Background of the invention

The object of the invention is a method for joining coated metal sheets by welding, as defined in the preamble of claim 1. The object of the invention is also a joint as defined in the preamble of claim 9. Prior art

Various solutions for joining coated metal sheets to each other are known in the art. For example, solutions are known in which sheets are joined to each other using an auxiliary sheet, which auxiliary sheet connects the sheets to be joined, and in which joint an adhesive substance is used for joining the sheets to each other.

A solution is known from publication EP 0538087A in which coated metal sheets are joined to each other with laser welding in two phases by welding, and with either a square butt joint or with a lap joint. In the method the first welding phase disintegrates the coating with a focused laser beam, which surface is repaired in the second phase with an unfocused laser beam. Prior-art solutions have certain drawbacks.

One problem, among others, in prior-art solutions is that when glueing is used it is complex and awkward, and requires a long drying time, and is therefore not a cost-effective method for joining. Correspondingly, when prior-art solutions in which sheets are welded together are used, the coatings of the sheets are damaged, and this causes the joint area to be susceptible to corrosion, such as crevice corrosion. Furthermore, welded joints according to prior art do not enable a structurally strong joint.

The purpose of the current invention is to achieve a new type of solution for joining coated metal sheets to each other, with which solution it is possible to avoid the problems occurring in prior-art solutions. Brief description of the invention

The method according to the invention is characterized by what is disclosed in the characterization part of claim 1. The method according to the invention is also characterized by what is disclosed in claims 2-8. The joint according to the invention is characterized by what is disclosed in the characterization part of claim 9. The joint according to the invention is also characterized by what is disclosed in claims 10-20. The solution according to the invention now being presented has some significant advantages when it is compared to prior-art solutions.

A new type of solution is achieved with the invention now presented for joining coated metal sheets, and with which crevice corrosion in the joint between the sheets can be effectively prevented. With the solution according to the invention, it is also possible to make the type of joints that previously have not been possible at all, or that have been awkward to make. By means of the solution according to the invention sheets, which are pre-coated, can be used more extensively and in more applications than in prior-art solutions, and especially work phases that are high in costs and are time-consuming (such as painting/coating the seam between the sheets) can be replaced with a cheaper and faster method. The method according to the invention does not break the protective coating on the exterior face of the coated metal sheet, in which case the corrosion resistance of the joint is excellent. The solution according to the invention enables the use of hot-dip galvanized sheets, in which case effective corrosion resistance of the joint is achieved. The joint achieved with the method according to the invention is also strong and durable. In the solution according to the invention the joint can be loaded and used immediately after its making.

Brief description of the figures

In the following, the invention will be described in more detail by the aid some embodiments with reference to the drawings la -lc, wherein

Fig. la and also Figs, lb and lc present a joint according to the invention and a method for achieving the joint. Detailed description of the invention

According to Figs, la and lb and also lc, in the joint and in the method according to the invention for joining coated metal sheets to each other by welding, the sheets to be joined are connected by laser welding in such a way that a joint 1 is achieved between the metal sheets with welding, which joint comprises at least one such section L (marked in the figures additionally also as sections LI and L2) in which the sheets to be connected 2 and 4 and also 3 and 4 overlap. In the method an additive 5 has been formed or is formed between the coated metal sheets to be joined by welding, for protecting the edges to be joined of the metal sheets from corrosion, which additive is activated for sealing the joint. Spots in which the coating is not quite on the edge of the sheet normally occur on the edges of coated metal sheets (when cutting them), and the purpose of the additive is to protect the edge of the sheet, where there is no coating, or where the coating has been damaged, from corrosion.

Activation of the additive can occur in different ways. Activation can occur e.g. either using UV light, or the additive can be activated by means of pressure, which pressure occurs when the sheets to be joined are connected by laser, in which case pressure (compression) is exerted on the additive when the sheets are pressed against each other, e.g. in a jig. The additive can also be activated by means of heat such that e.g. a laser is used to produce heat in the additive. When activation occurs, the additive can also swell/expand, and this can assist protection from crevice corrosion, because when it expands the additive densely covers the edges of the sheet/sheets, and those spots of the edges in which the coating is possibly damaged. The additive can also reinforce the joint between the sheets, when it increases the strength and durability of the joint.

The additive can be pre-fixed to one of the sheets, but the additive can be affixed/brought to the sheet e.g. only after welding, after which the additive can be activated/hardened e.g. with UV light containing certain desired wavelengths. If UV light is used in the activation, the wavelength of the light can be between 250-450 nm. The output power of the UV hardening apparatus can be e.g. approx. 50W. Correspondingly the output power of the laser to be used is between 0.5 kW - 5.0 kW, and the laser can be e.g. a spot laser. The output power of the laser can be varied or regularly pulsed during the formation/activation of the joint. The operating parameters of the laser are selected such that the coating of the first side, i .e. of the exterior face, is not damaged when welding from the other side, i .e. from the inside face. If necessary, a shielding gas, such as Argon, is used.

In the method according to the invention the welded seams between sheets are formed in essentially the longitudinal direction or in the transverse direction of the sheet, but the type of solution in which a welded seam travels at an angle as viewed from the surface of the sheet is also conceivable. Furthermore, there can be one or more welded seams in each sheet, always depending on need. In the method according to the invention welding is performed from the second side of the joint, i.e. from the inside. In this case the coating of actual exterior face, i.e. the coating of the first side, is not damaged in the welding. According to one embodiment of the invention welding is used together with glueing, in which case a joint to be treated is rapidly achieved with welding, but the strength of the final joint is ensured together with a glued joint. In this case glue is spread between the metal sheets to be overlapped preferably before welding.

The joint according to the invention is described below in more detail with reference to the drawings.

Figs, la-lc present a joint 1, with which joint at least two coated metal sheets 2,3 are joined by welding with laser welding in such a way that a joint is achieved between the metal sheets, which joint comprises at least one such section L, in which the sheets to be connected 2 and 4 and also 3 and 4 overlap. An additive is formed between the coated metal sheets 2,3 for protecting the edges to be joined of the metal sheets from corrosion, which additive is activated for sealing the joint. A depression or groove (not shown in figure) can be formed in the sheet that functions as an auxiliary sheet 4. The additive is a material activated by means of pressure and/or heat, or a material activated with UV light. The additive is preferably a polymer-based additive suitable for sealing, such as acrylic. The additive can also be tape-like, or the additive is affixed to tape, such as to shaped tape. The additive can also be a polyurethane-based additive. The edges of the coated metal sheets 2,3 are not quite touching each other, in which case there is a gap between them for the additive. Deviating from the figures, the coated sheets 2,3 can also be attached to the auxiliary sheet 4 without the gap that appears in the figure between them. Coated metal sheets 2,3 refers to a metallic sheet, onto the surface of which a protective coating is formed.

Preferably the coated metal sheet is a hot-dip galvanized steel sheet. Most preferably the coated steel sheet is coated after hot-dip galvanizing with plastic paint, on at least the first side, i.e. on the exterior face. The second side, i.e. the interior face, of the coated metal sheet can also be coated with a backing coat.

An uncoated metal sheet can also be used as an auxiliary sheet 4, because a joint according to the invention achieves tightness, in which case moisture does not easily pass from the first side to the second side of the sheet. According to one embodiment a hot-dip galvanized sheet is used as an auxiliary sheet. Preferably the thickness of the coated metal sheet and/or auxiliary sheet is 0.4 - 5 mm, most preferably 0.4 - 3 mm.

Preferably the coated metal sheets 2,3 settle in the joint to essentially the same level, in which case there is no difference in level between them. Thus owing to the joint according to the invention the first side, i.e. the exterior face, formed from coated metal sheets joined to each other forms a seamless entity. Preferably the welded seams are parallel with the seam between the coated sheets 2,3.

Fig. lc presents one embodiment of a solution according to the invention wherein masking tape 7 has been placed on top of the joint, the purpose of which masking tape is to protect the additive and the joint during the joining and during storage. The masking tape is removed, e.g. when the additive is activated. A solution can also be conceived wherein an additive is affixed to the masking tape itself.

Table 1 below describes various additives, which can be used in a joint according to the invention.

Table 1. Additives used in different joints.

The additive can be colored to be the color of the coated sheet, in which case the joint is e.g. of even color when viewed from above. Alternatively the additive can be formed to essentially differ from the color of the coated sheet, if it is desired to accentuate the seams.

The additive can be pre-fixed to the edge of either or both of the coated metal sheets, or to the auxiliary sheet to be used in the joint. The metal sheets to be used in a joint can also be e.g. painted or galvanized/zinc- coated, e.g. hot-dip galvanized sheets.

It is obvious to the person skilled in the art that the different embodiments of the invention are not limited solely to the examples described above, and that they may therefore be varied within the scope of the claims presented below.

In the solution according to the invention the additive to be used can also be gel-like, liquid or solid, which when activating seals the gap between the sheets to be joined.