DK179149B1

DK179149B1 - Spot welding of metal plates with inoculants

Info

Publication number: DK179149B1
Application number: DKPA201670272A
Authority: DK
Inventors: Lixin Zhou
Original assignee: Hebei Lixin Tech Co Ltd
Priority date: 2016-04-28
Filing date: 2016-04-28
Publication date: 2017-12-11
Also published as: DK201670272A1

Abstract

The present invention relates to a method of spot welding of two sheets mainly formed of aluminum alloys, including the steps of: predetermining the location area of the two sheets in contact with each other in which a weld spot is to be formed; forming a small hole at the center of said location area of at least one of the two sheets; introducing inoculant particles into the hole; spot welding said two sheets by passing electrical current through the sheets via a pair of electrodes, wherein the particles comprise at least one material selected from the group consisting of:(a) A1+Ti,(b) A1+C.

Description

Method of spot welding sheets formed of aluminum alloys FIELD OF THE INVENTION

The present invention generally relates to spot welding area and metal inoculation area, and more particularly to a method of spot welding combined with nugget inoculating in sheets formed of aluminum alloys in order to refine the weld grain structure, and thus to obtain metal welds with enhanced mechanical properties.

BACKGROUND OF THE INVENTION

Resistance spot welding (RSW) is a process in which contacting metal surfaces are joined by the heat obtained from resistance. Work-pieces are held together under pressure exerted by electrodes. Typically the sheets are in the 0.5 to 3 mm (0.020 to 0.118 in) thickness range. The process uses two shaped copper alloy electrodes to concentrate welding current into a small "spot" and to simultaneously clamp the sheets together. Forcing a large current through the spot will melt the metal and form the weld. The attractive feature of spot welding is that a lot of energy can be delivered to the spot in a very short time (approximately 10-100 milliseconds). That permits the welding to occur without excessive heating of the remainder of the sheet.

The amount of heat (energy) delivered to the spot is determined by the resistance between the electrodes and the magnitude and duration of the current. The amount of energy is chosen to match the sheet's material properties, its thickness, and type of electrodes. Applying too little energy will not melt the metal or will make a poor weld. Applying too much energy will melt too much metal, eject molten material, and make a hole rather than a weld. Another feature of spot welding is that the energy delivered to the spot can be controlled to produce reliable welds.

The most common application of spot welding is in the automobile and airplane manufacturing industry, where it is used almost universally to weld the sheet metal to form a car and aircraft. Spot welding is also used in the orthodontist's clinic, where small-scale spot welding equipment is used when resizing metal "molar bands" used in orthodontics.

Another application is spot welding straps to nickel-cadmium or nickel-metal hydride cells to make batteries. The cells are joined by spot welding thin nickel straps to the battery terminals. Spot welding can keep the battery from getting too hot, as might happen if conventional soldering were done.

Good design practice must always allow for adequate accessibility. Connecting surfaces should be free of contaminants such as scale, oil, and dirt, to ensure quality welds. Metal thickness is generally not a factor in determining good welds.

Because vehicle applications impose cyclic stresses on the body components over a long period of time, it is important that the metal welds possess adequate mechanical strength and resistance to fatigue.

In order to form resistance type spot welds between sheets of an metal, the sheets are clamped together under pressure between a pair of welding electrodes, typically copper, and an electrical current is passed between the electrodes so as to flow through an area or "spot" on the sheets. This current flow heats the metal material at the spot to its melting temperature, producing a molten weld nugget in which metal from the two sheets migrate toward each other to form a fusion weld when the molten nugget has cooled and solidified. The solidification process results from nucleation and growth of a new phase (a solid) at an advancing solid/liquid interface within the weld nugget. The solid phase within a molten weld nugget generally initiates by epitaxial growth from the surfaces of the material being welded, and proceeds by competitive growth toward the centerline of the weld. That is, grains with their easy growth direction oriented most preferentially along the heat flow direction gradient, tend to crowd out those grains whose easy growth directions are not as suitably oriented. The grain structure of the resulting weld is determined by the type of nucleation and growth of the solid phase. As the weld nugget cools, the solidification that begins at the walls of the substrate result the formation of grains that grow against the heat flux; these grains are known as columnar grains. Eventually, and depending upon the solidification conditions, equiaxed grains form in the central region the weld nugget. The columnar grain structures, i.e. structures in which the grains tend to be elongate and run parallel to each other, result in a weld that possesses less mechanical strength compared to a weld having an equiaxed grain structure where the grains are uniform in size and are arranged in a random orientation. Furthermore, the mechanical strength of the weld would degrade even more if the columnar grain structure is in the proximity of the high stress regions formed at the intersection of the weld nugget and the opening of the sheets. A solidified weld normally possesses both columnar and equiaxed grains, with the equiaxed grains being disposed in the center of the weld and surrounded by an outer boundary layer of columnar grains. In order to increase the mechanical strength of the weld as well as its resistance to fatigue, it would be desirable to maximize the volume of equiaxed grains, compared to the volume of the columnar grains. The present invention is directed toward achieving this objective.

SUMMARY OF THE INVENTION

According to one aspect of the invention, a method of spot welding of two sheets formed of aluminum alloys is provided, including the steps of: predetermining the location area of the two sheets in contact with each other in which a weld spot is to be formed; forming a small hole at the center of said location area of at least one of the two sheets; introducing inoculant particles into the hole; spot welding said two sheets by passing electrical current through the sheets via a pair of electrodes, wherein the particles comprise at least one material selected from the group consisting of: (a) Al+Ti, (b) Al+C. A significant advantage of the invention resides in its ability to not only increase the mechanical properties of metal weld, but also improve the consistency of weld quality by introducing an relatively inexpensive inoculant to the weld nugget.

Another advantage of the invention is conventional resistance welding equipment may be used to practice the inventive method without increasing weld cycle time.

These and other advantages and features of the invention will be made clear or will become apparent during the course of the following description of a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention involves a fusion bond such as a weld between two sheets formed of aluminum alloys. The weld is commonly referred to as a spot weld that may be produced using conventional resistance welding equipment well-known in the art. Such equipment typically includes a power supply, and a pair of electrodes between which the sheets are clamped with a pre-determined force. With the sheets in face-to-face contact under pressure, the power supply delivers an electrical current to the electrodes which flows through the facing, contacting surfaces of the sheets to produce a molten weld nugget. This weld nugget solidifies and cools to form a weld which ideally possesses a mechanical strength approaching that of the metal sheet material itself.

As weld nugget cools, the molten metal crystallizes as it changes state from a liquid to a solid. During the cooling process, crystallization of the outer boundary layers occurs first, and solidification proceeds inwardly toward the center of the weld until the weld has completely crystallized. Crystallization of an outer boundary layer in the weld nugget results in a columnar grain structure in which the individual grains tend to be elongate with their longitudinal axes extending parallel to each other and oriented in the direction of the heat flow. The outer boundary transitions into a central area where the grain structure is equiaxed, i.e. the individual grains have equal dimensions, rather than being elongate, and have their axes randomly oriented relative to each other. As will be later discussed, the equiaxed grain structure of the central area tends to provide the weld with superior mechanical strength and fatigue resistance compared to a weld where columnar grain structure are predominate within the weld nugget.

In accordance with the present invention, it has been found that the strength of a weld formed between sheets can be improved by inoculating the molten weld nugget with certain materials which tend to be particularly effective in promoting the nucleation of equiaxed grains as the nugget solidifies.

The inoculant is applied to one or both of the facing surfaces of the sheets to be welded. The inoculant may be in the form of a liquid or paste that is sprayed or brushed onto the sheet surface, or the inoculant may be incorporated into a carrier formed into a film or foil which is interposed between the workpiece surfaces before they are clamped and welded.

In an example, the location area of the two sheets in contact with each other in which a weld spot is to be formed is predetermined. A small hole can be formed at the center of the location area of at least one of the two sheets. The inoculant particles can be put into the hole. The two sheets are joined by passing electrical current through the sheets via a pair of electrodes. The particles comprise at least one material selected from the group consisting of: (a) Al+Ti,(b) Al+C.

The diameter of the hole can be 0.1-lmm. The depth of the hole can be 1/20-1/5 of the sheet. The particles can be in the form of powder. The two sheets can have the same or different depths.

The inoculant particles can be applied onto the surface of at least one of the two sheets at the location area. The materials can be applied in a form of a film onto the surface. The materials can be applied in a form of a paste onto the surface. The thickness of the film can be in the range of 0.01-0.1mm. The two sheets can have the same or different depths. A series of tests were performed to compare the properties of welds produced, namely welds produced with inoculation according to the method of the present invention and a weld produced without inoculation. These test results clearly show that the mechanical properties of the inoculated welds were superior to that which did not receive inoculants.

Table 1 Mechanical property of aluminum alloys sheets with or without the inoculation agents.

The results of these tests are represented in the table 1 which clearly show that the mechanical properties of inoculated welds according to the method of the present invention are markedly superior to the weld not having inoculation. Again, it can be seen that welds provided with inoculation in accordance with the present invention exhibited superior shear strength compared to the weld without inoculation.

From the foregoing, it may be appreciated that the weld nugget inoculation described above not only provides advantages over the prior welding methods, but does so in a particularly effective and economical manner. It is recognized, of course, that those skilled in the art may make various modifications or additions chosen to illustrate the invention without departing from the spirit or scope of the present contribution to the art. Accordingly, it is to be understood that the protection sought and to be afforded hereby should be deemed to extend to the subject matter claimed and all equivalents thereof fairly within the scope of the invention.

Claims

Spot welding of metal plates with inoculants

A method of spot welding two sheets formed of aluminum alloys, the method comprising the steps of: predetermining the location area of the two plates in contact with each other, in which a welding point is to be formed; forming a small hole at the center of the placement area of at least one of the two plates; introducing graft particles into the hole; spot welding the two plates by passing electrical current through the plates via a pair of electrodes, the particles comprising at least one material selected from the group consisting of: (a) Al + Ti, (b) Al + C.

The method of claim 1, wherein the diameter of the hole is 0.1-mm.

The method of claim 1, wherein the hole depth is 1 / 20-1 / 5 of the plate.

The method of claim 1, wherein the particles are in powder form.

The method of claim 1, wherein the two plates have the same or different depths.