Generating Lap Joints Via Friction Stir Spot Welding on DP780 Steel

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Summary

Here, we present a friction stir spot welding (FSSW) protocol on dual phase 780 steel. A tool pin with high-speed rotation generates heat from friction to soften the material, and then, the pin plunges into 2 sheet joints to create the lap joint.

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Hsu, T. I., Tsai, M. H. Generating Lap Joints Via Friction Stir Spot Welding on DP780 Steel. J. Vis. Exp. (150), e58633, doi:10.3791/58633 (2019).

Abstract

Friction stir spot welding (FSSW), a derivative of friction stir welding (FSW), is a solid-state welding technique that was developed in 1991. An industry application was found in the automotive industry in 2003 for the aluminum alloy that was used in the rear doors of automobiles. Friction stir spot welding is mostly used in Al alloys to create lap joints. The benefits of friction stir spot welding include a nearly 80% melting temperature that lowers the thermal deformation welds without splashing compared to resistance spot welding. Friction stir spot welding includes 3 steps: plunging, stirring, and retraction. In the present study, other materials including high strength steel are also used in the friction stir welding method to create joints. DP780, whose traditional welding process involves the use of resistance spot welding, is one of several high strength steel materials used in the automotive industry. In this paper, DP780 was used for friction stir spot welding, and its microstructure and microhardness were measured. The microstructure data showed that there was a fusion zone with fine grain and a heat effect zone with island martensite. The microhardness results indicated that the center zone exhibited a greater degree of hardness compared with the base metal. All data indicated that the friction stir spot welding used in dual phase steel 780 can create a good lap joint. In the future, friction stir spot welding can be used in high-strength steel welding applied in industrial manufacturing processes.

Introduction

Friction stir welding (FSW) was first reported in 1991 at TWI, Abington, UK1. In 2003, Piccini and Svoboda determined a superior method of enhancing the advantages of FSW called friction stir spot welding (FSSW) for use in commercial automobile manufacturing processes2. The FSSW method involves creating a spot lap joint with no bulk area melting. The most important development for the use of FSSW has been in aluminum alloys as Al alloys deform in the welding process under high temperature conditions. The first successful example was in the automotive industry, where FSSW was used in manufacturing the entire rear door of the Mazda's RX-81,3,4.

Meanwhile, high strength steel is the dominant material of the car body, specifically dual phase steel. The literature indicates that DP600 produced with FSSW can have the same properties as the base metal, where all welding regions have similar microstructures and degrees of hardness5. FSSW methods for the use of DP steel on their microstructure of the stir zone (SZ), the thermos-mechanically affected zone (TMAZ), and the failure model of DP590 and DP600 steel have been studied by a few researchers. They observed differences in the consistency of the microstructure (ferrite, bainite, and martensite) of DP590 and DP600 steel at various rotation speeds6,7,8,9,10. Some researchers conducted comparative studies of FSSW and RSW for DP780 steel8,9. They reported that longer joining times and higher tool rotation speeds resulted in an increased bonding area for all plunges, which led to a higher shear force and shifted the mode from interfacial to pull out. They also concluded that FSSW had a higher strength than RSW. The FSSW process includes 3 steps: plunging, stirring, and retraction. The first step is plunging with a rotation tool pin close to the sheet of the lap joint and plugged into the sheet. The rotating tool shoulder in the FSSW process can generate frictional heat. In the second step, the heat can soften the sheet and facilitate plugging of the tool pin into the sheet, as well as dwell in the materials to stir two workpieces together and mix around the pin area. Finally, the pressure from the tool shoulder press on the workpieces can enhance the bonding. After the welding process, the pin can be retracted from the keyhole. The benefits of FSSW compared with RSW are a lower welding temperature, no splashing, and more stability in the manufacturing process.

Even though studies on the FSSW of advanced high-strength steels (AHSS) have been reported by various researchers, studies on the FSSW of DP590, DP600, and DP780 have focused on the microstructure and on the mechanical and failure models using various process parameters. In the present study, the FSSW of DP780 steel was considered. The protocol of the FSSW process was reported in detail, and the individual hardness in the stir zone, the thermos-mechanically affected zone, and the heat-affected zone, as well as the base metal were evaluated based on the measured microhardness.

With the continuous growth and heavy demand for weight reduction in the automotive and aerospace industries, the automotive industry has shown an increasing interest in AHSS and lap joints. For example, the conventional steel body of a car, on average, has more than 2,000 spot weld lap joints11. There are 3 common welding processes for lap joints used in the industry, including resistance spot welding, laser spot welding, and friction spot welding12. One way to decrease the weight is by using advanced high-strength steels (AHSS). The most popular materials are dual-phase and transformation-induced plasticity (TRIP) steels, which are being increasingly used in the automotive industry13,14,15,16. Because the automotive industry has increased the strength standards due to improved fuel consumption and crash energy absorption under a decreased vehicle weight, the use of different materials and welding processes is becoming an important issue.

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Protocol

1. Material preparation

NOTE: Machine the 1.6 mm thick DP780 sheets into 40 mm x 125 mm coupons. The FSSW joints are designed as lap shear specimens for the mechanical tests. Join two 125 mm by 40 mm sheets with a 35 mm by 40 mm overlap following RSW standard NF ISO 18278-2; 2005. A geometry design polycrystalline diamond tool with a truncated cone shoulder. The geometry design is shown in Figure 1a. The diameter of the pin is 5 mm; the length is 2.5 mm, and the shoulder width is 10 mm. The real tool pin is shown in Figure 1b.

  1. Safety guidelines
    1. Use devices such as a hood or baffle, goggles, and gloves for protection.
    2. Stand behind the hood or the baffle. Wear goggles and gloves to prevent splash contact or heat damage.
  2. FSSW machine setting
    1. Manufacture all joints using an MIRDC-made friction stir welder machine.
    2. Record the Z axial force and penetration depth during each joining operation using the embedded data acquisition (DAQ) system.
  3. Parameter settings
    1. In this study, use the following parameters: a tool pin rotation speed of 2,500 rpm, 4 s of tool pin dwell time, and a rate pf 0.5 mm/s of tool pin plunge into the sheet.
    2. Optimize the parameters for the operator. The range of the rotation speed is 1,000- 2,500 rpm. The range of the dwell time can be from 2-10 s, and the plunge rate can be 0.1-0.5 mm/s.

2. Procedure

NOTE: The work space is shown in Figure 2. All the manufacturing procedures are completed in the work space. Before the procedure, the welding process sequences are comprised of a combination of tool rotations and penetration depths, as well as a series of sequences including preheating, plunging, dwelling, retracting, and post heating. All steps are shown in Figure 3 in the form of a work flowchart.

  1. DP780 workpiece preparation
    1. Before the welding process, ensure that there are no impurity substrates contaminating the workpieces. Use knitted micro-fiber fabrics to wipe the surface of the workpiece to eliminate any small particles.
  2. Place the DP780 workpiece, and clamp 2 DP780 sheets (size: 125 mm x 40 mm) with an overlap of 35 mm. Fix the clean workpieces on an anvil to prevent shifting.
  3. Ensure that the pin is clean to prevent impure substrate contamination. Use knitted microfiber fabrics to wipe the surface of the tool pin to eliminate small particles.
  4. Fix the pin with a clamp on the machine.
    1. Screw on the tool pin tightly again for tool pin clamping.
    2. Pay attention to the pin clamping step. Ensure that the pin is clamped tight in the machine to avoid danger. The rotating tool is surrounded by a nonrotating clamping ring with which the workpieces are pressed firmly against one another before and during welding by applying a clamping force. The illustration shown in Figure 3a notes the clamp ring used to fix the tool pin. After this step, the production is shown in the flowchart.
    3. Ensure safety.
    4. Confirm that the high-speed rotation pin without a clamp ring loosens. When the tool pin is placed on the machine, ensure that the tool pin does not separate from the clamp during rotation for safety reasons. The tool pin uses a low rotation rate from 10 to 100 rpm in 1 minute. The speed can accelerate from 100 to 1,000 rpm within 1 minute (Figure 3b).
  5. Machine settings
    1. Use the following parameters: a rotational speed of 3,000 rpm, a dwell of 4 s, and a plunge rate of 0.5 mm/s (Figure 3c).
  6. Calibrate the welding location (Figure 3d and the real product shown in Figure 4a).
    1. Set the pin in the stir spot welder machine. The gap between the pin and the workpiece is smaller than 5 cm to calibrate the joint location. After the location is confirmed, move onto the welding process.
  7. During welding, wear goggles and gloves to avoid injury.
    1. Begin the welding process with the tool under high-speed rotation to plunge the tool pin into the workpiece. The tool shoulder contacts the workpieces and stops the rotation and retracts the pin.
  8. Plunging
    1. Turn the stir button on. When the machine warms up, confirm that the tool pin is consistently operating at a 2,500 rpm rotation speed. Ensure that the tool pin is clamped well under the high-speed rotation at 2,500 rpm. The pin plunges into the workpieces under a high-speed rotation and the shoulder contacts the workpieces at a high angular speed (Figure 3e). The real product is shown in Figure 4b.
  9. Stirring
    1. As the plunged tool pin continues stirring in the workpiece, soften the interface of the pin and the material from the friction heat to create the grain. When the shoulder of the tool pin comes into contact with the top of the workpiece, stop the process because the high rotation of the tool pin can generate high temperatures. It is important to wear protective gear that ensures operational safety (see Figure 3f.) The real product is shown in Figure 4c.
  10. Retracting
    1. Draw out the tool pin in the vertical direction. After the procedure, the pin creates the key-hole welding spot in the lap joint. Note that the friction stir spot weld stops in this step (Figure 3g). The real product is shown in Figure 4e.
  11. Remove the workpieces.
    1. Turn off the machine power.
    2. After the welding is finished, remove the workpieces from the anvil. Observe the samples for cracks and lack of fusion.
    3. Remove the tool pin.
    4. After the procedure, remove the tool pin from the clamp ring. The appearance of the tool pin is observed and checked (Figure 5).

3. Mechanical property evaluation

  1. Microscopy examination of the FSSW welds (Figure 3h)
    1. Microscopic sample preparation
    2. Measure the cross-sectional area of the bonded region using an optical microscope image and a secondary electron image analysis. Prepare the microscopic samples using grounded silicon carbide paper with a grit size ranging from 200 to 2,000 starting with a grit size of 200 and increasing in sequence. Polish the samples with 0.03% alumina and etch with a 4% nital solution for 7–10 s at room temperature.
    3. Microscopy observation
    4. Observe and characterize the microstructures using optical microscopy and scanning electron microscopy. Use a voltage of 20 kV, and a working distance of 10 μm. From the optical microscopy, any tiny crack line or lack of a fusion zone can be determined. Use scanning electron microscopy to analyze the martensite and austenite distribution and the grain size.
  2. Microhardness
    1. Verify the microhardness experiments more than 3 times. The values were too small to clearly denote the standard deviation.
    2. Press the Vickers diamond indenter with a 300 g test load sample and 0.5 mm per test.
    3. Conduct the microhardness testing of the DP780 steel sheet using a microhardness testing machine with a 300 g load and a holding time of 15 s. The microhardness testing revealed the hardness distribution and the individual hardness values in the stir zone, the thermomechanical affect zone, the heat affected zone, and in the base metal of the welds.

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Representative Results

There is a diagram in Figure 3 that demonstrates that the friction stir spot welding process is comprised of 3 parts: plunging (Figure 3e), stirring (Figure 3f), and retracting (Figure 3g). In our research, the welding spot could be generated. The penetration depth is one factor that was evaluated. In Figure 6a, the FSSW creates the keyhole in the center to create the joint for 2 sheets. The measurement depth of the keyhole is from the sheet top to the keyhole bottom surface (Figure 6b). The measurement values are shown in Figure 6c, for which the setting values are 2 cm and the real values are 1.92 to 1.98 cm. In Figure 7, the image shows the key-hole overall view of the welding spot in the DP780 sheet. The analysis of the base metal microstructure showed martensite islands in a ferrite matrix (Figure 8a). The microstructures of the TMAZ near the keyhole show a mixture of needle-like martensite and fine acicular ferrite (Figure 8b,c). The stir region around the keyhole revealed a fine grain martensite and porosity (Figure 8d).

Hsu et al.25 studied the hardness of a base metal compared with the original material property. In the HAZ intercritical region, the hardness value was found to be in a range of approximately 310 to 330 Hv. The hardness of TMAZ was approximately 360 Hv. The hardness in the stir zone of friction stir spot welds is significantly higher than in other regions; the values were found to be 370 Hv (Figure 9, modified from Hsu et al.25). If the welding process is not successful, there will be some cracks and a lack of fusion in the weld zone.

Figure 1
Figure 1. A diagram of the tool pin.
(a) The size and geometry of the tool pin (b) the actual tool pin Please click here to view a larger version of this figure.

Figure 2
Figure 2. A diagram to demonstrate the work space. Please click here to view a larger version of this figure.

Figure 3
Figure 3. A flowchart to illustrate the friction stir spot welding process.
(a) clamp pin (b) safety confirmed (c) machine setting confirmed (d) calibration (e) plunging (f) stirring (g) retracting (h) validation of the mechanical properties of the joints Please click here to view a larger version of this figure.

Figure 4
Figure 4. The welding process. (a) calibration (b) plunging (c) stirring (d) retracting Please click here to view a larger version of this figure.

Figure 5
Figure 5. A diagram showing the used pin. The pins are consumed at high temperatures. Please click here to view a larger version of this figure.

Figure 6
Figure 6. Confirmation of the dwell depth using a comparison of the settings.
(a) The macro view of the FSSW creating the keyhole. (b) A diagram illustrating where the depths are measured (c) The dwell depths are set at 2 cm. The actual measurement values range from 1.92 to 1.98 cm. Please click here to view a larger version of this figure.

Figure 7
Figure 7. An overall view of the friction stir spot welding. The analyzed area contained 4 parts: (I) base metal (II) HAZ (III) TMAZ, and (IV) the stir zone. Please click here to view a larger version of this figure.

Figure 8
Figure 8. The microstructure composition of the joint created using FSSW. (a) base metal: the base metal of the workpieces is comprised of DP 780 sheets. The base metal shows no change in material properties (b) HAZ: the thermal cycle around the welding site with heat transfer. HAZ zone shows the martensite islands. (c) TMAZ: thermomechanically affected zone around the stir zone. The needle-like martensite and fine acicular ferrite shown in the TMAZ zone. (d) Stir zone: the pin hole created in the welding process with the formation of recrystallization grains. Fine grain smaller than 10 µm appeared in the stir zone. Please click here to view a larger version of this figure.

Figure 9
Figure 9. The microhardness values of the workpiece examined using a Vickers test machine with a loading weight of 300 g was held for 15 s. This figure was modified from Hsu et al.25. Please click here to view a larger version of this figure.

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Discussion

The plunging stage is the most important during the FSSW process. Without enough friction heat coming from the shoulder of the pin to soften the workpiece, the pin will fracture. Tool geometry, rotation speed, dwell time, and tool penetration depth26 parameters of the FSSW process play a critical role in determining the joint integrity. TPD and tool geometry27 particularly have an important effect on the weldability and joint properties was reported.

The geometry of the pins were cylindrical, Whorl, MX Triflute, Flared-Triflute, A-skew, and Re-stir designed by TWI28. They are suitable for butt welding but not for lap welding because the tool motion and the welding torque can be reduced by the traversing force caused by the intense stirring. Flared-Triflute, A-skew, and Re-stir tool pins are suitable for lap welding; the design is intended to increase the swept volume of the pin in order to expand the stir region to form a wider worked lap joint29. Meanwhile, during FSSW, friction generates heat at the interface of the rotating tool and the work piece. The tool geometry and FSSW parameters affect the strength of the FSSW welds4. The tool shoulder and pin are the main parts of the FSSW tool5. The pin generates friction heat, deforms the material around it, and stirs the heated material6. The size7, angle8, thread orientation9, length10 and profile11 of the pin depends on nugget formation. Meanwhile the tool shoulder generates heat during the FSSW process, forges the heated material, prevents material expulsion, and assists material movement around the tool12. The size and concavity of the shoulder are also important factors in friction stir spot welding13.

The pin materials are comprised of the following components: 12% Cr steel, low carbon steel, Mo and W alloy, W alloy, polycrystalline cubic boron nitride (PCBN), and polycrystalline cubic boron. Because tool wear occurred in the plunging period at the initial stage of welding, tool deformation and rubbing wear could be found in the tool. This problem can be resolved by choosing a suitable material for the pin that is hard and can withstand elevated temperatures compared to the workpieces for increasing the tool lifetime. In our research, we used the polycrystal diamond to weld the workpiece.

The pin length and the penetration depth are also factors that can influence the maximum loading in the welding process. It has been indicated that there will be an increased tool penetration depth and decreased pin length, resulting in a higher2.

The rotation rate is an important factor that leads to pin friction on the workpieces to begin the welding process. A speed ranging from 300-1,000 rpm can be used to detect the peak temperature from approximately 430 to 470 °C in the welding center zone. Far from the welding zone, the heat effect zone exhibited a decrease in temperature to 350 °C for the Al alloy (6061Al-T6)30. From other references, the friction situation at a low rotation speed with a stick can transform to a stick/slip at high speeds. The rotation rate is the key factor leading to the generation of the heat necessary to forge the workpiece. In the past, studies have been focused on Al alloy. However, in our study, the focus is on DP steel. There is no test value by which to identify the temperature. However, based on the fact that the microstructure at the centerline exhibited fine grain martensite, it can be inferred that the substrate temperature exceeded the Ac3 standard.

The study of FSSW workpieces in the past has concentrated on aluminum alloys because low melting temperature in metal welding leads to deformities and low strength that require being fixed via FSSW. Since the FSSW was developed, different materials have been used, including lightweight steel. Different kinds of DP steel welded with Al alloys are new areas for investigation. Based on commercial applications, the FSSW can be a useful method for different component alloys used in industrial production due to savings in terms of both time and cost.

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Disclosures

The authors have nothing to disclose.

Acknowledgments

We thank Dr. K. C. Yang in the China-Steel Company for material support and wish to express our gratitude to Mr. L.D. Wang, C. K. Wang, and B. Y. Hong at the MIRDC for assistance with the experimental FSSW. This research was supported by the Metal Industries Research and Development Centre, Kaohsiung, Taiwan, ROC.

Materials

Name Company Catalog Number Comments
anvil MIRDC made by MIRDC
DP780 China steel Corporation CSC DP780
stir spot welder machine MIRDC made by MIRDC
tool pin KINIK COMPANY DBN2B005B

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