Patent ID: 12208461

DETAILED DESCRIPTION

The designation press hardened steel part means a hot-formed or hot-stamped steel sheet having a tensile strength up to 2500 MPa and more preferably up to 2000 MPa. For example, the tensile strength is above or equal to 500 MPa, advantageously above or equal to 1200 MPa, preferably above or equal 1500 MPa.

The invention relates to an assembly of at least two metallic substrates spot welded together through at least one spot welded joint, said assembly comprising:a first metallic substrate being a hardened steel part coated with:an alloyed coating comprising from 0.1 to 11.0% by weight of zinc, from 0.1 to 20% by weight silicon, optionally 0.1 to 20% by weight of magnesium, optionally additional elements chosen from Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Zr or Bi, the content by weight of each additional element being inferior to 0.3% by weight and optionally residuals elements from feeding ingots or from the passage of the steel substrate in the molten bath including iron, the balance being aluminum, directly topped byA native oxide layer comprising ZnO and optionally MgO,said spot welded joint comprising a nugget; and said spot welded joint being such that on its top, at least a part of the native oxide layer and/or alloyed coating is not present.

Without willing to be bound by any theory, it seems that when the assembly comprises the above specific coating on the hardened part comprising among others 0.1 to 11.0% by weight of zinc, the welding range is equal or above to 1 kA. Indeed, it seems that ZnO and optionally MgO are naturally present on the surface of the hardened steel part due to the oxidation of the hardened steel with air. It is believed that the thickness of the native oxide layer comprising ZnO and optionally MgO is more important when the zinc content is outside the scope of the present invention, i.e. above 11.0% by weight, leading to a poor welding quality. Preferably, the alloyed coating of the hardened steel part comprises from 3.0 to 9.5% and more preferably from 6.5 to 9.5% by weight of zinc. Indeed, without willing to be bound by any theory, it is believed that when the coating comprises these amounts of zinc, the scope of the welding range is further improved.

Preferably, the alloyed coating of the hardened steel part comprises from 0.1 to 12.0%, more preferably between 0.1 and 6.0% and advantageously between 2.0 and 6.0% by weight of silicon.

Advantageously, the alloyed coating of the hardened steel part comprises from 0.1 to 10.0%, preferably from 0.1 to 4.0% by weight of magnesium.

Optionally, the coating comprises up to 5% by weight of iron.

In a preferred embodiment, the second metallic substrate is a steel substrate or an aluminum substrate. Preferably, the second steel substrate is a hardened steel part according to the present invention.

In another preferred embodiment, the assembly comprises a third metallic substrate sheet101(shown schematically inFIG.1) being a steel substrate or an aluminum substrate. In this case, two or several spot-welded joints are present.

The invention also relates to a welding method for the manufacture of the assembly according to the present invention, comprising the following steps:A. The provision of at least two metallic substrates wherein a first metallic substrate is a hardened steel part coated with:an alloyed coating comprising from 0.1 to 11.0% by weight of zinc, from 0.1 to 20% by weight silicon, optionally 0.1 to 20% by weight of magnesium, optionally additional elements chosen from Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Zr or Bi, the content by weight of each additional element being inferior to 0.3% by weight and optionally residuals elements from feeding ingots or from the passage of the steel sheet in the molten bath, the balance being aluminum, directly topped byA native oxide layer comprising ZnO and optionally MgO,B. The application of a spot welding cycle with a spot welding machine, comprising welding electrodes and a spot welding power source applying an inverter direct current, through the at least two metallic substrates of step A), said spot welding cycle comprising the following sub-steps:i. at least one pulsation having a pulsation current (Cp) applied through said at least two metallic substrates joined together using welding electrodes connected to the spot welding power source and directly after,ii. a welding step having a welding current (Cw) applied through the at least two metallic substrates andwherein the current Cp is different from the current Cw and wherein the pulsation duration is below the welding duration.

Without willing to be bound by any theory, it seems that the welding method according to the present invention performed on two metallic substrates comprising at least a hardened steel part coated with the specific coating comprising from 0.1 to 11.0% by weight of zinc allows for a welding range equal or above 1 kA and a decrease of splashing on the assembly surface. Indeed, it is believed that the at least one pulsation breaks the ZnO and optionally MgO barrier layer present on the coated hardened steel part opening a path to the welding current. However, if the zinc content is outside the scope of the present invention, it is believed that the ZnO and optionally MgO barrier layer is too thick to be broken by the at least one pulsation.

As illustrated inFIG.1, a spot welding machine100, comprising welding electrodes1,1′ and a spot welding source2, is used. In this Example, the electrodes permit to join two hardened steel parts3,3′ coated with the coating according to the invention4,4′, and4″,4′″ respectively on top of which coatings forms native oxide layers7,7′,7″ and7′″ respectively. During the welding, a nugget5is formed between the two hardened steel parts through diffusion. The nugget is an alloy of the residual coatings and the steel parts. Thanks to the spot welding cycle according to the present invention, it is believed that at least a part of the coatings4,4′,4″,4′″ are removed in the nugget. Moreover, on the top of the spot welded joint6,6′, it is believed that at least a part of the native oxide layers7,7′,7″,7′″ and/or alloyed coating is not present. Indeed, it seems that the at least one pulsation breaks the native oxide layer and starts the welding between the coated two hardened steel parts by melting and removing the coatings on top of the spot welded joint and in the nugget. Thus, the current can flow through the two hardened steel parts allowing an improvement of the welding. Finally, it is believed that no cooling is needed between the at least one pulsation and the welding step. Indeed, if a cooling is performed between these steps, there is a risk to stop the formation of the nugget between the two hardened steel parts because the steel parts start to solidify. On the contrary when no cooling is performed, it seems that the steel parts stay in liquid form and can easily be joined together.

Preferably, in step B.i), the pulsation current (Cp) is between 0.1 and 30 kA, preferably between 0.1 and 20 kA, more preferably between 8.0 and 20 kA and advantageously between 8.0 and 15 kA.

Advantageously, in step B.i), the pulsation duration is from 5 to 60 ms, preferably from 4 to 30 ms.

Preferably, in step B.ii), the welding current (Cw) is between 0.1 and 30 kA, preferably between 0.1 and 20 kA, more preferably between 0.1 and 10 and advantageously between 1 and 7.5 kA.

Advantageously, in step B.ii), the welding duration is from 150 to 500 ms and more preferably from 250 to 400 ms.

In a preferred embodiment, the current Cp is below the current Cw.

In another preferred embodiment, the current Cp is above the current Cw. Indeed, without willing to be bound by any theory, the inventors have found that when Cp is above Cw, the welding range is further improved.

Preferably, the welding force is between 50 and 550 daN.

In a preferred embodiment, the welding force during the spot welding cycle is between 350 daN and 550 daN.

In another preferred embodiment, the welding force during the spot welding cycle is between 50 daN and 350 daN. In this case, it seems that there is a better localization of current at the electrodes centers allowing a better weldability.

Preferably, the welding frequency is between 500 and 5000 Hz, more preferably 500 and 3000 Hz and for example between 800 and 1200 Hz.

Preferably, the welding step B.ii) comprises a plurality of pulses, the at least one pulsation B.i being directly followed by the first pulse of the welding step. In this case, there is no cooling between the pulsation and the first pulse. The first pulse is followed by one or more pulse(s), a break duration being present between each subsequent pulse. Preferably, the break duration is from 20 to 80 ms and preferably from 30 to 60 ms.

The spot welding cycle according to the present invention can have different shapes.FIG.2illustrates one preferred embodiment wherein the spot welding cycle21has a rectangular shape comprising a rectangular pulsation peak22and a rectangular welding peak23.FIG.3illustrates another preferred embodiment wherein the spot welding cycle31has a parabolic shape comprising a parabolic pulsation peak32and a parabolic welding peak33.FIG.4illustrates another preferred embodiment wherein the spot welding cycle41has a triangular shape comprising a triangular pulsation peak42and a triangular welding peak43. According to other embodiments, the spot welding cycle has a parabolic and a rectangular shape comprising a parabolic pulsation peak and a rectangular welding peak or, a triangular and a rectangular shape comprising a triangular pulsation peak and a rectangular welding peak.

FIG.5illustrates one preferred embodiment wherein the spot welding cycle comprises one pulsation B.i being directly followed by a first pulse of the welding step. In this Example, the spot welding cycle51has a rectangular shape comprising a rectangular pulsation peak52and three rectangular welding peaks53,53″,53″.

Finally, the invention relates to the use of the assembly according to the present invention for the manufacture of automotive vehicle.

The invention will now be explained in trials carried out for information only. They are not limiting.

Example 1: Welding Test

Trial 1 being Usibor® 1500 steel sheet was hot-dip coated with a conventional coating comprising 9% by weight of silicon, 3% by weight of iron, the balance being aluminum.

Trial 2 to 10 being Usibor® 1500 steel sheets were hot-dip coated with a coating comprising 3% by weight of silicon, 2% by weight magnesium, zinc, the balance being aluminum. Depending on the Trial, the percentage of zinc varied from 5 to 12% by weight.

The steel sheets were then press hardened at an austenitization temperature of 900° C. for 5 minutes.

Then, for each Trial, two identical press hardened steel were welded together.

The welding range was determined according to the norm SEP1220-2. Welding test started from 3 kA and increased by 0.2 kA every two spot welds. When two consecutive splashings occurred at the same current level, the splash limit was found. When splash limit was reached, welding current decreased with the step of 0.1 kA to have three consecutive welded samples at the same current level without expulsion. This current level is defined as the upper welding limit of the current range: Imax.

After that, the lower limit Imin was found. Imin search was done by using the criteria of 4√t, where t is the sheet thickness. This criterion defines the minimum acceptable diameter value that guaranteed the weld quality and strength. For confirmation five consecutive welded samples were obtained with superior welding diameter than minimal welding diameter.

For Trials 1, 3, 5, 8 and 10, the welding cycle comprises only a welding step having a welding current Cw defined by Imin and Imax according to the norm SEP1220-2. For Trials 2, 4, 6, 7, and 9, the welding cycle comprises a pulsation having a pulsation current Cp and a welding step having a welding current Cw defined by Imin and Imax according to the norm SEP1220-2.

The frequency was of 1000 Hz. The obtained Imin, Imax and the welding current range are in the following Table 1.

Welding stepZinc percentageWeldingWeldingin theWeldingPulsationWeldingcurrentcurrentcoatingforcedurationPulsedurationIminImaxrangeTrials(wt. %)(daN)number(ms)(kA)(ms)(kA)(kA)(kA)104500——3404.55.20.7212450120103404.55.20.73124500—————04*10450120103404.25.215104500—————06*102001201034045.21.27*7.5450120103404.15.51.487.54500—————09*5450120103404.35.71.41054500—————0*according to the present invention

Trials 3, 5, 8 and 10 were not weldable, i.e. the criterions of Imin and Imax defined in the norm SEP1220-2 were not achieved. Trials according to the present invention have a welding range equal or above 1 kA.