KR20180047304A - Metal sheets spot welding with inoculation agents - Google Patents

Abstract

The present invention relates to a spot welding method of two sheets mainly formed with a titanium alloy, which comprises the following steps of: predetermining a position area of two sheets which are in contact with each other with a welding point; forming a small hole in a central portion of the position area of at least one of the sheets; introducing an inoculation agent particle into the hole; and spot welding the sheets by enabling a current to pass through the sheets via a pair of electrodes, wherein the particle has at least one material selected from a group composed of (a) Al+Si and (b) Si+Ba.

Description

{METAL SHEETS SPOT WELDING WITH INOCULATION AGENTS}

The present invention relates generally to spot welding and metal inoculation areas and more particularly to nugget inoculating metal sheets to improve the weld grain structure. To a method of obtaining such a metal weld having improved mechanical properties.

Resistance spot welding (RSW) is a process in which a metal surface is contacted by heat from a resistor. The work-pieces are held together under the pressure exerted by the electrodes. Typically, the sheet is in the range of 0.5 to 3 mm (0.020 to 0.118 in) thick. This process concentrates the welding current to a small "point" using two molded copper alloy electrodes to simultaneously fix the thin plates together. Applying a large current through the point will melt the metal and form the weld. An attractive feature of point welding is that a lot of energy can be delivered to the point in a very short time (about 10 to 100 milliseconds). This allows the welding to take place without undue heating of the remainder of the sheet.

The energy delivered to the point is determined by the resistance between the electrode and the magnitude and duration of the current. The amount of energy is selected to suit the physical properties of the foil, its thickness, and the type of electrode. Applying too little energy will not melt the metal, or it will form a bad weld. Applying too much energy will melt too much of the metal and drain the molten material to form holes rather than welds. Another feature of point welding is the ability to control the energy delivered to the point to create a reliable weld.

The most common applications of spot welding are in the automotive and aircraft manufacturing industries and are almost universally used to weld automobiles and planes by welding sheet metal. Spot welding is also used in orthodontic clinics, and a small spot welding device is used to size the metal "molar bands" used for orthodontics.

Another application is spot welding straps for nickel-cadmium or nickel-metal hydride batteries for making batteries. The battery is connected by spot welding a thin nickel strap to the end of the battery. Spot welding can protect the battery from being too hot, which can always be present when conventional soldering is complete.

Good design practice should always allow sufficient accessibility. Contact surfaces should be free of contaminants, such as water, oil, and dust, to ensure good quality welds. The thickness of the metal is generally not a factor in determining good welds.

It is important that the metal welds have sufficient mechanical strength and resistance to fatigue, since automotive applications apply repetitive stresses on body parts over time.

In order to form a resistive spot weld between the metal foils, a pair of welding electrodes, typically a copper foil under pressure, is clamped together and a current is passed between the electrodes to form a region or "point" Perfusion. This current flow causes the metal material to heat up to its melting temperature, creating a molten weld nugget, and the metals from the two foils move toward each other to form a melt weld when the molten nugget is cooled and solidified . The solidification process is responsible for the nucleation and growth of new phases (solid) at the advancing solid / liquid contacts in the weld nugget. The solid phase in the molten weld nugget is typically initiated by epitaxial growth from the surface of the material being welded and proceeds by competitive growth towards the centerline of the weld. That is, crystals having the easiest growth direction oriented most preferentially along the heat flow direction slope tend to push crystals that are not easily oriented in the easy growth direction. The crystal structure of the obtained welded portion is determined by the type of nucleation and growth of the solid phase. Since the weld nugget is cooled, the solidification starting from the walls of the substrate causes the formation of crystals growing against the heat flux; These crystals are known as columnar crystals. As a result, depending on the solidification conditions, the equiaxed crystals form a weld nugget in the central region. A columnar crystal structure, that is, a structure in which crystals tend to be long and tend to become parallel to each other, has a structure in which a crystal portion having a uniform size and a weld portion having a smaller mechanical strength than a weld portion having an equiaxed crystal structure arranged in an irregular orientation It causes. Further, if the columnar crystal structure is in the vicinity of the high stress region formed at the intersection of the weld nugget and the hole of the thin plate, the mechanical strength of the welded portion will be further lowered. The coagulated welds usually have both columnar crystals and equiaxed crystals, which are located in the center of the weld and are surrounded by the outer boundary layer of the columnar crystals. In order to increase the resistance to fatigue as well as the mechanical strength of the weld, it will be necessary to maximize the volume of the equiaxed crystals compared to the volume of the columnar crystals. The present invention is directed to achieving this object.

According to one embodiment of the present invention, there is provided a spot welding method of two thin plates, which is mainly formed of a titanium alloy, the method comprising the steps of:

Predetermining a location area of two thin plates to be in contact with each other where a welding spot is to be formed;

Forming a small hole in the center of at least one of the two thin plates;

Introducing the inoculant particles into the hole;

Contacting the two thin plates by passing an electric current through the thin plate through a pair of electrodes,

Here,

(a) Al + Si,

(b) Si + Ba

And at least one material selected from the group consisting of.

A significant advantage of the present invention lies in its ability to not only enhance the mechanical properties of metal welds but also improve the consistency of weld quality by introducing relatively inexpensive inoculants to the weld nugget.

Another advantage of the present invention is that a conventional resistance welding apparatus can be used without increasing the welding cycle time to implement the method of the present invention.

These and other advantages and features of the present invention will become apparent or will become apparent during the course of the following description of the preferred embodiments of the invention.

The present invention includes welds between two workpieces formed of a melt bond, for example, a metal. The welded portion generally refers to a spot welded portion that can be produced using a conventionally known resistance welding apparatus in the art. Such devices typically include a power supply, and a pair of electrodes between which thin plates are held at a predetermined force. For thin plates that are in contact against each other under pressure, the power supply transfers current to the electrodes by opposing and coming into contact with the surface of the foil to create a molten weld nugget. And solidifies and cools the weld nugget to form a weld having an ideal mechanical strength close to the mechanical strength of the thin metal sheet itself.

Since the weld nugget is cooled, the molten metal crystallizes as the state changes from liquid to solid. During the cooling process, crystallization of the outer boundary layer occurs first, and solidification progresses toward the center of the welded portion until the welded portion is completely crystallized. The crystallization of the outer boundary layer of the weld nugget results in a columnar crystal structure that extends parallel to one another and tends to lengthen the individual crystals in the longitudinal axis towards the direction of heat flow. The outer boundary moves to a central region where the crystal structure is isotropic, that is, rather than lengthening, where each crystal has the same size and whose axes are randomly directed towards one another. As will be discussed later, the equiaxed crystal structure of the central region tends to provide welds with excellent mechanical strength and fatigue resistance as compared to those welded in the weld nugget where the columnar crystal structure dominates.

According to the present invention, it has been found that the strength of a weld formed between workpieces can be improved by injecting a molten weld nugget with a specific material that tends to be particularly effective in promoting nucleation of equiaxed crystals, such as solidifying nugget .

An inoculum is applied to one or both sides when the opposite surface of the workpiece is welded. The inoculant may be in the form of a liquid or paste, may be inoculated with a carrier formed by a film or foil that is sprayed or brushed on the surface of the workpiece, or inserted between the workpiece surfaces before being fixed and welded.

The workpiece of the thin metal plate may be formed of various materials suitable for spot welding and may be made of at least one of mild steel, galvanized steel, quenched steel, stainless steel, aluminum-plated steel, copper alloy, Alloy. The present invention is also applicable to other metal sheet spot welding.

In one embodiment, the location area of the two thin plates contacting each other to form the welding spot is predetermined. A small hole may be formed in the center of at least one of the two thin plates. Inoculation agent can be put into the hole. Two thin plates are connected by passing an electric current between the thin plates through a pair of electrodes. Said particles comprising at least one material selected from the group consisting of:

(b) Al + Ti, (b) Al + C, (c) Ti + C, (d) Ti-B alloy + Al, Re alloy + C, (h) Al + Ca, (i) Sr + Zr, (j) Ba + Zr, (k) Si + C, Si + Na, (o) Al + Ba, (p) Al + Si,

The diameter of the hole may be 0.1 to 1 mm. The depth of the hole may be 1/20 to 1/5 of the thin plate. The particles may be in powder form. The two thin plates may have the same or different depths.

The inoculant particles may be applied on at least one surface of the two thin plates in the location region. The material may be applied in film form on the surface. A paste-like material can be applied on the surface. The thickness of the film may range from 0.01 to 0.1 mm. The two thin plates may have the same or different depths.

While the method of the present invention is preferably claimed, any of the listed inoculum particles may be used.

A series of tests are performed to compare the properties of the welds produced with and without inoculation according to the method of the present invention. These test results clearly show that the mechanical properties of the inoculated welds are superior to those without the inoculum.

The results of these tests are shown in Table 1 and clearly show that the mechanical properties of the inoculated welds in accordance with the method of the present invention are significantly superior to the mechanical properties of the inoculated welds. In addition, it can be seen that the welded part inoculated in accordance with the present invention exhibits excellent shear strength as compared with the uninoculated welded part.

From the above, it can be understood that the welding nugget inoculation described above is not only advantageous over previous welding methods, but also an effective and economical method in particular. Of course, those skilled in the art will appreciate that various modifications or additions may be made to the invention to further illustrate the present invention without departing from the spirit or scope of the present invention as defined by the appended claims. It is therefore to be understood that the protection sought and provided herein is to be considered as extending to the claimed subject matter, and all equivalents thereof are to be regarded as fair within the scope of the present invention.

Claims (5)

CLAIMS What is claimed is: 1. A method of spot welding two thin plates formed of a titanium alloy, comprising the steps of:
Predetermining a location area of two thin plates to be in contact with each other where a welding spot is to be formed;
Forming a small hole in the center of at least one of the two thin plates;
Introducing the inoculant particles into the hole;
Contacting the two thin plates by passing an electric current through the thin plate through a pair of electrodes,
Here,
(a) Al + Si,
(b) Si + Ba
≪ / RTI > at least one material selected from the group consisting of < RTI ID = 0.0 > The method according to claim 1, wherein the diameter of the hole is 0.1 to 1 mm. The method according to claim 1, wherein the depth of the hole is 1/20 to 1/5 of the thin plate. The method of claim 1, wherein the particles are in the form of a powder. The method according to claim 1, wherein the two thin plates have the same or different depths.