METHOD

FIELD OF INVENTION

The present invention relates to a method and an apparatus to produce coalescence of faying surfaces of circular spots at a temperature below the solidus line of two sheet lapping metallic work pieces which are placed or laid one of the sheets so as to overlap another without the addition of filler materials. In more specific, the process to join the work pieces involves diffusion and/or severe plastic deformation.

BACKGROUND OF THE INVENTION

Resistance Spot Welding (RSW) is a well-established metal joining process especially used in automotive industry. RSW employs electrical current that flows through a pair of electrode tips and two separate pieces of lapping metals to be joined. Owing to the resistance between the lapping work pieces, flow of the electrical current causes localized heating in the joint that creates molten nugget and a weld formed thereof. However, excessive application of heat in the RSW process deteriorates the quality of materials forming the weldment as well as the quality of heat-affected materials in weld area.

Another approach in spot welding process is the application of mechanical friction to generate heat by means of rotating tool tip on the top surface of two lapping pieces of metals to be joined. This method is known as Friction Stir Spot Welding (FSSW). In the FSSW, the rotating tool or stirrer probe is forced against the top surface of the lapping work pieces perpendicularly in its axial direction in order to generate heat through mechanical friction. The generated heat will soften the pieces of lapping metals allowing the axially loaded rotating stirrer probe to penetrate into the softened weld area of the two lapping metallic work pieces to be joined and stir the material in order to form the weldment. Penetration of the rotating stirrer probe displaces the softened work piece that find exit available on the surface of weld area. Retraction or discontinuation of the mechanical friction by then cools the splashing softened work piece materials and join the two lapping pieces together at the treated spot in a circular fashion. The FSSW is a solid state welding process with the advantage that metals joined are not experiencing microstructure changes due to phase transformation in the metals during melting. However, the energy consumed to soften the lapping metallic work pieces is high thus the stirrer probe has to be made of material with excellent thermo mechanical properties in order to sustain through the FSSW process. Considering the drawbacks mentioned above, improved device and methods have been devised to achieve spot welding of more efficiency.

Japanese patent publication no. 2007268543 claims hybrid spot welding equipment which transmits an electric current through a rotary tool to generate resistance heat thereby and the rotary tool meanwhile rotates in high speed to further render frictional heat at the weld zone. The metals in the weld zone soon attain plastic flow state to produce weld joining at the weld zone.

Similarly, European patent application 1430986 discloses another apparatus for friction stir welding. The disclosed apparatus conducts a current flow in between the work pieces and the apparatus to create resistance heating on the work pieces while providing a friction force at the weld zone in the meantime. Through combined use of friction and resistance heating, the stir friction welding process becomes less demanding on the stirrer probe.

United States patent application no. 7150389 describes a method to improve stir friction welding by providing additional heat energy to the weld zone to expedite the welding process. Further, improved friction stir welding method is disclosed in another United States patent publication no. 20060065698 that portion of the work pieces surrounding the stirrer probe is heated by an electromagnetic heating means. SUMMARY OF THE INVENTION

The present invention aims to provide a method for spot welding. Specifically, the disclosed welding method combines both electrical resistance heating and rotational stirring-mixing action to weld at least two work pieces lapped together.

Another object of the present invention is to provide a method of spot welding that the stirrer probe, which also functions as anode used in the present invention, is subjected to less abrasion and frictional damage, as less mechanical friction is required to carry out an efficient heating of weld area prior to welding. More specifically, the weld area is firstly softened via resistance heating only then subjected to perform rotational-stirring mixing action to weld the lapping pieces rather than conducting both resistance and friction heating simultaneously.

Further object of the present invention is a method to detect the softening of the work pieces by employing a displacement sensor to determine the best timing to begin the rotational stirring-mixing action thus optimally protecting the tip of the stirrer probe from any frictional wear. Exploitation the relative changes in displacement renders an automatic mechanism in the present invention to initiates rotational stirring-mixing action.

Still another object of the present invention is to disclose an apparatus capable of performing the disclosed method with slight modification. The disclosed apparatus is less likely to spoil the stirrer probe by pre-heating the weld zone to a softened state prior to penetration into the weld zone using thrust force applied through tool axis.

Further object of the present invention includes providing an apparatus with thermo sensor towards the weld area to detect temperature changes that the apparatus retracts the stirrer probe or the conducting shaft once the temperature drops below a preset threshold or reheating the weld area to protect the stirrer probe. At least one of the preceding objects is met, in whole or in part, by the present invention, in which one of the embodiment of the present invention is an apparatus of spot welding comprising a clamp structure having a hollow bottom clamping arm and a hollow upper clamping arm that a gap exists in between the opposing clamping arms for positioning of at least two layers of lapping metallic work pieces and at least one of the opposing clamping arms is moveable towards or away in relative to another for clamping or releasing the lapping metallic work pieces; a bottom conducting shaft and an upper conducting shaft housed within the bottom clamping arm and the upper clamping arm respectively with the upper conducting shaft capable of performing axial rotation; a controlling means regulating the clamp structure and the conducting shaft as well as providing an user interface for management of the apparatus; wherein the bottom conducting shaft and the upper conducting shaft contacting weld area of the lapping metallic work pieces under a predetermined pressure that an electrical current passes through thereto generating heat to soften the weld area of the lapping metallic work piece without melting the lapping metallic work pieces, and the upper conducting shaft starts axial rotation to thrust into the weld area of the lapping metallic work pieces upon detecting of softening in the weld area; characterized in that a displacement sensor, in communication of controlling means, is used in conjunction with the opposing conducting shafts to detect slight axial movement of at least one conducting shaft towards the weld area to determine softening of the weld area and initiates the upper conducting shaft to rotate.

In order to protect the upper conducting shaft and the upper arm of the clamp, the present invention may further comprise a thermo sensor mounted adjacent to the opposing conducting shafts and/or the clamps to monitor temperature at the weld area. The thermo sensor prompts the upper conducting shaft to retract away from the weld area once detecting drop of temperature below a preset threshold or prompts the upper conducting shaft to rotate in higher speed to generate greater friction heating at the weld area once detecting drop of temperature below a preset threshold. Further, another embodiment may have the current reconnected to pass through the weld area once detecting drop of temperature below a preset threshold that the weld area will be re-heated and soften again.

In another aspect, a cooling means is mounted on the disclosed apparatus to introduce a flowing cooling medium into contact with a portion of the upper conducting shaft under pressure to transfer heat from the upper conducting shaft to the flowing cooling medium. Further the portion of the upper conducting shaft in contact with the flowing medium is fabricated with extended surface to enhance heat transfer. In another aspect, the cooling means comprises a detachable housing hermetically mounted onto the portion of the upper conducting shaft having at least one inlet for the cooling medium to flow in and at least one outlet for the cooling medium to flow out.

To avoid electrocution or damage to the apparatus, the clamp structure is completely or partially covered with an electric insulating material.

An embodiment of the present invention is a method of spot welding of two lapping work pieces at a weld area comprising the steps of heating the weld area by conducting an electrical current via a pair of opposing conducting shafts contacting the weld area, wherein the opposing conducting shafts press against the weld area under a pressure; verifying softening of the weld area by detecting axial movement in at least one conducting shaft via a displacement sensor coupled to the opposing conducting shafts; and stir welding into the softened weld area by axially rotating the conducting shaft, which is detected to have the axial movement, to thrust the conducting shaft into the softened weld area to form a weld joining the two lapping work pieces; wherein the weld area is heated until to become softened without melting the work pieces. Another further embodiment of the method may also comprise a step of detecting temperature changes at the weld area through a thermo sensor and retracting the conducting shaft from the weld area upon dropping of the temperature below a preset threshold. In another aspect, an additional step of cooling the rotating conducting shaft by bringing portion of the rotating conducting shaft into contact with a flowing cooling medium via a cooling means may be included in the welding method. Optionally, the electrical current is discontinued upon initiation of the stir welding step or after a predetermined duration upon initiation of the stir welding step.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic diagram of one embodiment of the disclosed apparatus prior to performing the resistance heating;

Figure 2 is a schematic diagram showing the frictional and resistance heating performed by the embodiment shown in Figure 1 ;

Figure 3 is a cross-sectional view of the joined work pieces after the disclosed method;

Figure 4 is an embodiment of the disclosed apparatus which that the upper arm of the clamp structure is fabricated with extended surface to accelerate heat dissipation in (a) perspective view and (b) cross-sectional view; Figure 5 shows one of the embodiments of the cooling means in the present invention which a compartment is available for channeling in the cooling medium to be in contact with the upper arm of the clamp structure in (a) exploded view and (b) perspective view; Figure 6 is the cross-sectional view of the embodiment shown in Figure 5;

Figure 7 is another embodiment of the (a) cooling means with (b) cross-sectional view with extended surface fabricated at the external surface of the cooling means;

Figure 8 is another embodiment of the (a) cooling means with (b) cross-sectional view that extended surface is formed at both external and internal surface of the cooling means as well as the external surface of the upper clamping arm;

Figure 9 shows another embodiment of the (a) cooling means with (b) cross-sectional view; and

Figure 10 shows another cooling means derived from the embodiment shown in Figure 9 that the conducting shaft is fabricated with extended surface to be contacted by the cooling medium.

DETAILED DESCRIPTION OF THE INVENTION

It is to be understood that the present invention may be embodied in other specific forms and is not limited to the sole embodiment described herein. However modification and equivalents of the disclosed concepts such as those which readily occur to one skilled in the art are intended to be included within the scope of the claims which are appended thereto.

The terms "arm" and "clamping arm" are used interchangeably in the description herein unless defined otherwise.

One of the major embodiment of the present invention is a method of spot welding of two lapping work pieces at a weld area comprising the steps of heating the weld area by conducting an electrical current via a pair of opposing conducting shafts contacting the weld area, wherein the opposing conducting shafts press against the weld area under a pressure; verifying softening of the weld area by detecting axial movement in at least one of the opposing conducting shafts via a displacement sensor coupled to the conducting shaft; and stir welding into the softened weld area by axially rotating the conducting shaft, which is detected to have the axial movement, to thrust the rotating conducting shaft into the softened weld area to form a circular spot of weldment joining the two lapping work pieces; wherein the weld area is heated until to be softened without actually melt the work pieces. The work pieces are clamped or hold together tightly during the process to accurately weld the materials at the specific spot decided. The work pieces have to be abutting one another in order to successfully passing the current through the weld area for heating. Relying on the material and physical dimension of the work pieces, the electrical current conducted through can be varied in the range enough to soften the materials of work pieces.

It is the electrical resistance of the work pieces towards the electrical current that leads to the localized heating at the weld area. Preferably, the work piece herein refers to metal or alloy having a melting temperature lower than the material used to fabricate the conducting shaft. This avoids the conducting shaft having it softened or melted prior to the softening of the weld area.

As setting forth, pressure is exerted on the surface of the weld area via the opposing conducting shaft, at the weld area of the top or bottom lapping work piece or the surface of the weld area contacted by the conducting shaft acted as the anode. The applied pressure not necessary of high magnitude but should be in sufficient amount to cause a detectable displacement into softened area to initiate subsequent procedures in the disclosed method. In more specific, the contacted surface applied with pressure should be the initial area where the work piece become softened to permit the conducting shaft to rotate at the desired timing stir welding into the work piece to mix the soften nugget and further join the work pieces together. Further, the rotation of the conducting shaft or one of the opposing conducting shafts is actuated via a motor or the like. . In respect to another embodiment, the method of the present invention preferably discontinues the conducted electrical current upon initiation of the stir welding step to avoid melting the work pieces, particularly for the work pieces made of material which has melting temperature close to the softened state. On the other hand, the electrical current may only be terminated after a predetermined duration upon initiation of the stir welding especially the work pieces made of material have wider gap in between softened state and the melting point. Generating heat through this extended period, after the initiation of the stir welding, ensures the stir welding process to last longer for mixing the softened area of two lapping work pieces to finally form weldment.

Nevertheless, it is important to be noted that the disclosed method is temperature caution. Changes in temperature may elicit various subsequent procedures to effectively weld the work pieces together. In one embodiment, the disclosed method may further include a step of detecting temperature changes at the weld area through a thermo sensor and retracting the conducting shaft from the weld area upon dropping of the temperature below a preset threshold. This embodiment particularly aims to avoid breakage of the rotating conducting shaft since the weld area of the work pieces maybe getting hardened upon dropping of the temperature. Continuingly stirring the conducting shaft in hardening weld area may fracture the conducting shaft and possibly inflict weld defects to the weld area rendering the joined work pieces is not usable.

Another approach to avoid damages caused by the hardening weld area employed in the present invention is to reheat the weld area, during the stir welding step, by re-conducting the electrical current to the weld area. To prevent melting of the work pieces at the weld area upon reconnection of the electrical current, the coupled thermo sensor is used in the disclosed method to actively monitor the temperature changes at the weld area and prompt the electrical current to be discontinued before the temperature at the weld area exceeds the melting point of the work pieces. To protect the conducting shaft from overheating caused by the friction and the electrical resistance, the present invention also includes a step of cooling the rotating conducting shaft by bringing portion of the rotating conducting shaft into contact with a flowing cooling medium via a cooling means. In one preferred embodiment, the cooling means comprises a detachable housing hermetically mounted onto the portion of the upper conducting shaft having at least one inlet for the cooling medium to flow in and at least one outlet for the cooling medium to flow out. The cooling medium used in the present invention can be air, oil or water-based coolant. Such cooling step is particularly important to soften work pieces having melting temperature higher than the conducting shaft. More specifically, this material has softened state temperature much higher than the conducting shaft itself that softening of this material can only be carried out by cooling the conducting shaft throughout process to transfer heat away from the conducting shaft.

Another aspect of the present invention is an apparatus of spot welding, as shown in Figure 1 , comprising a clamp structure having a hollow bottom clamping arm (3) and a hollow upper clamping arm (5), arranged in a tip to tip fashion, that a gap exists in between the opposing clamping arms (3 and 5) for positioning of at least two layers of lapping metallic work pieces (1 and 2) and at least one of the opposing clamping arms (3 and 5) is moveable towards or away in relative to another for clamping or releasing the lapping metallic work pieces (1 and 2); a bottom conducting shaft (4) and an upper conducting shaft (6) housed within the bottom clamping arm (3) and the upper clamping arm (5) respectively with the upper conducting shaft (6) capable of performing rotation at its elongated central axis; wherein one of the tips of each conducting shaft (4 and 6) contacting weld area of the lapping metallic work pieces (1 and 2) under a predetermined pressure to conduct an electrical current thereto generating heat to soften the weld area of the lapping metallic work piece (1 and 2) without melting the lapping metallic work pieces (1 and 2), and the upper conducting shaft (6) starts axial rotation to thrust into the weld area of the lapping metallic work pieces (1 and 2) upon detecting of softening in the weld area; a controlling means (24) regulating the clamp structure and the pair of opposing conducting shafts (4 and 6) as well as providing a user interface for management of the apparatus; characterized in that a displacement sensor (12), in communication with the controlling means (24), is used in conjunction with the opposing conducting shafts (4 and 6) to detect slight axial movement of at least one conducting shaft towards the weld area to indicate softening of the weld area and initiates the upper conducting shaft (6) to rotate. In more specific, the hollow bottom clamping arm (3) and the hollow upper clamping arm (5) are more like a tubular structure having one end of the arm facing one another to provide the gap thereof for positioning the work pieces (1 and 2). Though illustrated in the Figures (1 and 2), the portion of the hollow bottom clamping arm (3) and hollow upper clamping arm (5) around the opposite facing ends are in one narrow gap, the opposite facing end may be fabricated to bear enlarged tips to offer better clamping force onto the work pieces (1 and 2) disposed thereon in one embodiment. To securely position the work pieces (1 and 2) prior to clamping or after retraction of the upper conducting shaft (6) and upper clamping arm (5), the bottom clamping arm (3) is made relatively larger in diameter compared to the upper clamping arm (5) that more surface area of the bottom work piece (2) is supported by the upper rim of the bottom clamping arm (3). More preferably, the disclosed apparatus employs additional clamping devices to secure the work pieces (1 and 2) especially work pieces of irregular in shape that are supporting the work pieces with single pair of clamping device is hardly possible. Moreover, the clamp structure may be completely or partially covered with an electric insulating material to avoid directly contact with the conducting shaft. The insulating material shields the clamp structure from the electrical current preventing occurrence of electrical leakage or electrocution to the user. There are void area provided in between the opposing conducting shafts (4 and 6) and the opposing clamping arms (3 and 5) to permit limited air flow which assists dissipating heat off the shafts, particularly at the upper conducting shaft (6).

The bottom (4) and upper (6) opposing conducting shafts of the present invention are preferably made of material with good electrical conductivity with low resistance to effectively soften the weld area in the lapping work pieces (1 and 2), while it is less likely to generate heat on its own due to low electrical resistance. The fabricating material can of any metal, ceramic material, carbon material or alloy derived thereof. In the preferred embodiment, the upper conducting shaft (6) is anode (8) and the bottom conducting shaft (4) is cathode (9). The electrical current is passed from the upper conducting shaft (6) to the bottom conducting shaft (6). Illustrated in Figure 2, the weld area becomes heated and softened first in between the contacting surface of the anode (8) to upper working piece (1), followed by in between the contacting surface of the upper work piece (1 ) and bottom work pieces (2), but not at the contacting surface of the bottom work piece (2) to cathode (9). In connection to that, in the more preferred embodiment, the disclosed apparatus applies a pressure load (13) onto the upper conducting shaft (6) and/or the upper clamping arm (5) not only to clamp the lapping work pieces (1 and 2) but also aiming to cause detectable displacement onto the softened weld area to initiate upper conducting shaft (6) rotation. Further, a pressure gauge (7) connects to the disclosed apparatus to reveal the amount of pressure load applied onto the upper conducting shaft (6) and upper clamping arm (5). The higher the reading indicates greater clamping force holding the lapping work pieces (1 and 2) together.

The electrical current used in the disclosed apparatus to heat the lapping work pieces is in the range enough to soften the work piece materials. In one embodiment, the upper conducting shaft (6) acting as the anode bears a relatively smaller contacting surface to the upper work piece (1) compared to bottom conducting shaft (4). The smaller contacting surface at the anode leads to greater resistance at the contact point and greater heat generation to soften the weld area easier. The current pulse train (10) for the resistance heating in the disclosed apparatus can be configured by the user via the user interface available on the controlling means (24) to only soften the work pieces. More specifically, the strength of the current can be controlled by manipulating the amplitude of the pulse train, while adjusting the high pulse width and the low pulse width for supply duration and dwell duration respectively.

As in the foregoing description, the stirring or rotation of the upper conducting shaft (6) is initiated upon displacement of the conducting shaft towards the soften weld area. The displacement sensor used in the present invention is preferably a Linear Velocity Displacement Transducer (LVDT) (20). Under the pressure load (13), the upper conducting shaft (6) advances into the soften weld area (14) but the stirring or rotation may not be initiated until the volume of displacement exceeds a preset threshold. Nevertheless, the displacement sensor may be used in conjunction with the pressure gauge to prompt the upper conducting shaft (6) to rotate. More specifically, signal of pressure drop recorded in the pressure gauge is sent to the controlling means (24) to verify the axial displacement of the conducting shaft and starts the rotating and/or stirring action performed by the upper conducting shaft (6).

According to the preferred embodiment, the disclosed apparatus may opt one of the two actions once the rotation or stirring started that the current is either discontinued from passing through the weld area upon rotation of the upper conducting shaft (6) or the current is continued from passing through the weld area after the rotation of the upper conducting shaft (6) is initiated for a duration. Termination of the electrical current at the specified moment ensures that the work pieces are not overly heated to reach the liquid state. For the materials having wider temperature range in between the soften state and the melted liquid state, the electrical current can be passed through at the weld area to heat the material to a higher temperature allowing more stored heat energy at the weld area promoting longer work duration for the stirring or stir welding process.

The disclosed apparatus, in one embodiment, is equipped with a thermo sensor (1 1) mounted adjacent to the upper conducting shaft (6) inside the upper clamping arm (5) to monitor temperature at the weld area. Likewise, the thermo sensor is in communication to controlling means (24) that any temperature changes in the weld area will be recorded and sent to the controlling means (24) for processing. Depending on the temperature changes, the controlling means (24) may prompt other structures in the disclosed apparatus to react accordingly. For example, in one embodiment, the thermo sensor (1 1), via the controlling means, prompts the upper conducting shaft (6) to retract away from the weld area once detecting drop of temperature below a preset threshold. Through the user interface, the threshold can be set in accordance with the material type of the work pieces and protects the conducting shaft or the weldment from damage especially when the weld area becomes cold and hard. In respect to another embodiment, the thermo sensor (1 1), again via the controlling means (24), prompts the upper conducting shaft (6) to rotate in higher speed to generate greater friction heating at the weld area (14) once drop of temperature below a preset threshold is detected. An abrasion and friction resistance material is used for fabrication of the conducting shaft (6) in this embodiment in order to generate sufficient heat to soften the weld area again without sacrificing much of the conducting shaft (6).

To soften again the hardening weld area (14), another approach adopted by the disclosed apparatus is to reconnect the current to pass through the weld area (14) upon detecting the drop of temperature below a preset threshold. The electrical current employed (10) may be lower than the one applied in previous occasion just to maintain the temperature of the weld area at the preferred level, while the weld area is soft to be displaced without melting the weld area. In this phase, the thermo sensor (1 1) actively monitors the temperature at the weld area (14) to avoid melting of the weld area (14) and terminates the electrical current before the temperature reaches to point where the work pieces are going to be melted.

Pursuant to the preferred embodiment, the apparatus has further included a cooling means (19a and 19b) for introducing a flowing cooling medium into contact directly or indirectly with a portion of the upper conducting shaft (6) under pressure to transfer heat from the upper conducting shaft (6) to the flowing cooling medium. The cooling means can be air, oil or water-based coolant. Figures 5 to 10 show various applicable forms and embodiments of the cooling means. In one embodiment, the cooling means (19a and 19b) comprises a detachable housing, which has casing (35) and a cover (35), to be hermetically mounted onto the portion of the upper conducting shaft (6) having at least one inlet (32) for the cooling medium to flow in and at least one outlet (33) for the cooling medium to flow out bringing the heat away from the upper conducting shaft (6). As illustrated in Figure 5b and 6, the cooling means (19) is a two pieces assembly having no direct contact with the upper conducting shaft (6) but rather in communication with the upper clamping arm (5) which housing the upper conducting shaft (6) within. The casing (35) is a circular ring body with tapered bottom end and a flat top end. The tapered bottom end can be snugly fitted onto the upper clamping arm (5) that no cooling medium leaks from the fitting, while the flat top end engages to the cover to form a substantially hermetic housing to channel the cooling medium. The circular ring body is relatively larger in diameter than the upper clamping arm (5) creating a void area (37) in between the outer surface of the upper clamping arm (5) and the inner surface of the ring body. The void area (37) temporarily stores the introduced cooling medium. Correspondingly, the cover (36) is fabricated circular in shape as well bearing a central aperture thereto to slot through the upper conducting shaft to engage with the casing (35). Preferably, the inlet (32) and outlet (33) are evenly disposed around the central aperture. Referring to Figure 7, a modification can be made to the cooling means that heat sink or extended surface (30) is carved at the external surface of the ring body to accelerate the heat-dissipating rate from the cooling means (19). In Figure 8, both external surface of the upper clamping arm (5), surrounded by the casing (35), and the casing (35) carry the extended surface (30) to expedite the heat transferred from the upper clamping arm (5) to the cooling medium, then from the cooling medium to casing (35) and to the environment.

Further embodiment of the cooling means (19) is shown in Figure 9. The cooling means (19) in this embodiment actually formed parts of the upper clamping arm (5) that the cooling means is in direct contact with the upper conducting shaft (6). Similarly, the cooling means and/or the conducting shaft (6) may bear the extended surface to promote heat dissipation as illustrated in Figure 10. Specifically, the portion of the upper conducting shaft (6) in contact with the flowing medium is fabricated with extended surface to enhance heat transfer. Nonetheless, a much simpler design of the cooling means (19) is shown in Figure 4 where no cooling medium is used in this embodiment and the heat dissipation solely relies on the heat sink or extended surface (30) available on the upper clamping arm (5). The present disclosure includes as contained in the appended claims, as well as that of the foregoing description. Although this invention has been described in its preferred form with a degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of construction and the combination and arrangements of parts may be resorted to without departing from the scope of the invention.