Assembly of at least 2 metallic substrates

Abstract

A method for the manufacture of an assembly of at least two metallic substrates spot welded together through at least one spot welded joint, such method including two steps, the assembly obtainable according to this method and the use of this assembly for the manufacture of automotive vehicle.

Claims

1. A welding method for manufacture of an assembly, the method comprising the following steps: A. providing at least two metallic substrates including a first metallic substrate being a hardened steel part coated with: an alloyed coating including zinc and silicon, and optionally magnesium, a balance being aluminum, the alloyed coating being directly topped by a native oxide layer including ZnO and optionally MgO; B. applying a spot welding cycle, with a spot welding machine having welding electrodes and a spot welding power source applying an inverter direct current, through the at least two metallic substrates of step A), the spot welding cycle including the following sub-steps: i. applying one pulsation with a pulsation current through the at least two metallic substrates joined together using welding electrodes connected to the spot welding power source, the one pulsation having a pulsation duration, and directly after, ii. welding with a welding current applied through the at least two metallic substrates for a welding duration; wherein the pulsation current is higher than the welding current and wherein the pulsation duration is shorter than the welding duration.

2. The welding method as recited in claim 1 wherein in step B.i), the pulsation current is between 8.0 and 30.0 kA.

3. The welding method as recited in claim 1 wherein in step B.i), the pulsation duration is from 5 to 60 ms.

4. The welding method as recited in claim 1 wherein in step B.ii), the welding current is between 0.1 and 15 kA.

5. The welding method as recited in claim 1 wherein in step B.ii), the welding duration is from 150 to 500 ms.

6. The welding method as recited in claim 1 wherein a welding force during the spot welding cycle is between 50 and 550 daN.

7. The welding method as recited in claim 6 wherein the welding force during the spot welding cycle is between 350 daN and 550 daN.

8. The welding method as recited in claim 6 wherein the welding force during the spot welding cycle is between 50 daN and 350 daN.

9. The welding method as recited in claim 1 wherein a welding frequency is between 500 and 5000 Hz.

10. The welding method as recited in claim 1 wherein the welding step B.ii) includes a plurality of pulses, the one pulsation in step B.i) being directly followed by a first pulse of the plurality of pulses of the welding step B.ii).

11. The welding method as recited in claim 1 wherein the spot welding cycle shape is selected from the group consisting of: a rectangular form including a rectangular pulsation peak and a rectangular welding peak, a parabolic form including a parabolic pulsation peak and a parabolic welding peak, a triangular form including a triangular pulsation peak and a triangular welding peak, a parabolic and a rectangular shape including a further parabolic pulsation peak and a further rectangular welding peak, and a triangular and a rectangular shape including a further triangular pulsation peak and a yet further rectangular welding peak.

12. An assembly comprising: at least two metallic substrates spot welded together through at least one spot welded joint obtainable according to the method as recited in claim 1, the at least two metallic substrates including a first metallic substrate being a hardened steel part coated with an alloyed coating including zinc and silicon, and optionally magnesium, a balance being aluminum, the alloyed coating being directly topped by a native oxide layer including ZnO and optionally MgO; the spot welded joint including a nugget; and the spot welded joint being such that on a top at least a part of the native oxide layer or alloyed coating is not present.

13. The assembly as recited in claim 12 wherein the alloyed coating of the hardened steel part includes from 0.1 to 40.0% by weight of zinc.

14. The assembly as recited in claim 13 wherein the alloyed coating of the hardened steel part includes from 0.1 to 20.0% by weight of zinc.

15. The assembly as recited in claim 12 wherein the alloyed coating of the hardened steel part includes from 0.1 to 20.0% by weight of silicon.

16. The assembly as recited in claim 15 wherein the alloyed coating of the hardened steel part includes from 0.1 to 15.0% by weight of silicon.

17. The assembly as recited in claim 12 wherein the alloyed coating of the hardened steel part includes from 0.1 to 20.0% by weight of magnesium.

18. The assembly as recited in claim 17 wherein the alloyed coating of the hardened steel part includes from 0.1 to 10.0% by weight of magnesium.

19. The assembly as recited in claim 12 wherein a second metallic substrate of the at least two metallic substrates is a steel substrate or an aluminum substrate.

20. The assembly as recited in claim 12 wherein the second steel substrate is a further hardened steel part coated with a further alloyed coating including zinc and silicon, and optionally magnesium, a balance being aluminum, the further alloyed coating being directly topped by a further native oxide layer including ZnO and optionally MgO.

21. The assembly as recited in claim 12 further comprising a third metallic substrate sheet being a steel substrate or an aluminum substrate.

22. An automotive vehicle comprising the assembly as recited in claim 12.

23. A method for manufacturing an automotive vehicle comprising performing the welding method as recited in claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) To illustrate the invention, various embodiments and trials of non-limiting examples will be described, particularly with reference to the following Figure:

(2) FIG. 1 illustrates an embodiment according to the present invention.

(3) FIGS. 2, 3, 4 and 5 illustrate Examples of spot welding cycle according to the present invention.

DETAILED DESCRIPTION

(4) Other characteristics and advantages of the invention will become apparent from the following detailed description of the invention.

(5) The designation 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.

(6) The invention relates to a welding method for the manufacture of an assembly 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 zinc, silicon, optionally magnesium, the balance being aluminum, directly topped by A 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. 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 and wherein the current Cp is above the current Cw and wherein the pulsation duration is shorter than the welding duration.

(7) 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 zinc, silicon, optionally magnesium, the balance being aluminum, allows for a welding range equal or above 1 kA and a decrease of coating splashing on the assembly surface. 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 pulsation having a pulsation current (Cp) higher in intensity than the welding current (Cw) breaks at least a part of the ZnO and optionally MgO oxide layer and/or alloyed coating present on the coated hardened steel part opening a path to the welding current. However, if Cp is below Cw, i.e. outside the scope of the present invention, it is believed that the ZnO and optionally MgO barrier layer will not be broken by the pulsation due to the important amount of ZnO and optionally MgO. Additionally, it seems that the method according to the present invention comprising one pulsation is easy to implement on an R industrial scale.

(8) As illustrated in FIG. 1, a spot welding machine 100, comprising welding electrodes 1, 1 and a spot welding source 2, is used. In this Example, the electrodes permit to join two hardened steel parts 3, 3 coated with the coating according to the invention 4, 4, and 4, 4 respectively on top of which coatings forms native oxide layers 7, 7, 7 and 7 respectively. During the welding, a nugget 5 is 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 coatings 4, 4, 4, 4 are removed in the nugget. Moreover, on the top of the spot welded joint 6, 6, it is believed that at least a part of the native oxide layers 7, 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.

(9) 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.

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

(11) Preferably, in step B.ii), the welding current (Cw) is between 0.1 and 15 kA, advantageously between 0.1 and 7.5 and more preferably between 2.0 and 7.5 kA.

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

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

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

(15) 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.

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

(17) 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.

(18) The spot welding cycle according to the present invention can have different shapes. FIG. 2 illustrates one preferred embodiment wherein the spot welding cycle 21 has a rectangular shape comprising a rectangular pulsation peak 22 and a rectangular welding peak 23. FIG. 3 illustrates another preferred embodiment wherein the spot welding cycle 31 has a parabolic shape comprising a parabolic pulsation peak 32 and a parabolic welding peak 33. FIG. 4 illustrates another preferred embodiment wherein the spot welding cycle 41 has a triangular shape comprising a triangular pulsation peak 42 and a triangular welding peak 43. 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.

(19) FIG. 5 illustrates 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 cycle 51 has a rectangular shape comprising a rectangular pulsation peak 52 and three rectangular welding peaks 53, 53, 53.

(20) The invention relates also to an assembly of at least two metallic substrates spot welded together through at least one spot welded joint obtainable with the method according to the present invention, said assembly comprising: a first metallic substrate being a hardened steel part coated with: an alloyed coating comprising zinc, silicon, optionally magnesium, the balance being aluminum, directly topped by A 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.

(21) Without willing to be bound by any theory, it seems that when the assembly comprises the above specific coating on the hardened part welded using the welding method according to the present invention, the welding range is equal or above to 1 kA. Indeed, it seems that although the thickness of the native oxide layer is higher than the coatings of the prior art, the welding method according to the present invention breaks the native oxide layer and remove at least a part of the native oxide layer and/or alloyed coating allowing a good weldability of the assembly.

(22) Preferably, the alloyed coating of the hardened steel part comprises from 0.1 to 40.0% by weight of zinc, more preferably between 0.1 and 20.0% by weight of zinc and advantageously between 5.0 and 14% by weight of zinc and for example between 7.0 and 12.0% by weight.

(23) Preferably, the alloyed coating of the hardened steel part comprises from 0.1 to 20.0% by weight of silicon, more preferably from 0.1 to 15.0% by weight of silicon and advantageously from 0.1 to 6.0% by weight of silicon and for example between 2.0 and 6.0% by weight of silicon.

(24) Preferably, the alloyed coating of the hardened steel part comprises from 0.1 to 20.0% by weight of magnesium, from 0.1 to 10.0%, preferably from 0.1 to 4.0% by weight of magnesium.

(25) Optionally, the coating comprises 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. For example, the amount of iron is up to 5% by weight.

(26) 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.

(27) In another preferred embodiment, the assembly comprises a third metallic substrate sheet 101 (shown schematically in FIG. 1) being a steel substrate or an aluminum substrate. In this case, two or several spot welded joints are present.

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

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

EXAMPLES

(30) All Trials being Usibor 1500 steel sheets were hot-dip coated with a coating comprising 3% by weight of silicon, 2% by weight magnesium, 10-12% by weight of zinc, the balance being aluminum. The steel sheets were then press hardened at an austenitization temperature of 900 C. for 5 minutes.

(31) Then, for each Trial, two identical press hardened parts were welded together.

(32) The welding range was determined according to the norm SEP1220-2.

(33) 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 the splash limit was reached, the welding current was 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.

(34) After that, the lower limit Imin was found. Imin search was done by using the criteria of 4t, 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.

(35) For Trials 1 to 12, 17 and 18, the welding cycle comprises, optionally one pulsation having a pulsation current Cp, and one welding step having a welding current Cw defined by Imin and Imax according the norm SEP1220-2. For Trials 13 to 16, the welding cycle comprises one pulsation having a pulsation current Cp and three or four welding steps having a welding current Cw defined by Imin and Imax according the norm SEP1220-2, a temporary stop being performed between each welding step.

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

(37) TABLE-US-00001 Welding step Zinc Welding percentage Pulsation current Welding in the Welding Current (Cw) Cp current coating force duration (Cp) Number of duration Stop Imin Imax > range Trials (wt. %) (daN) Number (ms) (kA) welding step(s) (ms) (ms) (kA) (kA) Cw (kA) 1 12 200 0 1 0 2 12 450 0 1 0 3 12 200 1 20 4.5 1 0 4* 12 200 1 20 10 1 340 4.2 5.2 yes 1 5* 12 200 1 20 11 1 340 4.46 5.65 yes 1.19 6* 12 200 1 20 12 1 340 4.46 5.88 yes 1.42 7* 12 200 1 20 13 1 340 4.6 6.2 yes 1.6 8* 12 200 1 20 14 1 340 4.87 6.66 yes 1.79 9 12 450 1 20 6 1 0 10 12 450 1 20 7 1 0 11* 12 450 1 20 16 1 340 5.67 7.07 yes 1.4 12* 12 450 1 40 10 1 340 5.43 6.64 yes 1.21 13 12 450 1 20 10 3 160 40 5.6 6.1 yes 0.5 14 12 450 1 20 10 3 160 40 4.9 5.4 yes 0.5 15 12 450 1 20 10 3 160 40 4.5 4.6 yes 0.1 16 12 450 1 20 10 4 160 40 0 17* 10 200 1 20 10 1 340 4 5.2 yes 1.2 18* 10 400 1 20 10 1 340 4.2 5.2 yes 1 *according to the present invention

(38) Trials 1, 2, 3, 9, 10 and 16 were not weldable, i.e. the criterions of Imin and Imax defined in norm SEP1220-2 were not achieved. Trials according to the present invention have a welding range equal or above 1 kA.

Example 2: Heterogeneous Welding Test

(39) Usibor 1500 steel sheets were hot-dip coated with a coating comprising 3% by weight of silicon, 2% by weight magnesium, 12% by weight of zinc, the balance being aluminum. The steel sheets were then press hardened at an austenitization temperature between 900 C. for 5 minutes. They were welded with DP600 steel grade (C: 0.14 wt. %, Mn: 2.1 wt % and Si: 0.4 wt. %) coated with a zinc coating. The welding range was determined as in Example 1. The frequency was of 1000 Hz. The obtained Imin, Imax and the welding current range are in the following Table 2.

(40) TABLE-US-00002 Welding step Welding Welding Welding Pulsation Welding current Cp current force Duration Stop Current duration Imin Imax > range Trials (daN) Number (ms) (ms) (kA) (ms) (kA) (kA) Cw (kA) 19* 200 1 20 10 340 4.8 6.6 Yes 1.8 20* 300 1 20 10 340 5.2 6.4 Yes 1.2 *according to the present invention

(41) Trials according to the present invention have a welding range equal or above 1 kA.

Example 3: Electrode Life Test

(42) Electrode life is defined as the last weld number of a test strip before reaching more than two welds out of eight below a minimum weld diameter defined. The minimum weld diameter was of 4.7 mm.

(43) Two coated hardened steel parts prepared as Trial 4 were welded together with the welding method according to the present invention comprising one pulsation and the welding step. The pulsation current was of 10 kA during 10 ms. The welding current was Imax determined for Trial 4 in Example 1. A plurality of spot welds was performed with the electrodes on the two coated hardened parts and the weld diameter was measured for each spot weld. Results are in the following Table 3.

(44) TABLE-US-00003 Number Weld of spot diameter Trial welds (mm) 4* 10 5.2 100 5.2 200 5.2 300 5.2 400 5.2 500 5.3 600 5.4 *according to the present invention

(45) The weld diameter was always above the minimum weld diameter with Trial 4 according to the present invention.