Welding method

Abstract

The present 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 weld joint, comprising: A. provision of said substrates (3, 3'), said first substrate being a press-hardened steel part obtained by press-hardening of a steel sheet coated with an aluminium-based coating; B. application of a spot welding cycle by means of a spot welder comprising a welding electrode (1, T) and a spot welding power source (2) applying a current through said substrates, said cycle (21) comprising: - at least three pulsations (22, 32, 42), each having the same maximum pulsation current (Cp) applied through said substrates, the duration p of each pulsation being identical and set between 20 and 60 ms; The present invention relates to a welding method including an application including the same cooling time c following each pulsation, set to 30 to 50 ms, and a welding parameter Wp value of at least 0.8, Wp being defined as Wp=(t×c) / p, where t is the average thickness of the substrate in mm, c is the cooling time in ms, and p is the pulsation duration in ms.

Description

[Technical Field]

[0001] 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 weld joint. The invention is particularly well suited for the manufacture of automobiles. [Background technology]

[0002] To reduce vehicle weight, it is known to use high-strength steel plates to achieve lighter body construction and improve crashworthiness. Hardened parts are also used to reduce vehicle weight. The tensile strength of these steels ranges from a minimum of 1200 MPa to a maximum of 2500 MPa. Hardened parts can be coated with aluminum- or zinc-based coatings, which offer excellent corrosion resistance and thermal properties.

[0003] Typically, the process for the production of coated cured parts comprises the following steps: A) Providing steel sheets pre-coated with a metallic coating, which is a conventional coating based on aluminum; B) Cutting of coated steel sheets to obtain blanks; C) heat treatment of the blank at high temperature to obtain a fully austenitic microstructure in the steel; D) Transfer of the blank to the press tool; E) hot forming of the blank to obtain the part; F) Cooling of the part obtained in step E) to obtain a microstructure in the steel that is martensite or martensite-bainite or that consists of at least 75% equiaxed ferrite, 5% to 20% martensite and an amount of bainite not exceeding 10%.

[0004] After components are manufactured, they are assembled to other vehicle components by spot welding. However, welding hardened parts with aluminum-based coatings is difficult to achieve. Specifically, such materials typically do not have a wide welding range. The appropriate welding current range is the current at which the smallest nugget diameter is formed, up to the current at which expulsion occurs. A wide welding current range is also desirable because it allows the nugget diameter to be controlled within a given range even when the welding current fluctuates. A wide welding current range is also useful because it means the material is more resistant to electrode wear, misfit, and power line voltage fluctuations. Automotive manufacturers typically require a welding range of 1 kA or more, and the ability to operate their welding lines using high-quality welds without the need to frequently replace welding electrodes.

[0005] It has also been observed that the weld area of ​​press-hardened parts depends on the press-hardening parameters used to produce them. The higher the temperature and the longer the press-hardening time, the smaller the weld area. This is due to the presence of surface oxides created by the press-hardening process. Summary of the Invention [Problem to be solved by the invention]

[0006] It is therefore an object of the present invention to provide a welding method for the production of coated press-hardened parts that allows increasing the welding range to at least 1 kA and minimizing weld expulsion, independent of the press-hardening parameters, while maximizing electrode life. [Means for solving the problem]

[0007] This object is achieved by providing a welding method according to claim 1. The method may also include any or all of the features of claims 2 to 9.

[0008] Other features and advantages of the present invention will become apparent from the following detailed description of the invention.

[0009] To illustrate the invention, various embodiments and non-limiting example attempts will now be described with particular reference to the following drawings: [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 shows an apparatus for carrying out the present invention. [Figure 2] FIG. 2 is a diagram illustrating an example of a spot welding cycle according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] The present 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 weld joint.

[0012] As shown in Figure 1, a spot welding machine (not shown) is used, comprising welding electrodes 1, 1' and a spot welding power supply 2. In this example, the electrodes make it possible to join two press-hardened steel parts 3, 3', which have been produced by press-hardening steel sheets coated with an aluminium-based coating 4, 4', 4". During welding, a nugget 5 is formed between the two press-hardened steel parts by diffusion, which finally forms a spot weld joint 6, 6'. The current can be alternating current (AC) or direct current (DC). In a preferred embodiment, the current is mid-frequency direct current (MFDC), obtained by conversion of an AC current source.

[0013] The method according to the present invention further comprises applying a spot welding cycle 21, the spot welding cycle 21 comprising: - at least three pulsations 22, 32, 42, each having the same pulsation current (Cp) applied through the metal substrates to be joined together using a welding electrode connected to a spot welding power supply, the duration p of each pulsation being the same and set to 20 to 60 ms; - Each pulsation is followed by an identical cooling time, c, set to 30 to 50 ms. It consists of The welding parameter Wp value is at least 0.8, and Wp is Wp=(t×c) / p is defined as t is the substrate thickness in mm, c is the cooling time in ms, p is the pulsation duration in ms.

[0014] The number of pulsations used in the method according to the invention must be at least three, preferably at least five. In a preferred embodiment, the maximum number of pulsations can be set to nine. After using such pulsations separated by cooling times, the substrate is completely welded, i.e., no other welding cycles of any kind are performed in addition to these. The duration p of the pulsations is the same from one pulsation to the next and is set within the range of 20 to 60 ms, preferably 30 to 50 ms.

[0015] The maximum pulsation current (Cp) of all pulsations is the same, preferably set to 0.1 to 30 kA, while the welding method is preferably set to 50 to 650 daN, more preferably 250 to 500 daN.

[0016] The welding intensity is preferably set at 500 to 5000 Hz, more preferably 800 to 2000 Hz.

[0017] Spot welding cycles according to the present invention can include pulsations with various forms of current setpoints. Such pulsations can be the same or different in a given welding cycle. Figure 2 shows a preferred embodiment in which a spot welding cycle 21 is comprised of pulsation setpoints with rectangular shapes, i.e., the same rectangular pulsation peaks 22, 32, 42, 52, and 62. Other options for such pulsation setpoint shapes include: - Parabolic shape, - triangular shape, Or any other suitable shape, provided that the pulsations in a given welding cycle all have the same maximum pulsation current (Cp).

[0018] In order to reduce premature expulsion, which would significantly reduce the welding area, a specific cooling time c must be observed between each pulsation of the welding cycle according to the invention. Such cooling time is set to 30 to 50 ms. Also, the value of the welding parameter Wp is at least 0.8, preferably at least 0.9, or even more preferably at least 1.0, where Wp is Wp=(t×c) / p is defined as t is the average thickness of the substrate in mm, c is the cooling time in ms, p is the pulsation duration in ms.

[0019] Setting the value of the welding parameter Wp taking into consideration the thickness of the substrate contributes to achieving the improvement in welding characteristics that is the goal of the present invention.

[0020] Within the scope of the present invention, the term press-hardened steel part refers to a hot-formed or hot-stamped steel part which, after austenitization of the blank and further forming in a die and quenching, has a tensile strength of at most 2500 MPa, more preferably at most 2000 MPa, for example a tensile strength of at least 500 MPa, advantageously at least 1200 MPa, preferably at least 1500 MPa.

[0021] The method according to the invention is applied to press-hardened steel parts obtained by press-hardening of steel sheet coated with a so-called AlSi coating, said coating comprising 7 to 12% by weight of silicon, 2 to 5% by weight of iron, optionally additional elements selected from Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Zr or Bi, the content of each additional element being less than 0.3% by weight, and optionally remaining elements, the remainder being aluminum.

[0022] The press hardening process for such steel sheets is well known to those skilled in the art and involves austenitizing a blank cut from such steel at a temperature which may be, for example, 880 to 950°C, preferably 900 to 950°C, for 3 to 10 minutes, preferably 6 to 10 minutes, followed by quenching of the forming die. After press hardening, the aluminum coating is alloyed by diffusion of iron due to heating of the blank.

[0023] The average thickness of the steel substrate may be, for example, from 0.8 to 3 mm, preferably from 1 to 2 mm.

[0024] The welding method according to the invention can be used to weld such press-hardened parts to similar press-hardened parts (like welding) or to any steel part, and can also be used for hybrid welding of press-hardened steel parts to aluminum substrates.

[0025] The invention is described below with reference to trials carried out for informational purposes only and is not limiting.

[0026] example Steel sheets of different compositions and average thicknesses coated with aluminum-based alloys were prepared and press-hardened under the conditions summarized in Table 1.

[0027] [Table 1]

[0028] U1500 has a composition of 0.22 wt% carbon, 1.2 wt% manganese, 0.25 wt% silicon, 0.2 wt% chromium, 0.04 wt% aluminum, 0.04 wt% titanium, and 0.003 wt% boron.

[0029] The AlSi coating contains 9% by weight silicon, 3% by weight iron, and the remainder aluminum.

[0030] Next, two identical press-hardened parts were welded together for each test. The weld range was determined using standard ISO 18278-2:2016. The welding test started with a low current, such as 3 kA, and was increased by 0.2 kA, with two spot welds performed for each current level. When both welds met the minimum size requirement of 4√t, a third weld was performed at the same current, Imin, so that all three welds were equal to or greater than 4√t, where t is the plate thickness. This standard defines the minimum acceptable nugget diameter that ensures weld quality and strength. The current intensity was then increased in 0.2 kA steps until two of the three consecutive welds produced splashing at the same current level. This current level was defined as the upper weld limit, Iexp, of the current range. The weld range was then calculated as (Iexp - Imin). The pulsation setpoint was rectangular.

[0031] The frequency was set to 1000 Hz, and the welding force was set from 350 daN to 500 daN for various thicknesses according to ISO18278-2:2016. The results of the trials are summarized in Table 2.

[0032] [Table 2]

[0033] Trials 6, 8, 10, 13, 16, 17 and 18 were not weldable, i.e. the weld range as defined in standard ISO 18278-2 was not achieved. As is strikingly demonstrated by Trials 7, 9 and 11, all trials according to the invention have a weld range of 1 kA or more, even for parts produced at very high press hardening temperatures and times.

[0034] It has also been observed that when using the method according to the present invention, electrode life is dramatically improved, with the electrode being capable of over 1000 welding cycles compared to 100 welding cycles with conventional methods.

Claims

1. 1. A welding method for the manufacture of an assembly of at least two steel substrates (3, 3') spot-welded together through at least one spot weld joint, comprising: A. Providing at least two metal substrates (3, 3'), wherein a first steel substrate (3) is a press-hardened steel part obtained by press-hardening a coated steel sheet, said coating comprising, prior to press-hardening, by weight, 7 to 12 wt. % silicon, 2 to 5 wt. % iron, optionally additional elements selected from Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Zr or Bi, the content of each additional element being less than 0.3 wt. %, and optionally remaining elements, the remainder being aluminum; B. Applying a spot welding cycle by a spot welder comprising welding electrodes (1, 1') and a spot welding power source (2) for applying current through the at least two metal substrates of step A, said spot welding cycle (21) comprising: - at least three pulsations (22, 32, 42), each having the same maximum pulsation current (Cp) applied through the at least two metal substrates to be joined together using a welding electrode connected to a spot welding power supply, the duration p of each pulsation being the same and set to between 20 and 60 ms; Each pulsation is followed by the same cooling time c, set between 30 and 50 ms. It consists of The welding parameter Wp value is at least 0.8, and Wp is: Wp = (t × c) / p is defined as t is the average thickness of the substrate in mm, c is the cooling time in ms, p is the pulsation duration in ms, step A welding method comprising:

2. The welding method of claim 1, wherein the maximum pulsation current (Cp) is set to 0.1 to 30 kA.

3. 3. The welding method according to claim 1, wherein the number of pulsations is set to 3 to 9.

4. The welding method according to claim 1 or 2, wherein the welding pressure is set to 50 to 650 daN.

5. 3. The welding method according to claim 1, wherein the welding frequency is set to 500 to 5000 Hz.

6. The spot welding cycle is - rectangular shape, - parabolic shape, - Triangular shape 3. The welding method of claim 1, further comprising pulsation having a setpoint shape (21) selected from the group consisting of:

7. 3. A welding method according to claim 1 or 2, wherein the second metal substrate (3') is a steel substrate or an aluminum substrate.

8. The welding method of claim 7 wherein the second steel substrate is a press-hardened steel component.

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