Aluminum spot welding method

A welding tip (20) for spot welding a first part (22) formed of conductive metal, for example aluminum, to a second part (24) formed of aluminum or another conductive metal, such as steel, is provided. The welding tip (20) includes a notch (30) at a distal end (38) and a convex contact surface (28) extending radially outwardly and upwardly from the notch (30) for engaging a surface of the first part (22). The rotating welding tip (20) forms a depression (32) on the surface of the first part (22) during the welding process. The notch (30) creates a pin (34) in the center of the depression (32) which provides a fixed axis of rotation for the rotating welding tip (20) and prevents the welding tip (20) from moving radially relative to the fixed axis, thereby improving the quality of the final spot weld (36) and reducing process time.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to spot welding methods, tools used for spot welding, and parts joined by spot welding.

2. Related Art

Spot welding is oftentimes used to join a first part formed of aluminum to a second part formed of aluminum or another metal material. The parts are held together under pressure by a pair of welding tips, which also function as electrodes. Current is supplied to the welding tips and concentrated in a single spot to melt the surface and form the weld. One drawback of spot welding aluminum parts is that aluminum oxides typically form along the surfaces, which reduces the integrity of the weld.

To break the oxide surface and reduce the amount of aluminum oxides formed during spot welding, the welding tips can present a spherical radius at their terminal end, and rotate continuously or intermittently at a controlled rate as they spot weld the parts together. An example of this technique was developed by KUKA and Mercedes-Benz® and is referred to as robo-spinning. The robo-spinning technique uses a robot to rotate the welding tips and spot weld the parts together. However, due to the significant force applied and the shape of the part being welded, the rotating weld tips tend to move out of position during the spot welding process. In addition, the terminal ends of the rotating welding tips can melt the surfaces of the parts and create locking divots.

SUMMARY OF THE INVENTION

The invention provides a welding tip for spot welding parts formed of conductive metal, such as aluminum. The welding tip comprises a shaft extending to a distal end and presenting a notch at the distal end. The shaft also includes a contact surface extending radially outwardly from the notch.

The invention also provides a method for spot welding. The method includes providing a first part formed of conductive metal and a second part formed of conductive metal. The method then includes contacting the first part with the welding tip while rotating the welding tip around its center axis.

The invention further provides a spot welded structure formed using the welding tip. The spot welded structure comprises the first part formed of conductive metal joined to the second part formed of conductive metal by a spot weld. The spot weld comprises a depression and a pin extending upwardly from the depression.

DETAILED DESCRIPTION

The invention provides a welding tip20, as shown inFIGS. 1 and 2, for spot welding a first part22formed of conductive metal, typically aluminum, to a second part24formed of conductive metal, such as aluminum or another metal. The spot welding process is preferably a robo-spin or electromechanical motion spin process, for example the process illustrated inFIGS. 3 and 4, wherein the welding tip20rotates around its center axis A either continuously or intermittently. A high quality spot welded structure25including the first part22joined to the second part24by a spot weld36, such as the structure25shown inFIGS. 5 and 6, can be formed by the method of the present invention. The welding tip20and method of the present invention can also avoid forming divots along the surface of the parts22,24, which are often formed by traditional spot welding tips. The welding tip20can also reduce the total spot welding process time. More specifically, the time it takes to fix the rotating welding tip20to one of the parts22,24is reduced. In addition, the welding tip20requires less electrical current during the spot welding process, compared to a traditional welding tip.

The welding tip20may be formed of a copper alloy or another electrically conductive material so that when the welding tip20receives an electrical current the welding tip20functions as an electrode. The welding tip20ofFIG. 1is uncoated, but the welding tip20may alternatively be coated to reduce friction while rotating around its center axis A and thus experience less wear.

The welding tip20includes a shaft26which is typically disposed in a spot welding gun (not shown). The shaft26extends along the center axis A to a distal end38and includes a notch30at the distal end38. A contact surface28surrounds the notch30at the distal end38for engaging the parts22,24to be welded. As shown inFIGS. 1 and 2, the contact surface28extends radially outwardly and upwardly from the notch30. The contact surface28also has a spherical radius which provides a rounded surface adjacent the distal end38. In the example embodiment, the contact surface28presents a convex or semi-spherical shape. The area of the contact surface28and the size of the spherical radius can vary, depending on certain parameters, including, but not limited to, the thickness of the parts22,24to be joined. Alternatively, other shapes may be used instead of the convex surface, depending on the desired formation of the spot weld36to be formed.

In the example embodiment ofFIG. 1, the shaft26includes a slot27extending along the center axis A for receiving another component of the welding gun (not shown) which conveys the electrical current to the welding tip20during the welding process. The end of the slot27is spaced axially from the contact surface28of the spot welding tip20.

The notch30of the welding tip20, also referred to as a cavity, dimple, depression, or arbor, reduces the area of the surface in contact with one of the parts22,24. As a result, of the reduced area, the welding tip20requires less electrical current during the spot welding process, compared to a traditional welding tip. The notch30is preferably located at an apex of the convex contact surface28and extends inwardly along the center axis A away from the distal end38, as shown inFIGS. 1 and 2. The cross-section of the notch30typically has a circular shape, as shown inFIG. 2, but can comprise other shapes. The diameter D1of the notch30can vary depending on the size of the contact surface28and the parts22,24to be joined, or other factors. However, the cross-sectional area of the contact surface28surrounding the notch30is typically greater than the cross-sectional area of the notch30, as shown inFIG. 2.

The depth d1of the notch30can also vary depending on the size of the shaft26and parts22,24to be joined, or other factors. In the example embodiment ofFIG. 1, the depth d1of the notch30is contained within the spherical portion of the welding tip20and is spaced from the slot27which receives the component of the welding gun. For example, the depth d1of the notch30could be less than 30 percent (%), or less than 20%, or less than 10%, or less than 5% of the distance between the distal end38of the welding tip20and the slot27for receiving the welding gun. The depth d1of the notch30could also be less than 10%, or less than 5%, or less than 1% of the total length1of the welding tip20.

During the spot welding process, the contact surface28of the rotating welding tip20forms a depression32on the surface of one of the parts22,24to be joined. As the contact surface28forms the depression32, the notch30creates a pin34extending upwardly from the center of the depression32. The notch30fixes or secures the welding tip20to the surface of one of the parts22,24, and the pin34provides a fixed axis of rotation for the welding tip20. The pin34also prevents the welding tip20from moving radially relative to the center axis A while rotating around the center axis A. The notch30also allows for precise location of applied force and electrical current which further prevents the rotating welding tip20from moving out of position. As alluded to above, the notched welding tip20has much higher electrode force density and requires less initial electrical current during the welding process—compared to a traditional welding tip, since the contact surface28is reduced. The depression32and pin34remain on the final spot welded structure25as a witness to the process quality. It can be measured as a quality indicator relating to roundness in shape and surface indentation depth.

The invention also provides a method for joining the first part22formed of conductive metal to the second part24formed of conductive material by a spot welding method using the notched welding tip20and thus forming the spot welded structure25. The method preferably includes the robo-spinning technique, but can comprise another method that involves rotating the welding tip20around its center axis A, either continuously or intermittently.FIG. 3illustrates phases of an example method used to spot weld the parts22,24, including the degree of force F, electrical current I, and electrical resistance R applied during each phase of the spot welding process.

The method begins by providing the first part22and the second part24to be welded. The first part22is formed of conductive metal, such as aluminum, and the second part24is also formed of conductive metal, which is typically aluminum, but may be another conductive metal, such as steel. The size and shape of the parts22,24can vary depending on the intended application of the finished spot welded structure25. For example, the parts22,24can be designed for use as a component of an automotive vehicle. In addition, the parts22,24can be pre-conditioned in any manner know in the art to improve the integrity of the spot weld36ultimately joining the parts22,24. The conductive metal of the parts22,24can also be coated or uncoated. Coating thicknesses are becoming increasingly thicker to cope with corrosion issues. Example coatings include aluminum, zinc, and combinations of alloys to protect the conductive metal from corrosion.

Typically, the method employs two of the notched welding tips20, including the first welding tip20and a second welding tip20′, as shown inFIG. 4. A pair of welding guns (not shown) each including one of the notched welding tips20,20′ are used to spot weld36the parts22,24. As shown inFIG. 4, the first and second welding tips20,20′ are aligned on opposite sides of the parts22,24to be joined. The welding tips20,20′ preferably have the same design and perform the same function at the same time. For example, the first welding tip20engages the first part22while the second welding tip20′ engages the second part24, or vice versa. Accordingly, although the following description refers to only the first welding tip20and the first part22in several instances, the description also applies to the second welding tip20′ and the second part24.

The method begins with a first phase including supplying power to the welding gun, which drives the welding tip20to rotate around its center axis A, preferably before contacting the part22. In the example embodiment, the rotating step begins before the welding tip20contacts the surface of the part22in order to reduce process time. The first phase of the example spot welding process also includes crimping the parts22,24before any electrical current I or heat is applied to the welding tips20,20′ or the parts22,24. This cold crimping first phase can be applied in any situation, but is typically applied when a gap between the parts22,24is present, for example, due to manufacturing tolerances. The first phase comprises a first period of time at the start of the welding process, during which the rotating welding tips20,20′ first contact a spot along the surface of each of the parts22,24. As shown inFIG. 3, no electrical current I is applied to the welding tips20,20′ during the first phase. Once the rotating welding tips20,20′ contact the parts22,24, the first phase includes applying a significant force F to the parts22,24by the welding tips20,20′. The center axis A of the first welding tip20is aligned with the center axis A′ of the second welding tip20′, as shown inFIG. 4, as the welding tips20,20′ rotate.

The welding tip20can rotate continuously or intermittently during the first phase. As the rotating welding tip20develops force, any oxide layer present on the surface of the part22is removed. The rotating welding tip20can also score, remove, condition, or scrub any coating on the surface of the part22. At the end of the first phase, the force F applied to the welding tip20is typically reduced in preparation for the second phase.

The second phase of the example method shown inFIG. 3includes softening the part22. During the second phase, which is a second period of time immediately following the first period of time, the force F is still applied to the rotating welding tip20at a constant level. The electrical current I is then turned on and applied to the welding tip20in order to soften the part22.FIG. 3shows that the electrical current I initially increases and then stays at a constant level throughout the second phase, while the electrical resistance R is highest at the beginning of the second phase and decreases continuously throughout the second phase.

The temperature of the part22also increases during the second phase as the welding tip20continues to rotate while in contact with the part22. Thus, the spot along the surface of the part22engaged by the rotating welding tip20begins to melt, and the welding tip20begins forming the depression32and the pin34extending upwardly from the center of the depression32. Once the pin34forms, the welding tip20rotates about the pin34. The notch30and pin34fix the axis of rotation at the center axis A of the welding tip20and prevent the welding tip20from moving radially relative to the center axis A during the rotating step. In other words, the notch30and pin34fix or secure the welding tip20to the part22and prevent the welding tip20from moving or shifting radially relative to its center axis A during the rotating step of the second phase.

The welding tip20can rotate continuously or intermittently during the second phase. In either case, the welding tip20rotates quickly enough to prevent the melted aluminum or other conductive metal of the part22from sticking to the contact surface28or notch30of the welding tip20. The lack of oxides on the surface of the part22also prevents the melted metal from sticking. Thus, the service life of the welding tip22is improved.

The third phase of the example method shown inFIG. 3is the welding phase. During the third phase, which is a third period of time immediately following the second period of time, the force F is maintained at the same level as in the second softening phase. However, the electrical current I increases sharply to its highest level and stays at that level during the majority of the third phase, while the electrical resistance R continues to slowly decrease. As the welding tip20continues rotating, the temperature continues to increase and the spot along the surface of the part22continues to melt. During the third phase, the notch30continues to fix the welding tip20to the surface of the part22, while the pin34provides the fixed center axis A of rotation for the rotating welding tip20. Thus, the notch30allows for precise location of the applied force F and electrical current I, which leads to a higher quality spot weld36in the finished structure25. The notch30also continues to prevent the welding tip20from moving, sliding, or skidding out of position. Towards the end of the third phase, the spot weld36, also referred to as a weld nugget is typically formed between the two parts22,24. At the end of the third phase, the electrical current I is sharply reduced to zero, the electrical resistance R is gradually reduced to zero, and the temperature of the welding tip20and the part22begins to decrease. Thus, the part22begins to cool at the end of the third phase and after the third phase.

In the example embodiment shown inFIG. 3, the fourth phase includes forging. The forging is beneficial to reduce cracking along the surface of the part22, especially when the part22is formed of an alloy, but the forging step is not required. During the optional fourth phase, which is a fourth period of time immediately following the third period of time, the electrical current I is turned off, and the welding tip20and part22continue to cool. The force F applied to the part22by the welding tip20during the fourth phase increases relative to the second and third phases. The force F applied during the fourth phase is approximately equal to, or at least equal to the force F applied during the first phase, and the force F remains at this high level for a majority of the fourth phase. A high capacity welding gun may be required to achieve this high level of force F during the first and fourth phases. In addition, the welding tip20can optionally rotate during the fourth phase.

The welding tip20typically stops rotating continuously around its center axis A at some point after the third phase. If the method includes the optional fourth phase, then the welding tip20stops rotating continuously before, during, or after the fourth phase. A cooling phase (not shown inFIG. 3) then begins either after the third phase or after the optional fourth phase, wherein the depressions32and pin30formed by the welding tip20can solidify and provide the finished spot weld36joining the first part22and the second part24. At the beginning of the cooling phase, the welding tip20is still in contact with the part22, and the method preferably includes “swiveling” or rotating the welding tip20less than 360 degrees around its center axis A in a first direction, and preferably followed by rotating the welding tip20less than 360 degrees around its center axis A in a second direction opposite the first direction. For example, the swiveling step can include rotating 10 degrees in one direction, or rotating 5 degrees clockwise followed by 5 degrees counterclockwise. This swiveling step further prevents the aluminum or other conductive metal from sticking to the welding tip20. The swiveling motion can be repeated a plurality of times, either continuously or intermittently. The swiveling step can also be incorporated into other phases of the spot welding process. The pin34formed on the surface of the part22remains disposed in the notch30of the welding tip20during the swiveling step and keeps the welding tip20in position during the swiveling step.

The invention further provides a structure25including the first part22formed of aluminum and the second part24formed of aluminum or another metal material joined together by the spot weld36, as shown inFIGS. 5 and 6. The spot weld36comprises the depression32on the surface of each part22,24, and the pin34extending upwardly from the center of each depression32. The depression32typically presents a concave surface and the pin34extends upwardly from the center of the concave surface. The pin34formed in the first part22is preferably aligned with the pin34formed in the second part24. The pin34makes it easy to identify spot welded structures25formed using the notched welding tip20of the present invention. The depth d2and diameter D2of each depression32can vary, depending on the size of the welding tip20and the pressures and temperatures of the spot welding process. However, the total cross-sectional area of each depression32is typically greater than the total cross-sectional area of each pin34, as shown inFIG. 5. The spot weld36formed using the notched welding tip20is higher quality than spot welds formed using other welding tips without the notch30.