Spot welding method

Abstract

DEPCT68 Welding methods for the fabrication of a set of at least two steel substrate components. (3,3') which are spot welded together and consist of: first steel substrate (3) which is Press-hardened steel parts are obtained by pressing hardening of steel plates. Coated with an aluminum-based coating; the coating consists of the following percentages: By weight, 7.0 to 9.0% zinc, 1.0 to 10% silicon, 1.0 to 10% magnesium. Up to 3.0% iron, alternative elements were chosen from Pb, Ni, Zr, Hf, Sr, Sb, Pb, Ti, Ca, Mn. Sn, La, Ce, Cr, or Bi; the weight of each element will be less than 0.3%, and impurities will be present. It's unavoidable down to 0.02%, the remaining amount being aluminum, the application of the cycle. A spot connection consisting of: 0. At least three pulses (22,32,42), each of which There is an equal maximum pulse current (Cp), and the pulse duration p for each value is the same and is set. The equal cooling time is set from 20 to 60 milliseconds, and the cooling time is set from 30 to 50 milliseconds. This separates each pulse from the next;

Description

[0001] A SPOT WELDING METHOD

[0002] The present invention relates to a welding method for the manufacture of an assembly of steel substrates spot welded together through at least one spot welded joint. The invention is particularly well suited for the manufacture of automotive vehicles.

[0003] With a view of saving the weight of vehicles, it is known to use high strength steel sheets to achieve lighter weight vehicle bodies and improve crash safety. Press- hardened steel parts are also used notably to reduce the weight of vehicles. Indeed, the tensile strength of these steels is of minimum 1200MPa and can be up to 2500MPa. Press-hardened parts can be coated with an aluminum-based coating having a good corrosion resistance and thermal properties.

[0004] Usually, the method for the manufacture of a coated press-hardened part comprises the following steps:

[0005] A) the provision of a steel sheet pre-coated with a metallic coating being conventional coating based on aluminum,

[0006] B) the cutting of the coated steel sheet to obtain a blank,

[0007] C) the thermal treatment of the blank at a high temperature to obtain a fully austenitic microstructure in the steel,

[0008] D) the transfer of the blank into a press tool,

[0009] E) the hot-forming of the blank to obtain a part,

[0010] F) the cooling of the part obtained at step E) in order to obtain a press-hardened steel part.

[0011] The pre-coated steel sheet of step A) is usually obtained by hot-dip coating of the steel sheet in a liquid metal bath.

[0012] Once the part is manufactured in step F), it is assembled to other parts of the vehicle through spot welding. However, the welding of aluminum based coated press- hardened parts is difficult to realize. In particular, such material does usually not allow to have a wide welding range. The suitable welding current range starts from the current under which a minimum nugget diameter is formed to that under which expulsion occurs. A wide welding current range is desirable because it is possible to control the nugget diameter within a prescribed range even if welding current fluctuates. A wide welding current range is also helpful because it means material is more resistant to electrode wear, misfit, and power line voltage fluctuation. The usual requirement from carmakers is to have a welding range equal or above 1 kA, to be able to run their welding lines with a good quality of welds and without having to change the welding electrodes too often.

[0013] Moreover, it was observed that the welding range of press-hardened parts depends on the press hardening parameters used to produce them. The higher the temperature and the time used for press hardening, the smallest the welding range will be. This is due to the presence of alloyed phases by diffusion of iron from the substrate into the coating. This is also due to the surface oxides generated by the press hardening process. Especially when the coating contains additional elements to aluminum, such as silicon, magnesium or zinc, complex surface oxides will develop in relation with the heat treatment parameters such as time and temperature. These oxides have to be removed before welding.

[0014] Thus, the purpose of the present invention is to provide a welding method for the manufacture of press-hardened steel parts that allows increasing the welding range up to at least 1 kA and minimizes welding expulsion, without having to remove the oxide layer resulting from the press hardening process.

[0015] This objective is achieved by providing a welding method according to claim 1. The method can also comprise any or all of characteristics of claims 2 to 10. Another object of the invention is a vehicle comprising such assembly according to claim 11 .

[0016] Other characteristics and advantages of the invention will become apparent from the following detailed description of the invention.

[0017] To illustrate the invention, various embodiments and trials of non-limiting examples will be described, particularly with reference to the following figures:

[0018] - figure 1 illustrates an equipment to carry out the present invention.

[0019] - figure 2 illustrates an embodiment of spot-welding cycle according to the present invention.

[0020] The invention relates to a welding method for the manufacture of an assembly of at least two steel substrates spot welded together through at least one spot welded joint.

[0021] As illustrated in Figure 1 , a spot-welding machine (not illustrated), comprising welding electrodes 1 , T and a spot-welding electric power source 2, is used. In this example, the electrodes permit to join two press-hardened steel parts 3, 3’ manufactured by press hardening of a steel sheet coated with an aluminium based coating 4, 4’. The current can be alternative current (AC) or direct current (DC). In a preferred embodiment, the current is mid frequency direct current (MFDC) obtained by conversion of AC current supply.

[0022] The method according to the invention further comprises the application of a spot-welding cycle 21 , consisting of:

[0023] - at least three pulsations 22, 32, 42, each having the same pulsation current (Cp) applied through the metallic substrates joined together using welding electrodes connected to the spot-welding power source, each pulsation duration being identical and set from 20 to 60 ms,

[0024] - an identical cooling time set from 30 to 50 ms, separating each pulsation.

[0025] The pulsations used in the method according to the invention must be present in a number of at least three and preferably at least five. In a preferred embodiment, the maximum number of pulsations can be set to nine of them. After using such pulsations separated by such cooling times, the substrates are fully welded, meaning that no other welding cycle of any kind is performed in addition to them.

[0026] The duration is identical from one pulsation to the others and is set within a range going from 20 to 60 ms, preferably from 30 to 50 ms.

[0027] If the pulsation duration is shorter than 20ms the minimum nugget diameter may not be achieved. If the pulsation duration is longer than 60ms, early expulsion may occur. The inventors have found that increasing the number of pulsations increases the current welding range.

[0028] The maximum pulsation current (Cp) is identical for all pulsations and is preferably set from 0.1 to 30kA. The welding force applied by the electrodes at the same time as the current is preferably set from 50 to 650 daN and more preferably from 250 to 500 daN.

[0029] The welding frequency is preferably set from 500 to 5000Hz and more preferably from 800 to 2000 Hz.

[0030] The spot-welding cycle according to the present invention can include pulsations of various shapes. Such pulsations shape can be identical in a given welding cycle or can be different. Figure 2 illustrates one preferred embodiment wherein the spot-welding cycle 21 consists of five pulsations with a rectangular form, namely identical rectangular pulsations peaks 22, 32, 42, 52 and 62. Other options of shape for such pulsations are: - a parabolic form,

[0031] - a triangular form or any other suitable form, provided that the pulsations of a given welding cycle all have the same maximum pulsation current (Cp).

[0032] In the frame of the invention, the term press-hardened steel part refers to a hot- formed or hot-stamped steel part having a tensile strength up to 2500 MPa, and more preferably up to 2000MPa, after austenitisation of a blank and further forming and quenching in a die. 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.

[0033] In case steel having high mechanical strength is needed, in particular for parts of structure of automotive vehicle, steel having a tensile resistance superior to 500MPa, advantageously between 500 and 2000MPa before or after heat-treatment, can be used. The weight composition of steel sheet is preferably as follows: 0.03% < C < 0.50% ; 0.3% < Mn < 3.0% ; 0.05% < Si < 0.8% ; 0.015% < Ti < 0.2% ; 0.005% < Al < 0.1 % ; 0% < Cr < 2.50% ; 0% < S < 0.05% ; 0% < P < 0.1 % ; 0% < B < 0.010% ; 0% < Ni < 2.5% ; 0% < Mo < 0.7% ; 0% < Nb < 0.15% ; 0% < N < 0.015% ; 0% < Cu < 0.15% ; 0% < Ca < 0.01 % ; 0% < W < 0.35%, the balance being iron and unavoidable impurities from the manufacture of steel.

[0034] For example, the steel sheet is 22MnB5 with the following weight composition: 0.20%

[0035] < C < 0.25%; 0.15% < Si < 0.35%; 1.10% < Mn < 1 .40%; 0% < Cr < 0.30%; 0.020% < Ti < 0.060%; 0.020% < Al < 0.060%; 0.002% < B < 0.004%, the remainder being iron and unavoidable impurities from the manufacture of steel.

[0036] In another embodiment, the steel sheet has the following weight composition: 0.24% < C < 0.38%; 0.40% < Mn < 3%; 0.10% < Si < 0.70%; 0.015% < Al < 0.070%; Cr < 2%; 0.25% < Ni < 2%; 0.015% < Ti < 0.10%; Nb < 0.060%; 0.0005% < B < 0.0040%; the remainder being iron and unavoidable impurities resulting from the manufacture of steel.

[0037] Alternatively, the steel sheet can have the following weight composition: 0.30%

[0038] < C < 0.40%; 0.5% < Mn < 1.0%; 0.40% < Si < 0.80%; 0.1 % < Cr < 0.4%; 0.1 % < Mo < 0.5%; 0.01 % < Nb < 0.1 %; 0.01 % < Al < 0.1 %; 0.008% < Ti < 0.003%; 0.0005% < B < 0.003%; 0.0% < P < 0.02%; 0.0% < Ca < 0.001 %; 0.0% < S < 0.004 %; 0.0% < N < 0.005 %, the remainder being iron and unavoidable impurities resulting from the manufacture of steel. In another embodiment, the steel sheet has the following weight composition: 0.040% < C < 0.100%; 0.80% < Mn < 2.00%; 0% < Si < 0.30%; 0% < S < 0.005%; 0%

[0039] < P < 0.030%; 0.010% < Al < 0.070%; 0.015% < Nb < 0.100%; 0.030% < Ti < 0.080%; 0% < N < 0.009%; 0% < Cu < 0.100%; 0% < Ni < 0.100%; 0% < Cr < 0.100%; 0% < Mo

[0040] < 0.100%, the balance being iron and unavoidable impurities from the manufacture of steel.

[0041] In another embodiment, the steel sheet has the following weight composition: 0.06% < C < 0.1 %, 1 % < Mn < 2%, Si < 0.5%, Al <0.1 %, 0.02% < Cr < 0.1 %, 0.02% < Nb < 0.1 %, 0.0003% < B < 0.01 %, N < 0.01 %, S < 0.003%, P < 0.020% less than 0,1 % of Cu, Ni and Mo, the remainder being iron and unavoidable impurities resulting from the manufacture of steel.

[0042] In another embodiment, the steel sheet has the following weight composition: 0.015% < C < 0.25%; 0.5% < Mn < 1.8%; 0.1 % < Si < 1.25%; 0.01 % < Al < 0.1 %; 0.1 %

[0043] < Cr < 1.0%; 0.01 % < Ti < 0.1 %; 0% < S < 0.01 %; 0.001 % < B < 0.004%; 0% < P < 0.020%; 0% < N < 0.01 %; the balance being iron and unavoidable impurities from the manufacture of steel.

[0044] Alternatively, the steel sheet has the following weight composition: 0.2% < C < 0.34%; 0.5% < Mn < 1 .24%; 0.5% < Si < 2.0%; 0% < S < 0.01 %; 0% < P < 0.020%; 0%

[0045] < N < 0.01 %, the balance being iron and unavoidable impurities from the manufacture of steel.

[0046] The method according to the invention applies to press-hardened steel parts obtained by press hardening of a steel sheet coated with an aluminum-based coating and containing zinc, silicon and magnesium.

[0047] The steel sheet used to manufacture the press hardened part can be manufactured by hot dip galvanizing in a bath, the temperature of which is set from 600 to 700°C, preferably from 620 to 650°C.

[0048] The coating weight is set during the wiping process by gas knives in a range from 50 to 500 g / m2, possibly from 80 to 150 g / m2and preferably from 90 to 120 g / m2for the sum of both sides of the steel sheet.

[0049] Before being coated, the steel sheet according to the invention can be obtained by hot rolling and optionally cold rolling depending on the desired thickness, which can be for example between 0.5 and 3.0 mm, preferably from 1 .0 to 2.0 mm. The coating comprises by weight percent, from 7.0 to 9.0 wt.% of zinc and advantageously from 7.5 to 8.5 % of zinc.

[0050] Optionally, the coating comprises from 1 .0 to 10.0 % of silicon, and from 1 .0 to 10.0 % of magnesium.

[0051] Preferably, the coating comprises, in weight percent, from 1 .0 to 4.0 % of silicon and from 1 .0 to 4.0 % of magnesium, advantageously from 2.5 to 3.5 % of silicon and from 1 .5 to 3.0 % of magnesium.

[0052] Optionally, the coating comprises up to 3 % weight iron. Iron comes from the dissolution of the steel sheet in the hot dip coating bath and can vary during production. Optionally, the coating comprises additional elements chosen from Ni, Zr, Hf, Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, or Bi, the content by weight of each additional element being inferior to 0.3 wt.%.

[0053] In a preferred embodiment, up to 100 ppm in weight of calcium is added.

[0054] Finally, the coating may contain unavoidable impurities up to 0.02 wt%, preferably up to 0.01 wt. %.

[0055] The press-hardening processing of such steel sheets is well known to the man skilled in the art and includes an austenitisation of a blank cut out of such steel at a temperature that can, for example, from 840 to 950°C, preferably from 900 to 950°C, during 3 to 10 minutes, followed by a quenching in the forming die. After presshardening, the coatings described above will get alloyed by diffusion of iron due to the heating of the blanks. An oxide layer will form on top of the alloyed coating layer, said oxide layer containing aluminum, zinc, magnesium.

[0056] The welding method according to the invention can be used to weld such a press-hardened to a similar press-hardened part (homogenous welding) or to any steel part, without removing top oxides. It can also be used in a hybrid welding between a press-hardened steel part and an aluminum substrate.

[0057] The invention will now be explained in trials carried out for information only. They are not limiting.

[0058] Examples

[0059] Steel sheets of different average thicknesses coated with aluminium based alloys were prepared and press hardened under the conditions gathered in table 1 . Table 1

[0060] 111500 has a composition of 0.22 wt.% of carbon, 1 .2 wt.% of manganese, 0.25 wt.% of silicon, 0.2 wt.% of chromium, 0.04 wt.% of aluminum, 0.04 wt.% of titanium and 0.003 wt.% of boron.

[0061] The coating composition is also shown in table 1 .

[0062] Then, for each trial, two identical press-hardened parts were welded together. The welding range was determined as described below. These methods will now be explained. For all methods, the welding range is the difference between the maximum current at which no splashing occurs and the minimal current ensuring the minimum required nugget size.

[0063] According to ISO 18278-2:2016, the welding test starts from a current of at least 3kA and current is increased by steps of 0.2 kA, three spot welds being made for each current level. When two out of three welds meet the minimum size requirement of 4 t, where t is the sheet thickness at the same current, this current Im in is reached. This criterion defines the minimum acceptable diameter value of the nugget that guaranteed the weld quality and strength. The current intensity is then increased further by 0.2kA steps, until two out of three consecutive welds have splashing occurring at the same current level. This current level is defined as the upper welding limit of the current range I max.

[0064] According to SEP 1220-2:2011 , the welding test starts from 3kA and current increased by steps of 0.2 kA, two spot welds being made for each current level. When both welds show expulsion at the faying interface, the current is decreased by steps of 0.1 kA. When there is no splashing, a second and a third spot weld is performed without changing current. Imax is achieved when three consecutive welds have no splashing occurrence at the same current level. For searching Im in, one starts from the spot welds performed during the first current increase sequence. Im in is obtained when 5 spotwelds at the same intensity satisfy to the minimal size requirement of 4> / t.

[0065] As the standardized methods take time and consumes a lot of material, simplified variants have been used for the purpose of effectiveness.

[0066] Simplified SEP 1220 starts at 4 kA and current is increased by steps of 0.4 kA. After expulsion, current is decreased by steps of 0.2 kA to define Imax with two spot welds showing no expulsion. Im in is then searched and achieved when 2 spotwelds at the same intensity satisfy to the minimal size requirement (4 t).

[0067] Additionally, a gross method with steps of 0.5 kA has been used to check for approximate current range between an intensity ensuring the minimum size requirement of 4 t (> Imin) and a higher intensity free of expulsion (< Imax). Such current welding range obtained by the gross method is at least of the same magnitude of the current welding range obtained by the standardized method. The latter may be larger.

[0068] For all methods, the current welding range, calculated as (Imax - Imin), has to be of 1 kA or more. The pulsations were of rectangular form.

[0069] The frequency was set to 1000Hz and the welding force was set according to ISO 18278-2:2016 for various thicknesses from 350 daN to 500 daN.

[0070] The results of the trials are gathered in Table 2.

[0071] Table 2

[0072] Trials 3, 4, 5 and 16 were not weldable, i.e. the minimum welding range of 1 kA was not achieved. Trials according to the present invention all have a welding range equal or above

[0073] 1 kA, even for parts produced with high press hardening temperatures and long time as demonstrated notably by trial 17.

Claims

DEPCT681. Welding method for the manufacture of a set of at least two (3,3') steel substrate parts that are spot welded together through at least one spot welded joint, which includes the following steps: - Supply of at least two such (3,3') metal substrates, where the first (3) steel substrate is a press-hardened steel part obtained by press hardening of a steel sheet coated with an aluminum-based coating, such coating contains by weight 7.0 to 9.0% zinc, 1.0 to 10% silicon, 1.0 to 10% magnesium, up to 3.0% iron, alternative elements chosen from Pb, Ni, Zr, Hf, Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr or Bi, the weight of each element is less than 0.3% and unavoidable impurities are less than 0.02%, the remaining amount which is aluminum - Application of a spot welding cycle with a spot welding machine consisting of a welding electrode (1,1') and a spot welding power source (2) which applies current through at least two such metal substrates to such a spot welding cycle (21) which consists of: 0 at least three pulses (22,32,42), each with the same maximum pulse current (Cp) applied through at least two such metal substrates connected together.

1. Welding method according to claim 1, where in step A) the coating contains by weight percentage from 7.5 to 8.5% zinc, from 1.0 to 4.0% silicon, from 1.0 to 4.0% magnesium, up to 3.0% iron, alternative elements selected from Pb, Ni, Zr, Hf, Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, or Bi, the weight of each element shall be less than 0.3%, and unavoidable impurities shall be down to 0.02%, the remainder being aluminum.

3. Welding method according to claim 1 or 2 where the maximum pulse current (Cp) is set from 0.1 to 30 kA.

4. Welding method according to any one of claims 1 to 3 where the number of pulses is set from three to nine.

5. Welding method according to any one of claims 1 to 4.

6. Any welding method under claims 1 to 5 where the welding frequency is set from 500 to 5000 Hz.

7. Any welding method under claims 1 to 6 where the spot welding cycle includes pulses with a pulse shape chosen from among: rectangular, parabolic, triangular.

8. Any welding method under claims 1 to 7 where the second metal substrate (3') is a steel substrate or an aluminum substrate. 9.

10. Any of the welding methods under claims 1 to 9 in which the first (3,3') substrate is obtained by press hardening of a pre-heated steel plate at temperatures of 840 to 950 degrees Celsius for 3 to 10 minutes; 11. A vehicle which is assembled with at least one set of parts obtained by any of the welding methods under claims 1 to 10;

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