Patent Description:
The technique described in the present invention allows dissimilar metal materials to be welded by means of the action of a laser beam.

Due to environmental problems, reduction of greenhouse gas (GHG) emissions has become a necessity. The Intergovernmental Panel on Climate Change (IPCC) has called for an urgent reduction of emissions to prevent irreversible environmental damage. In this sense, reducing vehicle weight is fundamental for reducing GHG emissions in the transportation industry, which is one of the main sources of said gases. Lightweight materials play an important role in weight reduction, and aluminum alloys stand out mainly as a result of their high strength-to-weight ratio and relatively low production costs.

Although aluminum production generates between <NUM> and <NUM> times more GHG than steel production, the use of aluminum generally results in a net reduction of GHG emissions throughout its life cycle. The use of vehicles with a high aluminum content compensates for increased GHG emissions in aluminum production, and this leads to a net reduction of GHG emissions.

A strategy to achieve weight reduction, which is receiving increasingly more attention, is the combination of materials with different properties. A significant vehicle weight reduction can thereby be achieved when several materials are used, which translates directly into a cost reduction. There is therefore a need to develop technologies for joining different or dissimilar materials so as to make vehicle design and manufacture based on the use of several materials viable.

One of the most interesting combinations of dissimilar materials is the combination of steels with aluminum alloys. This is due to the fact that when joining these two materials, properties such as the low weight, good formability, and corrosion resistance of aluminum can be combined with the hardness, mechanical strength, and ductility of steel.

Although there is a growing trend to replace mild steel with high resistance steels, mild steel is still largely used in the transportation industry. Mild steel is still extremely appealing for the transportation industry due to its low cost along with a good shock resistance behavior, such that most bumper components of a passenger vehicle are formed by aluminum and mild steel.

Moreover, the joining of these two materials is a very complicated task due to the significant differences between their properties. The difference between the melting points of two different materials to be joined causes defects in the welded joint. These defects include, among others, the appearance of cracks and porosity.

However, the main problem when welding materials having a different chemical composition, such as when welding aluminum with steel, relates to the formation of intermetallic compounds (IMC) in the interface between these materials, which can cause the embrittlement of the joint and the subsequent failure during service.

Some earlier inventions resort to the solution which uses the overlap welding or lap welding configuration. In that sense, document <CIT> describes a method for welding two dissimilar materials in an overlap welding configuration by means of a laser beam. Likewise, document <CIT> describes another method for welding dissimilar materials in an overlap configuration by means of applying a high-energy source and subjecting both materials to a high temperature simultaneously.

Patent <CIT> describes a method for welding dissimilar materials in which a bevel or groove having curved shapes is established between the two parts to be joined. This entails an additional step compared with the direct welding of sheets in a butt welding configuration.

One of the techniques used for welding dissimilar metal materials in a butt welding configuration is the simultaneous use of two laser beams. In this case, the ends of the parts to be welded must be machined into a V shape. The simultaneous use of two beams entails a not insignificant technical complexity, whereas the process of machining the parts to be welded beforehand entails an increase in time and costs that is not insignificant.

Another one of the techniques used in the attempt of welding dissimilar metal materials in a butt welding configuration is the simultaneous application of a strong magnetic field and a laser beam. The use of a strong magnetic field entails a practical limitation of the system as regards its industrial implementation and an increase in the cost of the equipment required for carrying out the welding.

To solve all these difficulties, the present invention proposes a simpler method that does not require the prior machining of the parts to be joined, does not require using two laser beams simultaneously, and does not require using an additional magnetic field as indicated in the mentioned state of the art.

In particular, the method of the present invention uses a single laser beam as the main tool for joining dissimilar metal materials in a butt welding configuration.

The present invention provides a method for the welded joining of dissimilar metal materials in a butt welding configuration according to claim <NUM>. The dependent claims define preferred embodiments of the invention.

In a first and only inventive aspect, there is provided a method for the welded joining of two parts made of dissimilar metal materials in a butt welding configuration by means of a laser beam, said method comprising the following steps:.

It shall be understood that two materials are dissimilar when their chemical composition is different.

It shall be understood that the welding configuration is a butt welding configuration when the two welded parts are facing one another edgewise, without the superimposition of any of the parts, being joined to one another by one of their ends. The auxiliary material applied in the first step of the described method is a material in powder form and having a composition different from the composition of the two parts to be joined. Advantageously, this allows providing an additional material having a different chemical composition to the joining area of the two parts, with this auxiliary material being welded together with a portion of the part made of a material having a lower melting point in order to form the welded joint between the two parts to be joined.

In one embodiment, the auxiliary material applied in powder form is made up of alloys containing Si, alloys containing Si and Al, alloys containing Si, Al, and Mg, and more preferably alloys AlSi5, AlSi10, AlSi12, AlSi10Mg, Al7Si0. <NUM>, Ni11Cr3.7Si2.75Fe2.2B0.5C, Al5Si, Al10Si0. <NUM>, and even more preferably alloys AlSi5, AlSi10, or AlSi12.

In an embodiment of the method of the present invention, the step of applying a laser beam on the part made of a material to be welded with a lower melting point, said laser beam is from a laser source the wavelength of which is within the range of <NUM> to <NUM>, preferably within the range of <NUM> to <NUM>.

In an embodiment of the method, the mean power of the laser source is within the range of <NUM> W to <NUM> W, and more preferably within the range of <NUM> W to <NUM> W. In one embodiment, the irradiance of the laser beam on the material having a lower melting point is in the range between <NUM> W/mm<NUM> and <NUM> W/mm<NUM>, and more preferably in the range between <NUM> W/mm<NUM> and <NUM> W/mm<NUM>.

Advantageously, an irradiance comprised in this range allows an optimal welded joining of the proposed parts.

In one embodiment, the diameter of the area irradiated by the laser beam is in the range between <NUM> and <NUM>, and more preferably the diameter of the area irradiated by the laser beam is in the range between <NUM> and <NUM>.

In one embodiment, the laser source used for generating the radiation of the laser beam is selected from Nd:YAG, Nd:glass, Nd:YVO<NUM>, Er:YAG, Yb:YAG, Tm:YAG, diode, fiber, disk, CO<NUM>, CO, HeCd, copper vapor laser, iodine laser, argon laser, krypton laser, or chemical lasers (HF, DF).

In one embodiment, the interaction time between the laser beam and the material having a lower melting point (<NUM>) is in the range between <NUM> and <NUM>, and more preferably in the range between <NUM> and <NUM>.

In one embodiment, the specific point energy of the laser beam is in the range between <NUM> J and <NUM> J, and the specific energy relative to the offset distance is in the range between <NUM> and <NUM> J/mm, and more preferably the specific point energy is in the range between <NUM> J and <NUM> J, and the specific energy relative to the offset distance is in the range between <NUM> and <NUM> J/mm.

In one embodiment, the specific point energy of the laser beam is in the range between <NUM> J and 500J.

In one embodiment, the laser beam is focused on the material having a lower melting point at an offset distance from the contact area with the material having a higher melting point. In one embodiment, the offset distance at which the laser beam is focused is comprised between <NUM> and <NUM>, and more preferably between <NUM> and <NUM>.

In one embodiment, the specific energy relative to the offset distance of the laser beam is in the range between <NUM> and <NUM> J/mm, and more preferably the specific point energy is in the range between <NUM> J and <NUM> J, and the specific energy relative to the offset distance of the laser beam is in the range between <NUM> and <NUM> J/mm.

In an embodiment, the two parts to be joined through a welding process are in the form of a flat sheet, with their edges perpendicular to the corresponding surfaces of the sheets, i.e., with straight welding edges.

In an embodiment of the method, the dissimilar materials to be welded are, on one hand Fe-based materials, and on the other, Al-based materials. Preferably, the material having a higher melting point is steel and the material having a lower melting point is an aluminum alloy.

In an embodiment, the two materials to be joined are placed on a base or support which allows the vertical position of the joint to be adjusted. In an embodiment, this base or support allows excess heat energy to be discharged into the welding process.

Optionally, the method of joining two parts by means of a welding process object of the present invention is carried out in vacuum or in the presence of an inert gas atmosphere. In one embodiment, this inert gas can be Ar, He, N<NUM>, or mixtures thereof.

Advantageously, this inert gas atmosphere provides protection during the welding process against the formation of oxides which may be disadvantageous in the attainment of a good welded joint. In one embodiment, the welding wire provided on the joining area of the two parts to be joined is made up of the material having a lower melting point. In one embodiment, the welding wire used can be ER1100, ER <NUM>, ER <NUM>, ER <NUM>, ER <NUM>, ER <NUM>, or ER <NUM>, and more preferably ER4047 or ER5356.

In one embodiment, the welding wire feeding speed is in the range between <NUM> and <NUM>/s, and more preferably in the range between <NUM> and <NUM>/s.

The method of the present invention allows the welded joining of dissimilar metal materials in a butt welding configuration to be obtained in a very simple and quick manner compared with other alternative methods in which there is need to perform prior machining of the edge of the parts to be welded. In the present method, another remarkable advantage is that a prior machining of the two parts made of dissimilar metal materials to be joined by means of welding is not required.

The method of the present invention does not require the application of a double laser beam or the simultaneous application of a laser beam and a magnetic field. The results are obtained with a relatively simple system based on the use of a single laser beam on the parts to be welded without requiring prior machining and with the provision of a welding wire and powder placed beforehand. Welded joints with high mechanical properties are thereby obtained.

To complement the description that is being made and for the purpose of helping to better understand the features of the invention, the following figures are attached as an integral part of said description.

The present invention relates to a method for the welded joining of two parts of dissimilar materials (<NUM>, <NUM>) in a butt welding configuration.

<FIG> shows the basic geometric configuration for the implementation of the method object of the present invention. In that sense, the material having a lower melting point or temperature (<NUM>) is located facing the material having a higher melting point or temperature (<NUM>). The two parts made of dissimilar metal materials (<NUM>, <NUM>) are therefore in direct contact in a butt welding configuration. The laser beam (<NUM>) from a laser source not shown in the figure is focused on the part made of a material having a lower melting point (<NUM>). When a relative movement occurs between this laser beam (<NUM>) and the set of two materials to be welded (<NUM>, <NUM>), the welded joint is obtained.

Before contacting the two parts (<NUM>, <NUM>), an auxiliary material in powder form must be applied right in the contact area (<NUM>) between the two materials to be welded (<NUM>, <NUM>). As shown in <FIG>, the ends of the two parts of the two materials to be welded (<NUM>, <NUM>) do not require prior machining and are welded in a butt welding configuration, with the faces of these two materials (<NUM>, <NUM>) to be welded being perfectly perpendicular to the surface of the material itself. In other words, preparation of the joint in the shape of a V, a rounded groove, or a chamfer, is not required.

The laser beam (<NUM>) moves over one of the parts to be joined along a longitudinal direction (<NUM>).

<FIG> shows a detail of the basic geometric configuration for the implementation of the method object of the present invention. The laser beam (<NUM>) is focused on the material having a lower melting point (<NUM>) at an offset distance (<NUM>) from the contact area (<NUM>) with the material having a higher melting point (<NUM>). The laser beam (<NUM>) must be focused such that the diameter of the irradiated area (<NUM>) is such that the irradiance reached on the material having a lower melting point (<NUM>) allows melting this material (<NUM>), forming the welding bead (<NUM>) without the material having a higher melting point (<NUM>) melting. In addition to the laser beam (<NUM>), a welding wire, not depicted in the figure, suited to the composition of the two materials (<NUM>, <NUM>) to be welded must be provided in the irradiated area (<NUM>).

<FIG> shows a detailed cross-section view of the joining area between the two materials to be welded (<NUM>, <NUM>). It can be observed that the material having a higher melting point (<NUM>) has not melted. In contrast, the material having a lower melting point (<NUM>) has melted completely, overlapping the material having a higher melting point (<NUM>) in the upper and lower parts. The powder auxiliary material and the welding wire have also melted and mixed with the material having a lower melting point (<NUM>).

<FIG> shows, at higher amplifications than in <FIG>, a detail of the cross-section of the joining area between the two materials to be welded (<NUM>, <NUM>). In this case, the interface (<NUM>) between both materials can be seen. It can be seen how this interface is very thin, barely <NUM> micrometers thick. This thin interface (<NUM>) between the two materials to be welded (<NUM>, <NUM>) ensures a good metallurgical joint and good mechanical properties of the joint.

<FIG> shows in greater detail the interface (<NUM>) between two materials (<NUM>, <NUM>) welded by means of the method object of the present invention. In this case, the interface consists of mild steel S235 (which is the material having a higher melting point (<NUM>) of the two) and aluminum alloy AA6061 (which is the material having a lower melting point (<NUM>) of the two materials to be joined). The image in the center shows a micrograph obtained by means of an electron scanning microscope. The different grains forming the interface (<NUM>) between the two materials to be joined (<NUM>, <NUM>) can be seen. The three remaining images depict the corresponding electron diffraction patterns corresponding to the three intermetallic compounds formed when welding steel S235 and the aluminum alloy. These intermetallic compounds are Fe2Al5, Fe4Al13, and Fe4Al17.5Si1. This Fe-, Al-, and Si-based ternary compound is responsible for the good mechanical properties of the welded joint (<NUM>) produced by means of the method object of the present invention.

In a preferred embodiment, the laser beam (<NUM>) is from a laser source having a wavelength within the range of <NUM> to <NUM>, an irradiance of between <NUM> W/mm<NUM> and <NUM> W/mm<NUM>, the diameter of the area irradiated (<NUM>) by the laser beam (<NUM>) of between <NUM> and <NUM>, the interaction time between the laser beam (<NUM>) and the material (<NUM>) of between <NUM> and <NUM>, the specific point energy of between <NUM> J and <NUM> J, the specific energy relative to the offset distance (<NUM>) of between <NUM> and <NUM> J/mm, and the welding wire feeding speed of between <NUM> and <NUM>/s. And more preferably, the laser beam (<NUM>) is from a laser source having a wavelength within the range of <NUM> to <NUM>, an irradiance of between <NUM> W/mm<NUM> and <NUM> W/mm<NUM>, the diameter of the area irradiated (<NUM>) by the laser beam (<NUM>) of between <NUM> and <NUM>, the interaction time between the laser beam (<NUM>) and the material (<NUM>) of between <NUM> and <NUM>, the specific point energy of between <NUM> J and <NUM> J, the specific energy relative to the offset distance (<NUM>) of between <NUM> and <NUM> J/mm, and the welding wire feeding speed of between <NUM> and <NUM>/s.

In a particular embodiment of the method of the invention, the laser beam (<NUM>) is generated in a laser source selected from Nd:YAG, Nd:glass, Nd:YVO<NUM>, Er:YAG, Yb:YAG, Tm:YAG, diode, fiber, disk, CO<NUM>, CO, HeCd, copper vapor laser, iodine laser, argon laser, krypton laser, or chemical lasers (HF, DF).

In a particular embodiment, the offset distance (<NUM>) at which the laser beam (<NUM>) is focused is comprised between <NUM> and <NUM>, and more preferably between <NUM> and <NUM>.

In a particular embodiment, the welding wire used can be ER1100, ER <NUM>, ER <NUM>, ER <NUM>, ER <NUM>, ER <NUM>, or ER <NUM>, and more preferably ER4047 or ER5356.

In a particular embodiment, the powder auxiliary material can be made of alloys containing Si, alloys containing Si and Al, alloys containing Si, Al, and Mg, and more preferably alloys AlSi5, AlSi10, AlSi12, AlSi10Mg, Al7Si0. <NUM>, Ni11Cr3.7Si2.75Fe2.2B0.5C, Al5Si, Al10Si0. <NUM>, and even more preferably alloys AlSi5, AlSi10, or AlSi12.

A fiber laser beam (<NUM>) (<NUM>) having a mean power of <NUM> W, operating at a working power of <NUM> W, an irradiance of <NUM> W/mm<NUM>, with the diameter of the area irradiated (<NUM>) by the laser beam (<NUM>) of <NUM>, the interaction time between the laser beam (<NUM>) and the material of <NUM>, and the offset distance (<NUM>) of <NUM>, was used. Under these conditions, a S235 mild steel sheet was welded in a butt welding configuration with another AA6061 aluminum alloy sheet without requiring the prior machining of the joining area (<NUM>) of the two parts to be welded (<NUM>, <NUM>). To that end, an auxiliary material in AlSi12 powder form was used in combination with a welding wire having an AlSi5 composition fed at a speed of <NUM>/s.

With the method object of the invention and under the described conditions, an excellent metallurgical joint, as shown in <FIG>, was obtained. The formation of a layer of binary FeAl intermetallic compounds, as well as the formation of an intermetallic Fe4Al17.5Si1. <NUM> compound can be observed. This Fe-, Al-, and Si-based ternary compound is responsible for the good mechanical properties of the welded joint (<NUM>) produced by means of the method object of the present invention. In a tensile test, <NUM> MPa of maximum breaking stress and <NUM>% elongation stress were obtained. This maximum breaking stress value is higher than the minimum threshold established by the National Aeronautics and Space Administration (NASA) of the United States of America to validate welded joints made of similar AA6061-T6 alloy materials. This demonstrates the merit of the method object of the present invention.

Claim 1:
A method for welding two parts made of dissimilar metal materials (<NUM>, <NUM>) in a butt welding configuration by means of a laser beam (<NUM>), wherein one metal material (<NUM>) has a lower melting point than the other metal material (<NUM>), characterized in that it comprises the following steps:
- before contacting the two parts applying an auxiliary material in powder form on the edge of one or of both parts made of dissimilar metal materials (<NUM>, <NUM>) to be joined, the auxiliary material being applied right in the contact area (<NUM>) between the two dissimilar metal materials (<NUM>, <NUM>) to be welded,
- placing the two parts made of dissimilar metal materials (<NUM>, <NUM>) to be joined facing one another in a butt welding configuration, establishing the contact area (<NUM>) between the two parts made of dissimilar metal materials (<NUM>, <NUM>),
- applying a laser beam (<NUM>) on the part made of a material having a lower melting point (<NUM>), the laser beam (<NUM>) being focused such that the diameter of the irradiated area (<NUM>) is such that the irradiance reached on the material having a lower melting point (<NUM>) allows melting this material (<NUM>) without the material having a higher melting point (<NUM>) melting,
- providing a welding wire in the irradiated area (<NUM>), simultaneously with the interaction of the laser beam (<NUM>) with the material having a lower melting point (<NUM>), wherein the auxiliary material and the welding wire also melt and mix with the material having a lower melting point (<NUM>).