MATERIALS TRANSACTIONS
Online ISSN : 1347-5320
Print ISSN : 1345-9678
ISSN-L : 1345-9678
Development of Penetrating Tool Friction Stir Incremental Forming
Wei JiangTakuya MiuraMasaaki OtsuMasato OkadaRyo MatsumotoHidenori YoshimuraTakayuki Muranaka
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2019 年 60 巻 11 号 p. 2416-2425

詳細
Abstract

To form sheet metals into concave-convex mixed shapes without using special machines or dies, a novel forming tool, referred to as a penetrating tool, and a new forming method, referred to as penetrating tool friction stir incremental forming were developed herein. The proposed penetrating tool is composed of two dome shape tools, called the top and bottom tools. The top and bottom tools are vertically symmetric and joined by a middle screw. Pure aluminum (JIS: A1050-O) sheets with a thickness of 2 mm were used for workpieces. Forming of concave, convex, and concave-convex mixed shapes were implemented by using penetrating tool friction stir incremental forming under clockwise and counterclockwise tool path direction. Experimental conditions which obtained by a preliminary experiment were tool gap between the top and bottom tool of 1.8 mm, tool rotation rates of 1000–3000 rpm and tool feed rates of 200–3000 mm/min. Formability by the developed method was evaluated by the formable height or depth. Not only the shapes but also the distribution of thickness of the PTFSIFed sheets were measured. Material flow was discussed by thickness in Z direction due to keeping the volume constant before and after forming. From the experimental result, the concave, convex, and concave-convex mixed shapes can be formed using the proposed method. However, the formable depth or height remained relatively shallow. For more dramatic depth or height forming, groove-like defects occurred in advancing side of the formed sheet and the sheet fractured due to penetration of the sheet by the groove-like defect. From the distributions of thickness in Z direction of formed sheet, the material flow from the advancing to retreating side was shown to cause the groove-like defects.

1. Introduction

To manufacture products with a short lead time, various rapid manufacturing technologies, including additive manufacturing, have been proposed.13) For sheet forming, incremental sheet metal forming (ISF) is a high-profile rapid manufacturing method with great potential for rapid sheet metal forming.46) The principle of ISF involves layer by layer modeling. A complex three-dimensional shape can be disassembled into contour lines and formed by locally pushing sheets using a hemispherical tool along the contour lines. Because no dies are used during this type of forming, the manufacturing cost and time can be significantly reduced. Therefore, this method is suitable for small batch production, rapid prototyping, and sheet metal product repair.

Single point incremental forming (SPIF) is a commonly studied variant of ISF. In SPIF, the peripheral region of a sheet is fixed to a table and a forming tool locally indents the sheet to induce plastic deformation. Because the tool pushes the sheet from one side, only a bulged shape can be formed. Concave-convex mixed shapes cannot be formed by conventional SPIF, which limits its applications. To form sheets into concave-convex mixed shapes by incremental sheet metal forming, two main variants have been proposed. The first is two points incremental forming (TPIF), in which a support or a die is used under the sheets.7,8) A support or a die can be a full or partial die. The other method is double-sided incremental forming (DSIF), which employs two tools to form the sheets from both sides simultaneously.9,10) Although TPIF and DSIF can form sheets into concave-convex mixed shapes, the fabrication of a half die is required for TPIF and a special forming machine with two tools on both sides of the sheet is also needed. Die making in TPIF loses an important advantage of incremental sheet metal forming and using a special machine negatives the advantage of using a common 3-axis milling machine. If concave-convex mixed shapes could be formed with a common 3-axis NC milling machine without dies or supports, incremental sheet metal forming could be used for a greater variety of applications.

Friction stir welding (FSW), which is characterized by high welding efficiency and easy operation, has attracted significant attention.11) Bobbin tool friction stir welding (BTFSW) is a type of FSW12) where a bobbin tool, owing to the shape of the two shoulders, is vertically symmetric and connected by a probe to be used as the tool. During the welding process, a sheet is clamped by the top and bottom shoulders. The probe penetrates the sheet, and a hole is generated by passing the probe and is filled again after passing of the tool.

Utilizing the characteristics of BTFSW with a bobbin tool penetrating a sheet, a novel forming method for producing concave-convex mixed shapes with one tool and without dies or supports could be developed by combining BTFSW and ISF. In this novel forming process, a “bobbin tool” with a larger radius corner was used and concave shapes were formed by pushing the sheet with the top tool. Convex shapes were formed by pulling the sheet with the bottom tool, and concave-convex mixed shape were formed by combining the use of the top and bottom tools. Otsu et al. combined FSW with ISF and developed a novel incremental forming method called friction stir incremental forming (FSIF). This combined method achieved great success, allowing magnesium and aluminum alloys sheets to be formed successfully without using external heating.13,14) Therefore, it is possible to combine BTFSW and ISF. Because the tool penetrates the sheet during forming, the tool was called the penetrating tool. This novel forming method involves a combination of BTFSW and ISF, and was called penetrating tool friction stir incremental forming (PTFSIF).

In this study, penetrating tool friction stir incremental forming is developed by combining BTFSW and ISF. An original tool was designed for forming the sheets into concave-convex mixed shapes using only one tool without a die, of course, concave shape and convex shape only can also be formed by this proposed method. Using the developed tool, commercial pure aluminum sheets with 2 mm thickness were formed into concave, convex, and concave-convex mixed shapes. In addition, the distributions of thickness in Z direction of the sheets formed by this method were measured and material flow during the forming was discussed.

2. Penetrating Tool Friction Stir Incremental Forming

2.1 Penetrating tool and its forming principle

The hemispherical ended tool generally used in ISF, typical tool shape used in BTFSW, and shape of the proposed penetrating tool are shown in Fig. 1. The bobbin tool contains two shoulders connected by a probe, generally composed of a screw thread. The hemispherical ended tool is typically used for incremental sheet metal forming. The penetrating tool has both characteristics of bobbin and an incremental sheet metal forming tools, but a larger corner radius of than a bobbin tool. Therefore, it is expected that the penetrating tool can perform both FSW and ISF.

Fig. 1

Three kinds of tool shapes. (a) Incremental forming tool, (b) bobbin tool, and (c) penetrating tool.

Figure 2 shows a schematic illustration of PTFSIF where a sheet is clamped in the gap between the top and bottom tools. The penetrating tool rotates at a high rate and travels along the desired tool path. During this process, the sheet is heated from the friction generated between the tool and sheet. Plastic flow occurs around the probe and under the shoulders due to the mechanical stirring by tool rotation. Thus, the tool can pass through the sheet freely without leaving a hole similar to general FSW. A concave or convex shape can be formed by pushing the sheet using the corner part of the tool and moving the tool upward or downward simultaneously similar to general ISF. In addition, a concave-convex mixed shape can be formed by changing the vertical direction motion during forming.

Fig. 2

Schematic illustration of PTFSIF.

2.2 Design of the penetrating tool

Two general structures of a penetrating tool can be, similar to a bobbin tool: a monolithic structure or separable structure. A separable structure was used due to the disadvantages of monolithic structure listed below. The first and most important disadvantage is that it is impossible to remove a monolithic structured tool from the sheet after forming. Second, a whole part of the tool must be discarded upon probe fracturing, which often occurs in BTFSW.15) Third, it is impossible to adjust the gap, necessitating the manufacture of various penetrating tools of differential gap sizes. Fourth, it is difficult to manufacture a monolithic penetrating tools with a small gap.

A separable penetrating tool that can overcome the abovementioned disadvantages was designed, as shown in Fig. 3. The separable penetrating tool was composed of six parts: tool shank, top tool, middle screw (probe), bottom tool, screw locker, and joint screws. A commercial hexagonal bolt was used as a middle screw to join the top and bottom tools, which were also jointed with a screw locker, forming a column with a groove. The screw locker was jointed with the middle screw using the groove as a key. The screw locker, middle screw, and bottom tool were joined together by two joint screws. The top tool jointed the tool shank using the double-nut tightening method. A hole for tightening and removing the top tool was also drilled. Using a separable penetrating tool, the gap between the top and bottom tools can be adjusted, usability is improved, and cost and changing time were reduced when the middle screw must be changed because tool fracturing occurs. All parts were composed of a stainless steel (JIS: SUS304.). It should be noted that the combination of screw thread orientations and tool rotation directions can only be selected in the fixed combination, which means a right-thread bolt with clockwise (CW) tool rotation direction and a left-thread bolt with counterclockwise (CCW) tool rotation direction. The top tool and also the middle screw will be loosen from the tool shank because the top tool and also middle screw will be subjected a torque of the opposite direction to the tool rotation direction when the other combination was applied. In this work, combination of a right-thread screw and tool rotation direction of CW was used. Figure 4 shows the appearance of the prepared penetrating tool.

Fig. 3

Structure of the separable penetrating tool.

Fig. 4

Photo of the designed separable penetrating tool.

3. Experimental Methods

3.1 Friction stir welding using the penetrating tool

It is not advisable to conduct the PTFSIF experiment from the beginning because welding and forming will have to be optimized simultaneously in a 3-dimensional space. Thus, a friction stir welding experiment with stir-in-plate using the new developed penetrating tool was conducted as a preliminary experiment for finding the successful welding conditions, referred to the values of a gap between the top and bottom tools, tool rotation rate, and tool feed rate. Here the successful friction stir welding means tool travels freely without leaving any defects in the sheet.

A machining center (Okuma Corp., MILLAC44V II) was used for the friction stir welding experiment and Fig. 5 shows the experimental setup. A sheet was placed on a table with a square hole and fixed in place using a blank holder and bolts. Because the tool penetrates the sheet during forming, a tool hole was drilled in the left bottom corner of the square hole to adjust the location of the penetrating tool and sheet in the Z direction at the starting point. The developed separable penetrating tool was used as a welding tool and its dimensions are shown in Fig. 6. Commercial pure aluminum (JIS: A1050-O) sheets were used for the workpiece with dimensions of 200 mm × 200 mm × 2 mm. Pure aluminum was selected because the penetrating tool material was stainless steel (JIS: SUS304), which cannot be used in friction stir welding due to its insufficient hardness and heat resistance. The main parameters in friction stir welding were the gap between the top and bottom tools, Gp, tool rotation rate for welding, ωw, and tool feed rate for welding, vw. The values of these parameters used in the experiments are shown in Table 1. The rotational direction of the penetrating tool was CW.

Fig. 5

Photo of the experimental setup.

Fig. 6

Dimensions of the penetrating tool.

Table 1 The experimental conditions of FSW.

When designing a tool path of friction stir welding experiment with stir-in-plate, following points should be considered. First, the welding length should be as long as possible because welding condition change with increase of welding length. Second, straight line, curve of CW direction and curve of CCW direction should be contained in the tool path. Third, the shape of the workpiece should be also taken into consideration. In this experiment, the used workpiece was a 200 mm × 200 mm square sheet. Considering above mentioned, a tool path as shown in Fig. 7 was designed for the FSW, and corner radius of R = 20 mm was the results of those considerations.

Fig. 7

Tool path of the FSW (Top view).

In the Fig. 7, the dotted line at the beginning of the tool path represents the FSW guide line. In BTFSW, setting a guide line is a common method for avoiding the tool fracture at the tool plunging stage. In the guide line, the tool was plunged into the sheet usually at a slower feed rate (comparing with tool feed rate for welding). In this work, tool was plunged into the sheet at a tool rotation rate for guide line of ωg = 1000 rpm and tool feed rate for guide line of vg = 200 mm/min. Subsequently, the tool rotation rate and feed rate were changed to tool rotation rate for welding, ωw, and tool feed rate for welding, vw. After welding, the tool was moved back to the starting point. Herein, since friction stir welding was used as a preliminary experiment for PTFSIF, the welding results were judged from appearance of the welded sheets and occurrence of tool fracture during FSW.

3.2 Penetrating tool friction stir incremental forming experiment

The forming machine, penetrating tool, and workpieces used for the forming experiments were identical as those used in the friction stir welding experiments described in Section 3.1. The forming of three objective shapes, as well as concave, convex, and concave-convex mixed shapes were performed under tool path direction of CW and CCW. For the concave and convex shapes, the objective shape was a truncated cone. The depth and height of the concave and convex cones were called d and h. The objective shape for the concave-convex mixed shape involved the overlap of a larger concave truncated cone with a smaller convex truncated cone at the concave cone center, where w was distance between the large concave and small convex cones. The bottom diameters of the objective concave, convex, and concave-convex mixed shapes were 114 mm with a wall angle θ of 45°. Figure 8 shows the tool path of PTFSIF of the three objective shapes. The guide line tool path for tool plunging into the sheet in PTFSIF was set to the same specifications described in Section 3.1, which the tool were plunged into the sheet at a tool rotation rate for guide line of ωg = 1000 rpm and tool feed rate for guide line of vg = 200 mm/min in the guide line. After leading the tool into the sheet, the tool rotation rate and feed rate were changed to the tool rotation rate for forming. ωf, and tool feed rate for forming, vf, using a contour line strategy. The gap, Gp, tool rotation rate for forming, ωf, and tool feed rate for forming, vf, were set based on the FSW experimental results reported in Section 4.1. Table 2 lists the experimental conditions of PTFSIF.

Fig. 8

Tool path of PTFSIF (e.g. tool path direction of CCW). (a) Top view tool path of concave, convex and concave-convex forming, (b) Front view tool path of concave forming. (c) Front view tool path of convex forming. (d) Front view tool path of concave-convex forming.

Table 2 The experimental conditions of PTFSIF.

Forming was stopped when the NC program ended or sheet fracturing occurred. After forming, the sheet and tool were removed by disassembling the tool. After the penetrating tool was removed from the sheet, a key hole was left in the sheet as well as BTFSW. A surface profile of the formed shape was measured using a laser displacement meter. The sheet thickness and thickness in Z direction of formed sheet were measured using the sectional profile photos via an image processing software.

4. Results and Discussion

4.1 Friction stir welding using the penetrating tool

Figure 9 shows the results of friction stir welding under different tool gaps, Gp, tool rotation for welding rates, ωw, and tool feed rates for welding, vw. Open circle marks indicate that FSW was successful. Cross marks indicate that sheet fractured. Triangle marks indicate that welded sheets with defects. Square marks indicate that tool fractured. It is clear that at Gp = 1.8 mm, the welding conditions was widest. Figure 10 shows the appearances of a successfully welded sheet, fractured sheet, and welded sheet with a defect in bottom surface. Thus, it was determined that the Gp should be set to a proper value smaller than the sheet thickness, but not excessively small. From these optimization results, Gp = 1.8 mm was used in the subsequent PTFSIF.

Fig. 9

FSW results using the penetrating tool under different gaps, tool rotation rates for welding, and tool feed rates for welding. (a) Gp = 1.6 mm, (b) Gp = 1.8 mm, and (c) Gp = 2.0 mm.

Fig. 10

Photo of the FSWed sheet by the penetrating tool. (a) Successfully welded sheet (Gp = 1.8 mm, ωw = 1000 rpm, νw = 200 mm·min−1), (b) enlarged view of the successfully welded sheet (Gp = 1.8 mm, ωw = 1000 rpm, νw = 200 mm·min−1), (c) fractured sheet (Gp = 1.6 mm, ωw = 3000 rpm, νw = 2000 mm·min−1), and (d) Sheet with a defect (Gp = 2.0 mm, ωw = 1000 rpm, νw = 1000 mm·min−1).

4.2 Penetrating tool friction stir incremental forming

Figure 11 shows the forming results of concave and convex shapes at different forming depths or heights under the tool path direction of CCW. The forming conditions involved a tool rotation rate for forming of ωf = 1000 rpm and tool feed rate for forming of νf = 200 mm/min. The open circle marks indicate that forming was successful, without defects or fractures after forming. Square marks indicate groove-like defects after forming. All of the groove-like defect was located in the bottom surface of the sheet. The cross marks indicate that the sheet fractured before forming completion. Figure 12 shows the appearances of a successfully formed sheet, groove-like defect in the sectional view, and the fractured sheet described in Fig. 11.

Fig. 11

Forming results of PTFSIF (ωf = 1000 rpm, νf = 200 mm·min−1).

Fig. 12

Appearance of the forming results of the PTFSIFed sheet with a concave shape. (a) Without defects, (b) with a groove-like defect, and (c) sheet fracturing.

The forming results at varied tool rotation rates for forming, ωf, tool feed rates for forming, νf, and tool path direction of CW followed the same trend as shown in Fig. 11, which the sheet was formed successfully only at shallow depths or heights, and defects occurred when the forming continued. From those results, it was determined that concave and convex forming were possible by PTFSIF, but the formable depth or height was quite shallow.

The height or the depth of 7 mm, which are height or depth just before sheet fractured in the experiment as show in Fig. 11, was used as samples of appearances and shapes of concave, convex and concave-convex mixed shape formed by PTFSIF. The appearances of concave, convex, and concave-convex mixed forming are shown in Fig. 13 and their measured surface profiles are plotted in Fig. 14. From Fig. 13, the surface conditions of the formed sheet were not optimal since the tool mark of each contour can be clearly observed. From Fig. 14, it is clear that an upheaval formed because the materials in the center did not sink or rise to the same extent as the top or bottom tool pushing. All achieved depths or heights were shallower than the ideal shape.

Fig. 13

Appearance of the three shapes formed by PTFSIF (ωf = 1000 rpm, νf = 200 mm·min−1). (a) Concave (d = 7 mm), (b) convex (h = 7 mm), and (c) concave-convex mixed (d = 7 mm, w = 12 mm, h = 7 mm).

Fig. 14

Sectional profile of the formed sheet by PTFSIF (ωf = 1000 rpm, νf = 200 mm·min−1). (a) Concave (d = 7 mm), (b) convex (h = 7 mm), and (c) concave-convex mixed shape (d = 7 mm, w = 12 mm, h = 7 mm).

4.3 Groove-like defect and material flow

Figure 15 shows the appearance of a groove-like defect at forming depths of d = 6, 7, and 8 mm for concave forming. Groove-like defects occurred at d = 6 mm, but only occupied a small area, whereas the groove-like defects occurred over all the formed area at d = 7 mm. The width and depth of the groove-like defects at d = 8 mm were wider and deeper than that at d = 7 mm. The sheet fractured at d = 8 mm, indicating that the depth of a groove was equal to the original sheet thickness. These results indicate that groove-like defects become wider and deeper as the forming proceeds, finally resulting in sheet fracture as the groove penetrates into the sheet. Therefore, it is clear that the formation of groove-like defects during forming ensured that only shallow forming can be achieved.

Fig. 15

Appearance of the groove-like defect at different forming depths (Concave forming, ωf = 1000 rpm, νf = 200 mm·min−1). (a) d = 6 mm, (b) d = 7 mm, and (c) d = 8 mm.

In FSW, the advancing side (AS) is the side at which the tool rotation and traveling direction are the same, whereas the retreating side (RS) is the opposite.16) In this experiment, the tool rotation direction was CW, so the location of AS and RS on the sheet depends on the tool path direction. The location of AS and RS on the sheet under tool path direction of CW and CCW were shown in Fig. 16. When tool path direction is CW, outer of the tool path is AS and the inner of the tool path is RS. From the Fig. 16, it should be noted that the area of AS increases with forming proceed. When tool path direction is CCW, outer of the tool path is RS and the inner of the tool path is AS. So, the area of RS increases with forming proceed. In PTFSIF, whether for tool path direction of CW or CCW, the groove-like defects formed at bottom surface because materials was transferred from bottom surface to top surface due to the forming conditions where the right-thread screw and tool rotation were CW.

Fig. 16

Location of AS and RS on the sheet. (a) tool path direction of CW. (b) tool path direction of CCW.

Figure 17 shows the ideal changing of sheet thickness during ISF in cross-sectional plane perpendicular to Y-axis. θ is the wall angle, t0 is the initial sheet thickness, t1 is thickness of formed sheet, and tz is thickness in Z direction of formed sheet. Thickness in Z direction, tz was defined as the distance between upper and lower surface in the same location in X and Y direction. Because no material was consumed, the volume of sheet should not change, which means tz equals t0. Volume distribution was defined as material’s volume along the radial direction. Because volume was effected by the thickness in Z direction of formed sheet, tz, can be used as an indicator of material flow in and out in the AS and RS. Figure 18 shows sectional profiles of PTFSIFed concave shape under tool path direction of CW and CCW. For a better understanding of material flow, the sectional profile was sectioned into unformed, formed, tool place, and center parts. It can be seen that under the tool path direction of CW, burr was generated in the bottom of the sheet due to the material flow.

Fig. 17

Sheet thickness changing in ideal incremental forming.

Fig. 18

Cross section of the formed sheet by PTFISF (concave shape, ωf = 1000 rpm, νf = 200 mm·min−1, d = 10 mm). (a) tool path direction of CW. (b) tool path direction of CCW.

The sheet thickness distributions of formed sheet, t1 are shown in Fig. 19. From Fig. 19, it can be seen that the t1 of formed part under tool path direction of CCW was thicker than t0 and t1 of formed part under tool path direction of CW was thinner than t0. This is different from the thickness formed by general incremental forming where the thickness was assumed to observe the “sine law”.17)

Fig. 19

Distribution of thickness of PTFSIFed sheet. (a) tool path direction of CW. (b) tool path direction of CCW.

As described above, tz can be used as an indicator of material flow in incremental sheet forming. And as shown in Fig. 16, the tool path was tool formed on the sheet from periphery to center. Therefore, the area of center part was reduced as forming proceeded. From this view, the sheet can be divided into 5 areas, an unformed part, formed part, RS of tool place, AS of tool place and the center. Figure 20 shows the distribution of tz of AS and RS under tool path direction of CW and CCW. The thickness in the Z direction, tz at the RS was larger than that at the AS whether the tool path direction was CW or CCW. This indicates that the material flows from the AS to RS during PTFSIF because the volume is constant. The formation of groove-like defects was caused by the material flow from the AS to RS, which is also the reason why groove-like defects only occurred in the AS.

Fig. 20

Distribution of thickness in Z direction of PTFSIFed sheet in sectional direction (concave shape, ωf = 1000 rpm, νf = 200 mm·min−1, d = 10 mm). (a) tool path direction of CW. (b) tool path direction of CCW.

5. Conclusions

Herein, friction stir incremental forming experiments were performed using a novel penetrating tool. The following results were obtained.

  1. (1)    Forming of a concave, convex and concave-convex mixed shapes by the developed penetrating tool friction stir incremental forming was possible.
  2. (2)    The formable depth or height was shallow since groove-like defects occurred during the forming and penetrated the sheet.
  3. (3)    The material flows from the AS to the RS during penetrating tool friction stir incremental forming process. Excessive material flow caused the groove-like defects and sheet fractures.
  4. (4)    The thickness distribution of the formed sheet by penetrating tool friction stir incremental forming was different from general incremental forming.

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