6061 is shown in Figure Weld integrity is so well that weld line was completely disappeared. No weld defects or inclusions were observed at weld interface. Figure 9.14 Microstructure of friction spot welding of Al 6061 Al 6061 (with Al 6061 as filler material)