APPLICATIONS

FIELD OF INVENTION

The present invention relates to resistance spot welding.

Particularly, the present invention relates to an electrically conductive layer which aids resistance spot welding.

DEFINITIONS OF TERMS USED IN THE SPECIFICATION

The term "weld current" used in the specification means the electrical amperage in the power system as the weld is being made.

The term "weld nugget" used in the specification means the fused weld metal in spot, seam, or projection welding.

The term "weld strength" used in the specification means the shear tensile strength at which the weld joint fails.

The term "welding cycle" used in the specification means the complete sequence of events involved in making a single resistance spot weld.

BACKGROUND

Resistance spot welding is the process of joining two contacting metal surfaces by the heat obtained from resistance to an electric current flow. The method is commonly used for aluminum alloy materials, and typically includes the steps of: a) superimposing on each other two aluminum alloy sheets to be welded together in lap joint configuration; b) pressing a pair of electrodes onto the aluminum alloy sheets, such that the electrodes are opposed to each other via the sheets; and c) applying an electric current to the electrodes to melt at the interface of the sheets interposed between the electrodes, due to heat resistance, to thereby form a weld nugget and join the sheets together. This welding technique is commonly employed for joining steel and/or aluminum sheets in automobile manufacturing industries, for, a lot of energy can be delivered to the spot in very little time which permits quick welding without excessive heating of the rest of the sheet, also, the amount of energy delivered to the spot can be controlled as per the metal requirements to produce desirable welds which are also reliable.

Where, aluminum materials, due to the high electrical and thermal conductivity of aluminum, require 2 to 3 times more energy than steel, the weld time for aluminum is only one-third of that of steel. Thus, for spot welding aluminum, a higher current must be delivered in 50 - 70 % lesser time, in comparison with steel materials. The electrodes formed of copper-chromium alloy, copper-chromium-zirconium alloy, and the like; are most suitable for resistance spot welding applications, in terms of their mechanical properties and electrical conductivity. When the above-described electrodes are used for continuous resistance spot welding of aluminum alloy work-pieces, at multiple spots in repetitive welding cycles, the tips of the electrodes are likely to be worn off rapidly, allowing only a small number of welding cycles, in each pair of electrode. The wear of the electrodes results in reduced strength of the welded parts of the work-pieces, relatively short life of the electrodes, and a comparatively small number of welding cycles. Also, aluminum sheets typically have a thin layer of aluminum oxide on the surface, which increases the contact resistance between the sheet and the electrode. To overcome this contact resistance, a higher electrode current and force is utilized during the spot welding process, which, drastically reduces the electrode service life, as compared to when spot welding uncoated steel. Further, aluminum alloy sheets when used for automobile body panels are spot welded after the sheets are formed in desired shapes by pressing, wherein, lubricating oil is applied on the aluminum sheets during pressing. The lubricating oil results in further shortened service life of the electrodes and a further reduced number of welding cycles. A similar problem is faced when continuous resistance spot welding galvanized steel, which is typically coated with zinc.

The above-listed problems largely curtail welding operations, especially in a mass production facility. Various attempts have been made to extend the period before which a welding operation is interrupted for cleaning or replacing the electrodes. Many techniques rely on reducing the contact resistance of the aluminum surfaces, through mechanical and/or chemical means, such as, cleaning the aluminum sheets to remove oxide film or providing protective coating on the aluminum sheets. These techniques aim at preventing electrode tip fouling, thus increasing the electrode service life. Some of the techniques used, include, electrode dressing, current stepping, arc-cleaning the sheet surface, surface treatment of work-pieces and incorporation of a bi-metallic process tape between the electrode and the work- piece. While these techniques enhance the electrode service life, they are challenging to implement and substantially increase the welding costs. Also, the available techniques might not be suitable for work-pieces of different materials, as some materials might develop a propensity to react with the copper electrodes. Therefore, there is felt a need to develop a reliable and economic solution for spot welding applications, especially for welding aluminum sheets, which overcomes the electrode fouling problem.

Several attempts have been made to provide techniques that overcome one or more of the drawbacks listed above. Some of the disclosures are listed in the prior art below:

US Patent No. 3751626 discloses providing the aluminum members to be joined with a reacted silicate coating which alleviates the electrode fouling problem. The silicate coating, having thickness between 100 angstroms to 0.00003 inch, is applied on at least a portion to be welded of the aluminum member, by immersing the aluminum member in a bath containing an aqueous solution of alkali metal silicate, such that, the coated metal surface has a silicate content of at least 0.7%, wherein, the member surface is preferably cleaned before coating.

JP Patent No. 3060876 discloses a method of spot welding metal sheets using a low current by superimposing the material sides to be welded and applying metallic super fine powder between the joining faces. Typically, Ni superfine powder is applied in a deliberate quantity to the material having an organic resin coating, at least to the face to be welded. A second sheet having an organic resin coating is fitted thereon, and with the chromium-copper electrodes being arranged welding is performed by applying current. In an initial pressurized state, the materials and the Ni powder are subjected to solid phase joining and thereby the dielectric breakdown resistance is further reduced.

US Patent No. 5391854 discloses a method for spot welding aluminum alloys which aims at increasing the number of welding cycles in a continuous spot welding operation. The method involves selecting an aluminum alloy work-piece comprising at least one element selected from copper, magnesium, manganese, silicon, zinc, and lithium, such that, the electrical conductivity of the alloy work- piece is preferably 23 - 57 % IACS. The desired electrical conductivity is obtained by suitably choosing the alloying element, its content, the heat treatment and processing. The work-pieces so obtained can be favorably spot welded even if coated with lubricating oil.

JP Patent No. 2006077101 discloses a coating for spot welding having an emulsion comprising 35 - 45 wt. % water and 3 - 8 wt. % thickening agent. It is possible to secure electro-conductivity on performing the spot welding by containing water. Since, the coating does not contain iron powder; rust is not formed even over a longer period.

The present invention aims at providing a reliable and effective solution for spot welding applications, which will increase the electrode service life, thereby increasing the number of welding cycles and providing an uninterrupted welding operation. The disclosures listed above discuss methods for enhancing spot welding of aluminum sheets. However, the present invention aims at providing a coating for spot welding applications which can be used for aluminum, aluminum alloys, galvanized steel, and the like. Further, the present invention aims at providing a technique which uses a low current, and is simple and cost-effective.

OBJECTS OF THE INVENTION

An object of the present invention is to provide an electrically conductive coating for spot welding applications. Another object of the present invention is to provide an electrically conductive coating for spot welding applications which is non-metallic.

Yet another object of the present invention is to provide an electrically conductive coating for spot welding applications which decreases the contact resistance between the work-piece and the electrode.

Still another object of the present invention is to provide an electrically conductive coating for spot welding applications which requires low weld current.

One more object of the present invention is to provide an electrically conductive coating for spot welding applications which prevents electrode tip fouling, thereby, increasing the electrode service life.

Yet one more object of the present invention is to provide an electrically conductive coating for spot welding applications 'which permits an uninterrupted welding operation.

Still one more object of the present invention is to provide an electrically conductive coating for spot welding applications which is easy-to-apply, reliable, and cost effective.

An additional object of the present invention is to provide an electrically conductive coating for spot welding applications of aluminum sheets, aluminum alloy sheets, galvanized steel sheets, and the like. Another additional object of the present invention is to provide an electrically conductive coating for spot welding applications which does not require any special/surface treatment to the work-piece to be welded.

SUMMARY OF THE INVENTION

In accordance with the present invention, is provided an electrically conductive coating for spot welding applications, said electrically conductive coating being non-metallic, comprising at least one non-metallic component selected from the group of components consisting of carbon black, carbon black feedstock, graphite, an ionic substance, and a mixture of ionic substances, and being in one form selected from the group of forms consisting of colloid, powder, film, slurry, suspension and liquid; said conductive coating being provided on at least the spot to be welded.

Typically, in accordance with the present invention, said electrically conductive coating has a thickness in the range of 10 to 1000 microns.

In accordance with the present invention, said electrically conductive coating has an electrical resistivity in the range of 10 to 50 micro-ohms-meter.

Preferably, in accordance with the present invention, the proportion of said non- metallic component in said electrically conductive coating is greater than 95%.

Typically, in accordance with the present invention, said electrically conductive coating comprises a bonding agent. Alternatively, in accordance with the present invention, said electrically conductive coating comprises a fluid substrate having proportion in the range of 0 - 5 %.

Typically, in accordance with the present invention, the ionic substance is selected from at least one substance from the group of substances consisting of butyl methyl imidazolium acetate and ethyl methyl imidazolium dibutyl phosphate.

In accordance with the present invention, is provided a process for coating a metallic substrate to be spot welded with an electrically conductive coating comprising a non-metallic component, the thickness of the coating being in the range of 10 to 1000 microns, by a technique selected from the group consisting of spraying, brushing, immersing, depositing, and dusting.

Alternatively, in accordance with the present invention, the process for coating a metallic substrate to be spot welded includes the step of bonding said electrically conductive coating on the metallic substrate.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

The invention will now be described with the help of the accompanying drawings, in which,

Figure 1 illustrates a graph showing the variation in weld strength with respect to weld current;

Figure 2 illustrates a graph showing the electrode face diameter measured from carbon print with respect to the number of spot welds for bare sheets and coated sheets; Figure 3 illustrates a graph showing the variation in weld strength with respect to electrode life for bare sheets and coated sheets; and

Figure 4 illustrates a cross-section of the spot weld portion showing the weld nugget dimension.

DETAILED DESCRIPTION OF THE INVENTION

The present invention envisages an electrically conductive coating for spot welding applications, wherein the coating is applied on at least the spot of the work-piece of a metallic substrate to be welded. The electrically conductive coating of the present invention can be used on metallic substrates that have a propensity to react with copper-based electrodes or have a barrier layer that increases the contact resistance causing an early deterioration of the electrodes. The electrically conductive coating is non-metallic and comprises at least one component selected from the group consisting of carbon black, carbon black feedstock, graphite, an ionic substance, a mixture of ionic substances, and the like. The coating is formed as colloid, powder, film, slurry, suspension, or liquid, having the non-metallic component in a proportion greater than 95%. The coating is applied on at least a portion of the work-piece of the metallic substrate, to be exposed to welding, by spraying, dusting, immersing, depositing or brushing, wherein the coating has an electrical resistivity of 10 to 50 micro-ohms-meter. Since, the conductive coating of the present invention, can be, optionally, applied only to the spot to be welded; the costs of the welding operation can be still reduced. The electrically conductive coating of the present invention is produced under temperature conditions ranging from 15 to 45 °C, wherein, the coating forms a thin, continuous, and adherent conductive layer on the metallic substrate that does not flake off on bending or flexing.

The electrically conductive coating of the present invention can be applied directly on the work-piece or at least the portion of the work-piece to be welded, before performing the welding operation. Alternatively, a heat treatment can be provided to the portion of the work-piece of the metallic substrate having the electrically conductive coating, to make the substrate speedily set for the welding operation. Still alternatively, the electrically conductive coating can be prepared separately and then bonded to the work-piece or at least the portion of the work-piece to be welded. A bonding agent can be optionally used in the coating composition to facilitate proper sticking of the coating on metallic substrate. When the electrically conductive coating is in the powder form, it is dusted on the work-piece so as to completely cover at least the spot to be welded, wherein, optionally, provisions can be made to ensure that the conductive powder bonds to the surface of the work- piece during the welding operation.

The electrically conductive coating of the present invention allows current to pass through from the electrode to the metallic substrate via the coating, even at a low weld current. Thus, by forming an interface between the electrode and the metallic substrate, the coating inhibits excessive interaction between the electrode and the substrate, preserving the electrode tip from fouling or damage. Also, since, the electrode face retains its shape and desired weld strength can thus be continually achieved at a low weld current, the metal substrate is only favorably melted, providing suitable weld nugget diameter. Further, the weld current may be altered as per the type of metallic substrate, the desired weld strength, and weld nugget diameter. An additional benefit of the electrically conductive coating of the present invention is that the coating helps in enlarging the process window for spot welding, which, eliminates any need for special/surface treatment to the metallic substrate to be coated and subsequently spot welded.

The thickness of the electrically conductive coating, although, varies depending upon the non-metallic component used, the method used to apply the coating on the metallic substrate, type of the metallic substrate, the electrical conductivity of the metallic substrate, and the like, it is desirable that the thickness of the electrically conductive coating be not less than 10 microns and not greater than 1000 microns. Further, the proportion of the non-metallic electrically conductive component in the electrically conductive coating of the present invention is essentially greater than 95%, such that, the coating has an electrical resistivity in the range of 10 to 50 micro-ohms-meter. Still further, a liquid/fluid substrate, typically selected from water, alcohol, acetone, and the like, can be used in the coating composition to form a suspension, solution or slurry, wherein the proportion of the liquid/fluid substrate is typically in the range of 0 - 5 %. Alternatively, the non-metallic component used can be an ionic substance, typically an ionic liquid, selected from butyl methyl imidazolium acetate and ethyl methyl imidazolium dibutyl phosphate and the like.

The electrically conductive coating can be used for aluminum sheets having high aluminum content and aluminum alloy sheets having other elements like zinc, magnesium, copper, manganese, chromium, silicon, and iron. Further, the electrically conductive coating can be used for galvanized steel sheets which comprise a layer of zinc for corrosion resistance. The electrically conductive coating of the present invention can be suitably used when spot welding extruded, wrought, or cast sheets of different materials without the need for cleaning prior to coating. The electrodes used are copper-based, typically, selected from copper- chromium or copper-chromium-zirconium. During the welding operation, initially a substantial amount of force is applied between the electrodes, such that, the work-piece to be welded is under compressive forces; the force is varied during the weld operation to manipulate the weld strength. Sufficient electric current is passed between the electrodes and through the spot to be welded, to cause a fusion in the work-pieces to produce a weld nugget of desirable dimension. Typically, when the electrically conductive coating of the present invention is provided, the electric current is varied between 15 kA - 25 kA. It is absolutely important that a coated spot of the work-piece be in contact with the electrodes while spot welding when the current is applied.

TEST RESULTS

The invention will now be described with respect to the following examples which do not limit the scope and ambit of the invention in anyway and only exemplify the invention.

EXAMPLE 1:

An electrically conductive coating of carbon black feed stock (CBFS) was applied to 1.0 mm aluminum 6xxx alloy sheet using a brush. Spot welds were made on the coated sheet using pedestal medium frequency direct current spot welding equipment having B-type (dome-type flat face) Cu-Cr-Zr electrode with 6 mm face diameter. Spot welding trials were carried out at different weld currents and cycle time. Welded samples were tested in a computerized tensile testing machine to measure the weld strength and construct lobe curves. Weld strength of above 1 kN was obtained at weld currents above 18kA. Weld strength was observed to increase linearly up to 2.25 kN on increasing the weld current to 23 kA. The graphical representation showing the variation in weld strength with respect to weld current is illustrated in Figure 1, generally represented by numeral 100. Liquid metal expulsion was observed at higher weld currents and weld time, which resulted in reduction of the weld strength. When bare sheets without the CBFS coating were employed as work-pieces, the process window was much smaller. At low weld current of 18 kA, no spot welds were formed. At slightly higher weld current of 20kA, the weld strength was found to be in the range of 0.2 to 1.6 kN (refer 100). To achieve comparable weld strength, for the bare sheets, a higher weld current was required, which lead to metal expulsion.

EXAMPLE 2:

Multiple welds were carried out by the method described in EXAMPLE 1, using CBFS coating, at a weld current of 20 kA and a cycle time of 100 millisecond (ms). A set of Cu-Cr-Zr electrode was used to make up to 300 spot welds, after which the electrodes showed visible aluminum pick-up and repeated sticking of sheet material to electrode, indicating requirement for electrode redressing. Visual inspection and carbon imprints of electrode surface showed minimal deterioration and no enlargement of the electrode face up to significant number of spots. Similar trials were conducted using bare sheets without any CBFS coating. The electrode showed visible sign of wear and repeated sticking of sheet material to the electrode only after 40 spots. Figure 2 illustrates a graph showing the electrode face diameter measured from carbon print with respect to the number of spot welds for bare sheets and CBFS coated sheets, generally represented by numeral 200; and Figure 3 illustrates a graphical representation showing the variation in weld strength with respect to electrode life for bare sheets and CBFS coated sheets, generally represented by numeral 300. Although, the weld strength did not show appreciable loss even with bare sheets, the trials had to be f equently stopped, when welding bare sheets, due to repeated sticking of sheet material to the electrodes and random metal expulsion. Weld nugget diameter was measured to be around 2.4 mm, by obtaining a cross-section of the spot welded samples and examining under a microscope, a pictorial view of which is illustrated in Figure 4, generally represented by numeral 400.

EXAMPLE 3:

Spot welding trials were carried out by the same method as described in EXAMPLE 1, using ionic liquid coating instead of the CBFS coating. The trial results showed that ionic liquids can also be most suitably used as an electrically conductive coating layer when spot welding aluminum or aluminum alloy sheets using copper-based electrodes.

EXAMPLE 4:

Several spot welding trials were carried out using the method as described in EXAMPLE 1, and graphite powder as the electrically conductive coating. Fine graphite powder was dusted on the surface of aluminum sheets to be welded so as to form a continuous layer, wherein, graphite powder was also applied on the face of the electrode. No visible deposition of aluminum on the copper-based electrode was observed and favorable spot welds were formed even at a weld current of 20 kA.

EXAMPLE 5:

Several spot welding trials were carried out using the method as described in EXAMPLE 1, on galvanized steel sheets instead of aluminum sheets. It was observed that the electrically conductive coating of the present invention was equally effective when used for galvanized steel sheets. TECHNICAL ADVANTAGES

An electrically conductive coating for spot welding applications comprising at least one non-metallic component selected from carbon black, carbon black feed stock, graphite, an ionic substance, and a mixture of ionic substances; and being in a form selected from colloid, powder, film, slurry, suspension and liquid; the conductive coating having thickness in the range of 10 to 1000 microns and being provided on at least the spot to be welded by spraying, brushing, immersing, depositing or dusting; as described in the present invention has several technical advantages including but not limited to the realization of:

• the electrically conductive coating for spot welding applications has an electrical resistivity in the range of 10 to 50 micro-ohms-meter, which decreases the contact resistance between the work-piece and the electrode, thereby, preventing the electrode tip from fouling, increasing the electrode service life and providing uninterrupted welding operation;

• the electrically conductive coating for spot welding applications requires low weld current, wherein, a weld strength of above 1 kN can be obtained at a weld current of 18 kA;

• the electrically conductive coating for spot welding applications which is easily applied on the work-piece surface by various methods and is cost effective; and

• the electrically conductive coating can be used for spot welding applications of aluminum sheets, aluminum alloy sheets, galvanized steel sheets, and the like, without the requirement for any special/surface treatment to the work-piece.

The numerical values mentioned for the various physical parameters, dimensions or quantities are only approximations and it is envisaged that the values higher/lower than the numerical values assigned to the parameters, dimensions or quantities fall within the scope of the invention, unless there is a statement in the specification specific to the contrary.

In view of the wide variety of embodiments to which the principles of the present invention can be applied, it should be understood that the illustrated embodiments are exemplary only. While considerable emphasis has been placed herein on the particular features of this invention, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing from the principle of the invention. These and other modifications in the nature of the invention or the preferred embodiments will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation.