RU2776711C2 - Contact welding of non-welded metals using intermediate layers applied by thermal spraying - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: present invention relates to a method for the lap connection of materials not directly welded to each other, in particular a sheet of stainless steel and a sheet of aluminum or aluminum alloy, or magnesium or magnesium alloy. At the first stage, a protective aluminum layer is applied to the surface of steel base (1) by thermal spraying. At the second stage, a method for contact welding is used for welding protective layer (2) to overlayed sheet (3), while the ratio of a thickness of thermally sprayed layer (2) to an achieved diameter of welding point (4) is less than or equal to 0.25, and a temperature in a contact area between base (1) and thermally sprayed coating layer is less than 350°C. Used welding parameters are selected in such a way that only a contact zone of aluminum with aluminum is melted, in the absence of fragile intermetallic phases between an aluminum layer and steel.

EFFECT: method provides for quality welding connection, while combining non-welded dissimilar metals.

10 cl, 4 dwg

Description

The present invention relates to the production of combinations of non-weldable metals such as aluminum with (stainless) steel, the resistance welding of which is achieved by thermally spraying a layer of aluminum onto the surface of (stainless) steel with a well-defined thickness, surface roughness and contact resistance. Then, at the second stage, the process of contact (spot) welding is carried out. The welding parameters used must be chosen in such a way that only the aluminum-to-aluminum contact zone is melted, in the absence of brittle intermetallic phases between the aluminum layer and the (stainless) steel.

Resistance welding is one of the most commonly used welding methods in the metalworking industry. Resistance welding can be performed, for example, by spot welding, roller welding, projection welding, or adhesive welding to weld large household appliances, fuel tanks, car bodies, railways, or trucks. But there are many materials, such as hot-formed martensitic stainless steels or combinations of two or three materials to be joined, that cannot be welded by resistance welding. For these materials, there are several ideas for creating mechanical solders or solder depots on the material. However, these solders or solder depots require deformation and/or cutting of the material in order to have a good bond or form a closure between the material and the mechanical solder or solder depot. When materials are further processed by conventional resistance welding processes, the spot weld typically results in a brittle fracture property with cold cracking and hence a low level of strength as well as low force transmission.

Throughout the automotive engineering industry, various metals such as (stainless) steels and aluminum are preferred for the car body in a multi-material structure having dissimilar material compounds. In the prior art, there is no method of thermal welding, only gluing and mechanical connection can be used. Conventional welding processes create weak intermetallic phases between aluminum and (stainless) steel. With the help of the invention it is possible to use cheap resistance spot welding with a short production cycle. In addition, corrosion (contact) resistance can be improved. With the invention it is possible to achieve a bond strength similar to aluminum joints for dissimilar combinations.

Patent application EP 2679328 A1 relates to a method of connecting the first object to be joined with the second object to be connected by means of resistance welding and permanent connection using the modern technological method of the two objects. The aim of the invention is to provide an alternative method for connecting the first object to be connected to the second object to be connected using an auxiliary connection and a welding method, in particular, to provide a method for adding various elements to be connected, in particular, various objects to be connected, of which at least one includes an oxide layer.

Patent application DE 102016106756 relates to thermal spraying for corrosion protection, for example thermal spraying of aluminium. In one embodiment, a corrosion resistant assembly is provided, including: a first metal element that is assembled with a second metal element, a connection between the first and second element, and an anti-corrosion layer covering at least a portion of the connection, where the anti-corrosion layer consists of aluminum and zinc.

Patent application DE 102014011599 relates to a method for connecting at least two elements, in which it is possible to ensure a reliable connection of two components simply in terms of technology and production. Weld points are formed in the center between the connecting auxiliary element and the inner side of the second component (steel). This eliminates the disadvantages of an eccentric material-closing connection, as well as poor lens formation, the tilt of the auxiliary connecting element, the formation of pores and voids, and isotropic strength properties. The diameter of the dome above the thickening of the material may be smaller than the diameter of the head of the element and have such a high height that only allowable clearance is obtained between the two components.

Patent application EP 1582283 relates to a process for spot welding two rigid steel members that are finally primed or painted in an electrophoretic bath, comprising placing small plates, preferably of an easily weldable metal such as low carbon iron, between the sheets at the weld points. The method provides spot welding of two sheet metal elements of high strength steel, which are then painted using an electrophoretic bath or they have already been painted.

Patent application DE 10 251 414 relates to a method for spot welding or soldering parts by means of resistive or induction heating, in particular sheet metal parts made from metallic materials, and also to a particular method for effecting the connection of an auxiliary element. The method of spot welding or soldering using resistive or induction heating includes determining the heat input by controlling the current strength and strength of the connection depending on the selected cross-sectional contour of the connecting auxiliary element inserted between the elements to be welded or soldered.

Patent applications DE 102004025493 and DE 102004025492 relate to a method for connecting components, a connecting element and a component for connecting components. Two or more connecting parts, in particular metal sheets and/or plastic sheets, are connected by means of connecting elements. When connected, the elements can be rivets or bolts. The components to be connected are provided with holes, in particular holes, into which the connecting elements can be inserted. The parts are aligned with each other and then connected by soldering the connecting elements. Preferably, a traditional welding gun can be used as conventional welding, but where a significantly lower heat input due to soldering is required compared to welding.

Patent application DE 102012013325 relates to a composite of three or more components that is easier to manufacture, in particular due to the simplified pre-fixation of two or more components to an incomplete composite before finally obtaining the composite component as a whole. The application also relates to a method for assembling components from various materials. An improved composite component of the first component, the second component and the third component is obtained by providing an auxiliary bonding agent, making a part of the composite from one of the two components with the connection aid, piercing the two components with the connection agent, and obtaining a movable or fixed connection between the two components and the connecting tool , and attaching the partial composite with the penetrating bonding aid to the remaining one of the components by pressure welding.

Patent application DE 102012013589 relates to a self-piercing fastener for welding a resistance element for connecting more than two components formed from a flat material. A self-piercing fastener of this type is suitable for connecting more than two components, at least one of these components may also be made of non-plastic deformable material.

Patent application WO 2010022709 relates to a method for producing a nodal joint, in which at least a sheet metal element made of steel, in particular hot-formed high-strength steel, is connected to at least one sheet metal component made of aluminum material, where the solder is first fixed with an arc or laser method to ensure safe transfer to one of the metal components, and then it is placed between the metal sheet members, and the metal components are heated by applying an electric current and applying a compressive force to the metal components, or heated by induction heating in the joint area so that between the sheet metal a brazed joint is formed between a metal member made of steel and solder, and a welded or brazed joint is formed between a metal sheet member made of aluminum material and the solder.

US 5,273,204 relates to a method that uses a butt joint configuration to thermally spray a weld into a v-butt weld. The thermally sprayed material is at the same time a weld.

US Patent Application 20100089977 describes a friction friction welding process in which a powder is applied in a loose layer between two metals to create a specific metallurgical composition and strengthen, for example, an aluminum alloy.

Currently, there is no welding method in the prior art for joining combinations of dissimilar metals such as aluminum and (stainless) steel, only bonding or mechanical joining (both with lower force transmission) works. In conventional (resistance) welding, the welding contact zone melts with the formation of brittle intermetallic phases FeAl ₃ and Fe ₂ Al ₅ in temperature ranges above 350°C.

In light of the increasing number of multi-material designs for the car body, the engineering industry is using aluminum and (stainless) steel at the same time. Resistance spot welding is the main connection method used for the manufacture of a car body.

The idea of the present invention is to create combinations of non-weldable metals, such as aluminum with (stainless) steel, or primarily non-weldable materials, such as hot-formed martensitic stainless steels, the weldability of which is ensured by thermal spraying of an aluminum layer on the surface of (stainless) steel with strictly certain thickness, surface roughness and contact resistance. Then, in the second stage, the contact (spot) welding process is carried out. The welding parameters used must be chosen such that only the aluminum-to-aluminum contact area melts, with no brittle intermetallic phases between the aluminum layer and the (stainless) steel.

The formula for phase growth can be formulated using a parabolic growth curve: X _m ² =k⋅t, where

x _m = average penetration depth = thickness of the intermetallic phase,

k = growth factor, literature value: 8.46⋅10-15 ^m2 /s at 873K,

t = welding time.

The parabolic growth curve formula gives information on the layer thickness for typical spot welds, which means a strength reduction of more than 50% compared to the aluminum base material.

The layer thickness characteristic of welded spots is greater than or equal to 15 µm. This corresponds to a strength reduction of more than 50% compared to the aluminum base material (280 N/mm ² ). As a result, the present invention should avoid any growth of the intermetallic phase during welding.

The thermomechanical coating method according to the present invention may consist of flame spraying, arc spraying, plasma spraying, laser spraying, cold gas spraying, detonation spraying, high speed oxyfuel spraying, or weld pool spraying.

In the spraying process, the material or consumable to be sprayed is supplied in the form of wire, rod, bar, rod, powder, or as a molten bath.

Materials not weldable directly to each other without a thermally sprayed layer and used in the method of the present invention may be, for example, steels such as non-alloyed, low-alloyed or stainless steels; aluminium, aluminum alloy; magnesium or an alloy based on magnesium.

The thermally sprayed layer is welded in accordance with the present invention to the overlay sheet by arc, laser, electron beam, stud or plasma welding, preferably by a resistance welding method such as resistance spot welding, adhesion welding, roller seam welding or projection welding. Naturally, other welding methods may be used in the welding process in accordance with the present invention.

The ratio of the thickness of the thermally sprayed layer to the achieved diameter of the weld point is lower/less than or equal to 0.25 mm.

The total thickness of the sheet, including the base, thermally sprayed layer, applied sheet, is from 1.5 mm to 6.0 mm, while the transition zone between the thermally sprayed layer and the base material is at a temperature below 350°C. One typical combination of thicknesses for thin parts of a seat structure in a transportation application might be:

T [mm] = t _base + t _{sprayed layer} + t _{overlay sheet} = 0.5 mM + 0.25 mM + 0.75 mM = 1.5 mM

Another example of design thickness can be obtained for crash-relevant structural parts of passenger cars, such as the B-pillar or battery case connection for an electric vehicle:

T [mm] = t _base + t _{sprayed layer} + t _{overlay sheet} = 1.7 mm + 1.5 mm + 2.8 mm = 6.0 mm,

where T is the total sheet thickness [mm] obtained by summing t _{of the base} + t of the _{sprayed layer} + t _{of the overlay sheet} , t _{of the base} is the sheet thickness [mm] of the base material, in the given example (stainless) steel. t of the _{sprayed layer} is the thickness [mm] of the thermally sprayed layer, in the above examples, obtained by the arc spraying method in which AlMg ₃ wire was used. At least t of the _{applied sheet} is the thickness of the sheet [mm] which is applied to the thermomechanical coating layer, which in these examples was EN AW-6081 aluminum sheet.

The thermal spray base may consist of a steel material, preferably unalloyed or low alloyed steel with a hot dip galvanized surface coating. More preferably it is stainless steel with a chromium oxide passivating layer. In addition, the materials used in the method of the present invention may be hot-formed or press-hardened steels with a flaked surface or a surface protection layer coated with aluminosilicon or zinc.

The method is particularly suitable for brittle materials such as high strength steels with a tensile strength of more than 800 MPa. Also, the basis for thermal spraying can be non-alloyed or low-alloyed steel having a carbon equivalent (CE) of more than 0.65%, where CE is calculated by the formula (element content in wt.%) CE=C+Mn/6+(Cu+Ni) /15+(Cr+Mo+V)/5.

Also, the surface of the substrate can be pre-treated in accordance with the present invention by cleaning or degreasing, as well as by shot-blasting or shot-peening the surface. In this case, the coarse grit abrasive for shot blasting or surface shot peening may also consist of corundum, steel wire, silicon carbide and hard cast gravel or glass beads.

The present invention shows the high energy transfer of these heterogeneous welds. The fracture behavior can be concentrated in the area of the superimposed sheet. Therefore, with the invention, a higher or equal energy transfer can be achieved with respect to a similar weld for one of the prior art materials compared to a combination of the same thickness.

The preferred resistance welding process used in the method of the present invention can be performed by various types of resistance welding, such as spot welding, roller seam welding, projection seam welding or adhesion welding.

In the following, the invention is described in more detail with reference to the drawings, where:

in FIG. 1 shows, as described in the prior art, a Fe/Al binary system of intermetallic phase growth between aluminum and steel;

in FIG. 2 shows, as described in the prior art, the relationship between the thickness of the intermetallic layer and the level of strength obtained;

in FIG. 3 shows a preferred embodiment of a thermally sprayed layer with a (stainless) steel backing according to the invention in side view and shows the first step of spraying an aluminum layer onto a base material/substrate;

in FIG. 4 shows another preferred embodiment of a thermally sprayed aluminum spot welded to a flat aluminum sheet according to the invention, schematically in side view showing a thermally sprayed layer/stainless steel welded to another flat aluminum sheet.

One example is the creation of a thermally sprayed layer of aluminum on the top of the surface of stainless steel, which uses the effect of diffusion, adhesion, mechanical adhesion of the layer, chemical bonding, where the layer on the stainless steel consists of an aluminum-based alloy, and this combination is welded by resistance welding with superimposed aluminum sheet to avoid intermetallic brittle phases.

On FIG. 1 presents the theoretical foundations of a binary system that works only for infinitely long cooling rates, and therefore, at a cooling rate of the welding point with a resistance of ≈1500 K/s, brittle intermetallic phases are formed: FeAl ₃ and Fe ₂ Al ₅ . The binary system * Fe/Al is presented with a temperature range.

*Source: Guimaraens, E., university of Bayreuth, 2005.

On FIG. 2 shows the theoretical background for phase growth, which is described using a parabolic growth curve. The relationship between the thickness of the intermetallic layer and the tensile strength is presented. As a result, the layer thickness for typical spot welds is greater than or equal to 15 µm. The curve shows a strength reduction of more than 50% compared to the aluminum base material (280 N/mm ² ). Because of this phenomenon, in the process of the present invention, it is necessary to avoid any growth of the intermetallic phase during welding.

On FIG. 3 is a schematic side view of a preferred embodiment of the invention. A thermomechanical or mechanical Al-alloyed surface coating in the form of a thermally sprayed intermediate layer (2) is placed on top of a base material (1) such as stainless steel, and these materials cannot be directly welded to each other.

On FIG. 4 is a schematic side view of a preferred embodiment of the invention in which a conventional resistance welding method is used between a thermally sprayed coating layer (2) on a substrate (1) and a superimposed aluminum sheet (3) to join the combination of materials. Between the base (1), the thermally sprayed intermediate layer (2) and the aluminum sheet (3), there is a spot welding zone (4). The temperature in the area of contact between the base (1) and the thermally sprayed coating layer is less than 350 degrees Celsius.

Claims (10)

1. A method of joining at least two materials that are not directly weldable to each other by means of thermal joining processes in a lap joint configuration, characterized in that a two-stage sequence is used, consisting of a first stage of applying a surface protective aluminum layer by means of thermal spraying onto a steel surface. base (1) and the second stage, in which a welding method is used to weld the protective layer (2) with a superimposed sheet (3) of aluminum, aluminum alloy, magnesium or magnesium alloy, in the absence of brittle intermetallic phases in the entire configuration of the material, while the ratio the thickness of the thermally sprayed layer (2) to the achieved diameter of the weld point (4) is less than or equal to 0.25 and the temperature in the contact area between the base (1) and the thermally sprayed coating layer is less than 350°C. 2. The method according to p. 1, characterized in that the application of the surface protective layer is carried out by thermomechanical coating. 3. Method according to any one of the preceding claims, characterized in that the thermally sprayed layer (2) is aluminium, an aluminum-based alloy or a combination of aluminum and carbides. 4. The method according to any of the preceding paragraphs. 1-3, characterized in that the thermally sprayed layer (2) is welded to the superimposed sheet (3) by a contact welding method, such as adhesion welding, spot welding, or projection welding, or roller seam welding. 5. The method according to any of the preceding paragraphs. 1-4, characterized in that the total thickness of the sheet, including the base (1), the thermally sprayed layer (2) and the superimposed sheet (3), is from 1.5 mm to 6.0 mm. 6. The method according to any of the preceding paragraphs. 1-5, characterized in that the thickness of the thermally sprayed layer (2) is from 0.25 to 1.5 mm. 7. The method according to any of the preceding paragraphs. 1-6, characterized in that the base (1) for thermal spraying is a carbon steel having a carbon equivalent (CE) of more than 0.65%, where the CE is calculated by the formula (element content in wt.%) CE = C + Mn /6 + (Cu + Ni)/15 + (Cr + Mo + V)/5. 8. The method according to any of the preceding paragraphs. 1-7, characterized in that the base (1) for thermal spraying is a carbon steel material, preferably carbon steel with a zinc surface coating. 9. The method according to any of the preceding paragraphs. 1-6, characterized in that the basis (1) for thermal spraying consists of stainless steel. 10. Method according to any one of the preceding claims, characterized in that the surface of the support (1) is pretreated by cleaning and shot blasting.

Images