Generating Lap Joints Via Friction Stir Spot Welding on DP780 Steel

The weight reduction demands in the automotive industry have generated a increasing interest in lap joints and the advanced high-strength steels, or AHSS. However, AHSS spot welding joints have challenges for process reliability. The benefits of friction stir spot welding, or FSSW, compared with resistance spot welding are a lower welding temperature, no splash, and the more stability in the manufacture process.

Demonstrating the procedure will be Cheng Chia-Ping and Hsu Hao, graduate students from my laboratory. During this procedure wear goggles and gloves to prevent splash contact or heat damage. Also stand behind the hood or the baffle for protection.

Manufacture all joints using a friction stir welder machine. Record the z-axial force and penetration depth during each joining operation using the embedded data acquisition system. Set a tool pin rotation speed of 25, 000 RPM, a tool pin weld time of four seconds, and a rate of 0.5 millimeters per second of tool pin plunge into the sheet.

Optimize the parameters for the operator. The range of the rotation speed is 1, 000 to 2, 500 RPM. The range of the dwell time can be from two to 10 seconds and the plunge rate can be 0.1 to 0.5 millimeters per second.

Before the welding process, ensure that there are no impurity substrates contaminating the work pieces. Wipe the surface of the work piece with knitted microfiber fabrics to eliminate any small particles. Place the DP 780 work pieces and clamp tWo DP 780 sheets with an overlap of 35 millimeters.

Fix the clean work pieces on an anvil to prevent shifting. Ensure that the pin is clean to prevent impure substrate contamination. Use knitted microfiber fabrics to wipe the surface of the tool pin.

To fix the pin with a clamp on the machine, first screw on the tool pin tightly. Pay attention to the pin clamping step. Ensure that the pin is clamped tightly in the machine to avoid danger.

The rotating tool is surrounded by a non-rotating clamping ring, with which the work pieces are pressed firmly against one another before and during welding by applying a clamping force. 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 one minute. The speed can accelerate from 100 to 1, 000 RPM within one minute. Set the machine to a rotational speed of 3, 000 RPM, a dwell of four seconds, and a plunge rate of 0.5 millimeters per second.

Calibrate the welding location and the real product. Set the pin in the stir spot welder machine. The gap between the pin and the work piece is smaller than five centimeters to calibrate the joint location.

After the location is confirmed, move on to the welding process. Begin the welding process with the tool under high-speed rotation to plunge the tool pin into the work piece. The tool shoulder contacts the work pieces, stops the rotation, and retracts the pin.

For plunging, first 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 in the work pieces under a high-speed rotation and the shoulder contacts the work pieces at a high angular speed. As the plunged tool pin continues stirring in the work piece, soften the interface of the pin and material from the friction heat to create the grain. When the shoulder of the tool pin comes into contact with the top of the work piece, 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. 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. Turn off the machine power. After the welding is finished, remove the work pieces from the anvil.

Observe the samples for cracks and lack of fusion. After the procedure, remove the tool pin from the clamp ring. The appearance of the tool pin is observed and checked before the mechanical property evaluation as described in the text protocol.

The friction stir spot welding creates the key hole in the center of the DP 780 sheets to create the joint for two sheets. The dwell depth of the key hole is measured from the sheet top to the key hole bottom surface. The dwell depth measurement values are shown, for which the setting values are two centimeters and the real values are 1.92 to 1.98 centimeters.

This image shows the key hole overall view of the welding spot in the DP 780 sheet. The analyzed area contained four parts, the base metal, the heat affected zone, the thermos-mechanically affected zone, and the stir zone. The analysis of the base metal microstructure shows no change in material properties.

The heat affected zone exhibits the martensite islands. The microstructures of the thermos-mechanically affected zone near the key hole are characterized by a mixture of needle-like martensite and fine acicular ferrite, whereas the stir region around the key hole reveals a fine-grain martensite and porosity. Friction stir spot welding of AHSS weld lap joints has great potential.

On average, the conventional steel body of a car has more than 2, 000 spot weld lap joints. Following this procedure, materials and the design geometry of the tool pin can be performed for hard lap joints. A longer lifetime will be the demand of this process.