Patent Application
20130105446
The invention relates to a process for the laser/arc hybrid welding of steel parts comprising a surface coating based on aluminum, in particular a coating of aluminum and silicon, using a shielding gas formed of argon and/or helium to which small proportions of nitrogen or oxygen are added.
Certain steels, referred to as aluminized steels since they are coated with aluminum or with an alloy based on aluminum, such as USIBOR™ steels, have very high mechanical characteristics after hot-drawing and are, therefore, increasingly used in the field of the construction of motor vehicles, when a weight saving is desired.
Indeed, these steels are designed to be thermally treated then quenched during the hot-drawing operation and the mechanical characteristics which result therefrom enable a very significant lightening of the weight of the vehicle compared to a standard high yield strength steel. They are mainly used for manufacturing bumper beams, door reinforcements, center pillars, window pillars, etc.
Document EP-A-1878531 proposes to weld aluminized steels of this type by use of a laser/arc hybrid welding process. The laser/arc hybrid welding principle is well known in the prior art.
However, it has been observed in practice that after an operation for the hybrid welding, with a shielding atmosphere formed of an He/Ar mixture, of steel parts coated with aluminum or with an aluminum alloy, in particular an alloy of Al/Si type, and post-welding heat treatment comprising hot-drawing at 920° C. then a quenching in the tool (30° C./s), a phase with lower tensile strength than the base metal and than the weld metal zone often appeared in the welded joint.
However, this phase with lower tensile strength constitutes a brittle zone of the weld thus obtained, as explained below. These more brittle zones appear within the martensite zone in the form of islands of white phase containing aggregates of aluminum originating from the surface layer.
After analysis, it was determined that this phase contains a significant percentage of aluminum (>2%) which gives rise to no austenitic transformation of the steel during the heat treatment thereof before drawing, that is to say that this phase remains in the form of delta ferrite and the result thereof is a lower hardness than the rest of the part that has undergone a martensitic/bainitic transformation.
However, the phase not transformed into martensite phase may result, during a mechanical characterization of the joint after welding, and drawing followed by heat treatment, in cracks, or even in a rupture by shearing of the welded joint, since these zones that have incorporated aluminum have a lower resistance of the weld than that of the deposited metal.
The problem that is faced is hence to propose an arc/laser hybrid welding process that improves the mechanical properties of the welded joint, during an operation for welding steel parts coated with a layer comprising aluminum. More specifically, the problem is to be able to obtain a homogeneous microstructure of martensite type in the weld metal zone, that is to say in the weld joint, after hot-drawing, typically at around 920° C., and quenching in the drawing tool, typically with a cooling rate between 800° C. and 500° C. of the order of 30° C./s.
The solution of the invention is a laser/arc hybrid welding process using an electric arc and a laser beam that are combined together, in particular within a single weld pool, wherein the weld metal is provided by melting a consumable wire and the weld pool is produced on at least one steel part comprising an aluminum-based surface coating, and wherein use is furthermore made of a shielding gas, characterized in that the shielding gas consists of at least one main compound chosen from argon and helium, and of at least one additional compound chosen from nitrogen and oxygen.
According to the invention, the weld pool, and therefore subsequently the welding joint, is therefore obtained, at the joint plane formed by the bringing into contact, in particular end to end, of the parts to be welded, by melting the constituent steel of the parts under the simultaneous action of a laser beam and of an electric arc which combine with one another to melt the metal of the part or parts to be welded, while an additional provision of weld metal is obtained furthermore by virtue of a consumable wire which is also melted, preferably by the electric arc, the weld metal thus obtained being deposited in the weld pool formed on the part or parts to be assembled.
In order to solve the aforementioned problem, according to the invention, use is made, as a shielding atmosphere of the welding zone, in particular of the weld pool, of a gas mixture which is formed solely, on the one hand, of argon, helium or the two as main compound(s) of the gas mixture and, on the other hand, of nitrogen or oxygen, or even of the two, as additional compound(s), so as to constitute a binary gas mixture of Ar/N2, Ar/O2, He/O2 or He/N2 type, or a ternary gas mixture of Ar/He/N2 or Ar/He/O2 type, or even a quaternary gas mixture of Ar/He/O2/N2 type. In all the cases, the proportion of main compound (i.e. Ar or He) or the sum of the proportions of the main compounds (i.e. Ar and He) is greater than the proportion of additional compound (i.e. N2 or O2) or the sum of the proportions of the additional compounds (i.e. N2 and O2) present in the gas mixture.
Among these various gas mixtures that can be used, two gas mixtures are particularly preferred as they lead to very good results, as explained below, namely the Ar/N2 or Ar/He/N2 mixtures containing at most 10% of nitrogen (% by volume) and advantageously from 3% to 7% approximately of nitrogen. Generally, it should be noted that within the context of the present invention, unless otherwise indicated, all the percentages (%) given are percentages by volume (% by volume).
Indeed, the implementation of an arc/laser hybrid welding process using a shielding gas mixture formed of argon and/of of helium, on the one hand, and of nitrogen and/or oxygen, on the other hand, makes it possible to obtain, during the assembling of aluminized steel parts, a weld joint of martensitic microstructure free or virtually free of whitish ferrite islands, since the addition of O2 or of N2 makes it possible to trap the aluminum originating from the surface layer and which is released during the melting of said layer under the effect of the arc and of the laser beam.
The trapping of the aluminum by the O2 or N2 compounds leads to the formation of compounds of Al2O3 or AlN type thus avoiding the formation of ferrite or of other harmful intermetallic compounds. In fact, the aluminum oxides or nitrides thus formed float at the surface of the pool, thus preventing the dissolution of the aluminum in the weld pool.
The result of this is a suppression or at least a sizable reduction of the incorporation of aluminum into the weld, therefore an improvement of the tensile strength due to a total or virtually total disappearance of the whitish delta ferrite phase customarily observed.
Depending on the case, the process of the invention may comprise one or more of the following characteristics:
The invention will now be better understood owing to the following description and examples carried out to show the effectiveness of the arc/laser hybrid welding process of the invention.
The laser/arc hybrid welding process according to the invention gave good results during the implementation thereof for carrying out a hybrid welding, using a laser source of CO2 type and a MIG arc welding torch, of steel parts coated with a layer of around 30 μm of an aluminum/silicon alloy in respective proportions of 90% and 10% by weight.
The welded parts have a thickness of 2.3 mm.
Within the context of the tests carried out, the gas used, which is dispensed at a flow rate of 25 l/min and at a pressure of 4 bar, is:
The ARCAL 37 mixture is sold by Air Liquide.
The torch used is a MIG torch of reference OTC fed by a filler wire of Nic 535 type (0.7% C and 2% Mn) having a diameter of 1.2 mm, which is delivered at a rate of 3 m/min.
The welding voltage is 15 V approximately and the intensity is approximately 139 A, which are obtained by virtue of a generator of Digi@wave 500 type (short arc/short arc +) in synergic mode (EN 131) sold by Air Liquide Welding France.
The laser source is a CO2 laser oscillator having a power of 12 kW.
The welding speed achieved is 4 m/min.
The parts to be welded are square butt-positioned tailored blanks made of aluminized steel (Al/Si) of Usibor 1500™ type.
The results obtained show that the presence of N2 in an argon/helium mixture leads to much better results than the tests without nitrogen in the shielding gas.
Similarly, the presence of a small proportion of O2 in an argon/helium mixture makes it possible to counter the effect of suppressing austenitic transformation caused by the presence of aluminum in the weld metal zone.
Indeed, by using the Ar and/or He and N2 and/or O2 mixtures according to the invention, a significant improvement in the results is therefore observed, which improvement increases proportionally to the content of N2 or O2 in the mixture. Indeed, the micrographs show that, in both cases, the white phases have completely disappeared, whereas that is not the case with the ARCAL 37 mixture alone.
Moreover, with the additions of O2 or N2, the resistance to rupture of the joint, after austenitization and quenching, is equivalent to that of the base metal.
The results obtained during the tests show that an addition of nitrogen to argon and/or helium makes it possible to greatly improve the quality of the welding of steels coated with a surface layer of aluminum/silicon alloy, in particular a homogeneous microstructure of martensite type in the weld metal zone.
The improvement is even more significant when the nitrogen content increases but with an optimum of less than 10% by volume, which would encourage the use of around 6% to 7% of nitrogen in argon or in argon/helium.
The improvement is also even more significant when the oxygen content increases, but with an optimum of less than 10% by volume, which would encourage the use of around 3% to 5% of nitrogen in argon or in argon/helium.
The process of the invention is particularly suitable for the welding of tailored blanks used in the field of motor vehicle construction, of components of mufflers, in particular for vehicles, and for the welding of pipes.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1055691 | Jul 2010 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR2011/051015 | 5/5/2011 | WO | 00 | 1/10/2013 |
