WO2020212741A1 - Procédé de soudage par points par résistance d'aciers à haute résistance revêtus de zinc - Google Patents

Procédé de soudage par points par résistance d'aciers à haute résistance revêtus de zinc Download PDF

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Publication number
WO2020212741A1
WO2020212741A1 PCT/IB2019/053252 IB2019053252W WO2020212741A1 WO 2020212741 A1 WO2020212741 A1 WO 2020212741A1 IB 2019053252 W IB2019053252 W IB 2019053252W WO 2020212741 A1 WO2020212741 A1 WO 2020212741A1
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Prior art keywords
zone
resistance spot
spot welding
sheet
comprised
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PCT/IB2019/053252
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English (en)
Inventor
Yacine BENLATRECHE
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Arcelormittal
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Priority to PCT/IB2019/053252 priority Critical patent/WO2020212741A1/fr
Priority to PCT/IB2020/052903 priority patent/WO2020212781A1/fr
Publication of WO2020212741A1 publication Critical patent/WO2020212741A1/fr

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K11/00—Resistance welding; Severing by resistance heating
    • B23K11/10—Spot welding; Stitch welding
    • B23K11/11—Spot welding
    • B23K11/115—Spot welding by means of two electrodes placed opposite one another on both sides of the welded parts
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K11/00—Resistance welding; Severing by resistance heating
    • B23K11/16—Resistance welding; Severing by resistance heating taking account of the properties of the material to be welded
    • B23K11/163—Welding of coated materials
    • B23K11/166—Welding of coated materials of galvanized or tinned materials
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K11/00—Resistance welding; Severing by resistance heating
    • B23K11/30—Features relating to electrodes
    • B23K11/3009—Pressure electrodes
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B32B15/012—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of aluminium or an aluminium alloy
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K2101/00—Articles made by soldering, welding or cutting
    • B23K2101/006—Vehicles
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K2101/00—Articles made by soldering, welding or cutting
    • B23K2101/18—Sheet panels
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K2103/00—Materials to be soldered, welded or cut
    • B23K2103/02—Iron or ferrous alloys
    • B23K2103/04—Steel or steel alloys

Definitions

  • the invention relates to a resistance spot welding process making it possible to suppress or reduce the cracks formation due to Liquid Metal Embrittlement, particularly in the industrial conditions required by the automotive industry.
  • Zinc or Zinc-alloy coated steel sheets are very effective for corrosion resistance and are thus widely used in the automotive industry.
  • the welding of certain steels can cause the apparition of particular cracks due to a phenomenon called Liquid Metal Embrittlement (“LME”) or Liquid Metal Assisted Cracking (“LMAC”).
  • LME Liquid Metal Embrittlement
  • LMAC Liquid Metal Assisted Cracking
  • This phenomenon is characterized by the penetration of liquid Zn along the grain boundaries of underlying steel substrate, under applied stresses or internal stresses resulting from restraint, thermal dilatation and/or phases transformations. It has been recognized that a higher stress level increases the LME risk. Since the stresses that are present during the welding itself depend in particular of the strength level of the base metal, it is experienced that welds made out of steels with higher strength are in general more sensitive to LME.
  • Document JP2005002415 proposes to interpose between the coating and the steel substrate, a nickel-based layer for minimizing the diffusion of zinc and thus suppressing the generation of LME cracks in the Heat affected Zone.
  • the fabrication of such coated steel sheet is more complex and costly.
  • spot welds with a small number of LME deep cracks so not to reduce the mechanical performance of the welds.
  • spot welds fabricated by such process have:
  • the invention relates to a resistance spot welding electrode with a tip zone (Z) having an active diameter d EL allowing current flow during resistance spot welding, comprising:
  • annular peripheral zone (P) having a flat surface, the width of annular peripheral zone (P) being dp, the central zone (C) protruding with respect to the peripheral zone (P), the protrusion height between the central zone (C) and the external part of the annular peripheral zone (P) being h ,
  • annular circular transition zone (T) concentric with the central zone (C), located between the central zone (C) and the peripheral zone (P), the width of the annular transition zone (T) being d-r,
  • dc, dp, dr, h are such as:
  • the circular central zone (C) has a hemispherical surface with a curvature radius higher than 30 mm.
  • the annular transition zone (T) is flat.
  • annular transition zone (T) comprises at least two distinct surfaces.
  • the resistance spot welding electrode is such as dei . is comprised between 4 and 12 mm.
  • the circular central zone (C) and the annular peripheral circular zone (P) are parallel planes.
  • the circular central zone (C) and the annular peripheral circular zone (P) are non-parallel planes forming a dihedral with an angle 23 less than 30°.
  • Another object of the invention is a process for resistance spot weld comprising the steps of :
  • h is the intensity making it possible to obtain a nugget diameter d NG higher than 3.5 x t 1/2 , t being the thickness of the thinnest sheet of the at least two steel sheets S1 and S2,
  • l 2 is comprised between 0.9 and 1.1 x l exp , wherein l exp is the intensity at which expulsion appears during welding.
  • the indentation depth ID after spot welding is such as:
  • the force applied during welding through the said pair of electrodes placed sensibly perpendicular and on the outer sides of the said at least two superimposed steel sheets S1 and S2, is comprised between 170 and 750 daN.
  • the thickness ti and t 2 of the at least two steel sheets S1 and S2 is comprised between 0.5 and 3mm.
  • At least one of the sheets S i or S 2 is preferably a High Formable steel sheet with a tensile strength (TS) higher than 800 MPa and a total elongation (TEL) such as (TS)x(TEL)>14000MPa%.
  • TS tensile strength
  • TEL total elongation
  • the composition of the High Formable steel substrate of at least one of the sheets Si or S 2 contains, in weight: 0.05% ⁇ C ⁇ 0.4%, 0.3% ⁇ Mn ⁇ 8%, 0.010% ⁇ Al ⁇ 3%, 0.010% ⁇ Si ⁇ 2.09%: with 0.5% ⁇ (Si+AI) ⁇ 3.5%, and optionally 0.001% ⁇ Cr ⁇ 1.0%, 0.001 % ⁇ Mo ⁇ 0.5%, 0.005% ⁇ Nb ⁇ 0.1%
  • the high Formable steel has preferably a retained austenite surface fraction between 5 and 30%.
  • the zinc-coating or zinc-alloy coating is obtained by hot-dip coating, or electrodeposition, or vacuum deposition.
  • the process for resistance spot welding is such as the spot welds fabricated by such process have:
  • the average value (L m ) av of maximum cracks depths in the spot welds is less than 0.15mm.
  • Another object of the invention is a resistance spot weld comprising at least two partially superimposed steel sheets S1 and S2, at least one of the steel sheets having a zinc-coating or a zinc-alloy coating, the nugget having a diameter (17) d NG , the distance between the extremity P notch (25) of the weld nugget forming a notch, and the position P AC 3 (20) of Ac3 isotherm in steel S1 and S2, being d (notch-Ac 3 ), the distance between the extremity P notch (25) of the weld nugget forming a notch, and the extremity Pindentation (24) of the flat portion of the indentation zone, being d( no tch-indentation), wherein:
  • the thickness ti and t 2 of the at least two steel sheets S1 and S2 is comprised between 0.5 and 3mm.
  • At least one of the sheets Si or S 2 is a High Formable steel sheet with a tensile strength (TS) higher than 800 MPa and a total elongation (TEL) such as (TS)x(TEL)>14000MPa%.
  • TS tensile strength
  • TEL total elongation
  • the composition of the High Formable steel substrate of at least one of the sheets Si or S 2 contains, in weight: 0.05% ⁇ C ⁇ 0.4%, 0.3% ⁇ Mn ⁇ 8%, 0.010% ⁇ Al ⁇ 3%, 0.010% ⁇ Si ⁇ 2.09%:, with 0.5% ⁇ (Si+AI) ⁇ 3.5%, and optionally, 0.001 % ⁇ Cr ⁇ 1.0%, 0.001% ⁇ Mo ⁇ 0.5%, 0.005% ⁇ Nb ⁇ 0.1 %, 0.005% ⁇ V ⁇ 0.2%, 0.005% ⁇ Ti ⁇ 0.1 %, 0.0003% ⁇ B ⁇ 0.005%, 0.001% ⁇ Ni ⁇ 1.0%, the remainder being Fe and unavoidable impurities.
  • the High Formable steel has preferably a retained austenite surface fraction comprised between 5 and 30%.
  • the zinc-coating or zinc-alloy coating is obtained by hot-dip coating, or electrodeposition, or vacuum deposition.
  • the spot weld has no Liquid Metal Embrittlement cracks in sheet S1 deeper than L/2, no Liquid Metal Embrittlement cracks in sheet S2 deeper than t 2 /2, less than 20% of spot welds with a crack depth in sheet S1 comprised between 0.2mm and ti/2, less than 20% of spot welds with a crack depth in sheet S2 comprised between 0.2mm and t 2 /2.
  • the average value (L m ) av of maximum cracks depths in the spot weld is less than 0.15mm.
  • Another object of the invention is the use of a spot weld fabricated according to the method described above, for the manufacturing of structural part of automotive vehicle.
  • Another object of the invention is the use of a spot weld as described above for the manufacturing of structural part of automotive vehicle.
  • FIG. 1 presents schematically a cross-cut of the tip of a welding electrode illustrating characteristic geometrical features of the invention.
  • FIG. 2 presents schematically a cross-cut of the tip of a variant of welding electrode illustrating characteristic geometrical features of the invention.
  • FIG. 3 presents schematically a cross cut-of the tip of another variant of welding electrode illustrating characteristic geometrical features of the invention.
  • FIG. 4 the figure 4 illustrates LME cracks that can appear after the welding of a Zn-coated sheet.
  • FIG. 7 is a schematic cross-cut of a spot weld illustrating some characteristic geometrical features.
  • figure 8 illustrates how a specific combination of geometrical features defined in figure 7 makes it possible to obtain high LME resistance.
  • At least two steel sheets S1 and S2 with respective thickness ti and t 2 are provided and superimposed at least partly, in order to be in contact. These sheets may have the same thickness or different thicknesses.
  • ti and t 2 are comprised between 0.5 and 3mm, which is a typical thickness range used in the automotive industry.
  • At least one of these sheets S1 and S2 is a zinc or zinc-alloy coated sheet, the latter expression designating coatings wherein the Zn content is higher than 50% in weight.
  • the coating can be obtained by Hot-Dip- Galvanizing (“Gl”) or by hot-dip galvanizing immediately followed by a heat- treatment at about 500-570°C so to cause diffusion of iron in the coating and to obtain“galvannealed” or“GA” coating containing about 7-14%Fe.
  • It can be also a zinc or zinc-alloy coating obtained by an electroplating process or by a Vacuum deposition process, the latter being possibly a Jet Vapor Deposition process.
  • the Zn-alloy can be also a Zn-Mg-AI coating such as for example a Zn-3%Mg-3.7%AI, or a Zn-1 ,2%AI-1.2%Mg coating.
  • the Zn or Zn-alloy coated sheet is made out of a High-Formable steel with a tensile strength (TS) higher than 800 MPa and a total elongation (TEL) such as (TS)x(TEL)>14000MPa%.
  • TS tensile strength
  • TEL total elongation
  • the microstructure of the Zn-coated sheet or sheets contains, in surface fraction between 5 and 30% of retained austenite, such constituent making it possible to increase the formability of the steel sheet.
  • these coated steels may be for example TRIP (Transformation Induced Plasticity) steels, CFB (Carbides Free Bainite) steels, or Q-P (Quenched and Partitioned) steels.
  • TRIP Transformation Induced Plasticity
  • CFB Carbides Free Bainite
  • Q-P Quadenched and Partitioned steels.
  • the composition of the High- Formable steel sheets may contain:
  • the carbon content is in the range between 0.13 and 0.25%, which makes it possible to achieve a tensile strength higher than 1180 MPa.
  • Manganese is a solid solution hardening element which contributes to obtain a tensile strength higher than 800 MPa. Such effect is obtained when Mn content is at least 0.3% in weight. However, above 8%, its presence contributes to the formation of a structure with excessively marked segregation bands which can adversely affect the hardenability of the welds and the use properties of the automobile structural part. The coatability is also adversely reduced.
  • the manganese content is in the range between 1.4% and 4% to achieve these effects. This makes it possible to achieve satisfactory mechanical strength without increasing the difficulty of industrial fabrication of the steel and without increasing the hardenability in the welded alloys which would adversely affect the weldability of the sheet claimed by the invention.
  • silicon reduces the carbides precipitation during the annealing after cold rolling of the sheet, due to its low solubility in cementite and due to the fact that this element increases the activity of carbon in austenite.
  • the enrichment of austenite in carbon leads to its stabilization at room temperature and to the apparition of a Transformation Induced Plasticity (“TRIP”) behavior which means that the application of a stress, during forming for example, will lead to the transformation of this austenite into martensite.
  • TRIP Transformation Induced Plasticity
  • Si is higher than 2.09%, strongly adhering oxides could be formed during annealing before hot dip galvanizing, which could lead to surface defects in the coating.
  • Silicon content above 0.5% contributes to an efficient stabilization of austenite, while Si content above 0.7% contributes to obtain a surface fraction of retained austenite comprised between 7 and 30%.
  • Aluminum must be comprised between 0.010 and 3.0%. With respect to the stabilization of retained austenite, aluminum has an influence that is relatively similar to the one of the silicon. However, since aluminum promotes efficiently the formation of ferrite at high temperature, an excessive aluminum addition would increase the Ac3 temperature (i.e. the temperature of complete steel transformation into austenite during heating) during the annealing step, and would therefore make the industrial process expensive in terms of electric power required for annealing. Thus, Al content is less than 3.0%.
  • retained austenite between 5 and 30% at room temperature makes it possible to obtain high total elongation.
  • the formability is particularly high when the surface fraction of retained austenite is comprised between 7 and 30%.
  • a sufficient stabilization of the austenite is obtained through the addition of silicon and/or aluminum in the steel composition, in quantities such as : (Si+AI) > 0.5%. If (Si+AI) ⁇ 0.5%, the fraction of retained austenite could be below 5%, thus the ductility and strain hardening properties in cold-forming are insufficient. However, if (Si+AI)>3.5%, the coatability and the weldability are impaired.
  • the steel may also contain optional elements:
  • Chromium hardens and refines the microstructure and makes it possible to control the formation of proeutectoid ferrite during the cooling step after holding at the maximal temperature during the annealing cycle.
  • ferrite when present in surface fraction higher than 40%, increases the risk that the tensile strength is lower than 800 MPa.
  • the chromium content is higher than 0.001% and less than 1.0% for reasons of cost and for preventing excessive hardening.
  • molybdenum in quantity comprised between 0.001% and 0.5% is efficient for increasing the hardenability and stabilizing the retained austenite since this element delays the decomposition of austenite.
  • the steels may optionally contain elements susceptible to precipitate under the form of carbides, nitrides, or carbonitrides, thus able to provide precipitation hardening.
  • the steels may contain niobium, titanium or vanadium: Nb and Ti in quantity comprised between 0.005 and 0.1 %, and V in quantity comprised between 0.005 and 0.2%.
  • the steels may optionally contain nickel, in quantity comprised between
  • the steels may optionally content also boron, in quantity comprised between 0.0003 and 0.005%.
  • B By segregating at the grain boundary, B decreases the grain boundary energy and is thus beneficial for increasing the resistance liquid metal embrittlement.
  • the balance in the composition consists in iron and residual elements resulting from the steelmaking.
  • Cu, S, P and N at least are considered as residual elements or unavoidable impurities. Therefore, their contents are less than 0.03% for Cu, 0.003% for S, 0.02% for P and 0.008% for N.
  • the electrodes are made out of copper or copper-alloy, i.e. copper with a small amount of alloy such as Cr, Zr, Mo... so to modify hardness or conductivity of the electrode.
  • the electrodes are water-cooled in order to limit their progressive shape deformation because of the successive heat welding thermal cycles, and to efficiently cool the weld nugget during the holding step of the welding cycle.
  • the electrodes have a tip zone (Z) with an active diameter dei . in contact with the steel sheet during the welding cycle. According to the usual thickness of Zn-coated steel sheets implemented in the automotive industry, the active diameter dei . is generally comprised between 4 and 12 mm.
  • the figure 1 illustrates the schematic cross-cut of the tip of an electrode which is axisymmetric and comprises different zones:
  • figure 1 illustrates the case of a flat zone, it is understood that the zone (C) can have also an hemispheric shape. In this case, the radius curvature of the hemispheric zone is larger than 30mm so to avoid a contact zone with the sheet, with a surface which would be too much reduced.
  • annular peripheral zone (P) (numbered 2 in figure 1) concentric with the circular zone (C)
  • This zone has a flat surface, its width is dp (numbered 21 in figure 1) and such as: d P >0.
  • the central zone (C) protrudes with respect to the peripheral zone (P), the protrusion height between the central zone (C) and the annular peripheral zone (P) being h (referenced as 5 in figure 1) and such as: h>0.
  • the protrusion is measured by the difference between the height of the highest portion of the central zone (C) and the height of the external part of the peripheral zone (numbered 22 in figure 1)
  • the zones (C) and (P) can be parallel planes.
  • the planes (P) and (C) form a dihedral as illustrated in figure 3.
  • the angle 23 between these planes is less than 30°. If the angle 23 is higher, there is a risk that for some steels, the strain at external part of the zone (P) is too high, thus inducing deep LME cracks.
  • annular transition zone (numbered 3 in figure 1) which is concentric with the central zone (C) and located between the central zone (C) and the peripheral zone (P), the width of the annular transition zone (T) being d T (numbered 31 in figure 1)
  • the transition zone may be flat, as illustrated in figure 1. This geometry offers the advantage of simplicity for mechanical machining.
  • (T) can comprise at least two distinct flat surfaces, and in particular a staircase geometry.
  • Figure 2 illustrates the case wherein two horizontal flat surfaces are present in the annular transition zone.
  • LME cracks appear mainly in the zones located at the peripheral part of the nugget created by the welding operation. These cracks appear more likely in some Zn-coated steels with a tensile stress higher than 800MPa because higher tensile stress increases the level of applied stresses or strains during welding, thus the probability of LME occurrence is higher. Steels with higher carbon and silicon contents have been found more susceptible to LME.
  • Figure 4 illustrates an example of LME crack (numbered as 9 in figure 4) which has occurred at the periphery of the indented zone in a Zn-coated steel with tensile stress higher than 800MPa.
  • the initiation site (numbered as 10) of the deep crack corresponds to a highly strained zone, due to the use of a conventional electrode which has sensibly a flat tip surface.
  • the inventor has searched to reduce the level of applied stress or strains during spot welding through the kinematics of geometrical, mechanical and thermal interactions between the electrodes and the Zn-coated sheets during the welding sequence.
  • the figures 5 and 6 illustrate schematically the possible succession of phenomena that occur when welding according to the invention (figure 5) by comparison with a reference electrode (figure 6)
  • the welding cycle can be presented in five successive steps (l-V)
  • the figures 5 and 6 illustrate schematically the sequence of two sheets 27 and 28 that are spot welded.
  • At least sheet 27 is a zinc-coated or a zinc-alloy coated steel sheet.
  • stage (I) (figure 5 and 6)
  • the sheets are pressed together with the electrodes. No current flow occurs at his stage.
  • stage (II) the current begins to flow, heat generation is mainly initiated at the interface between the sheets since the electric resistance is the highest in this contact zone.
  • the current flows mainly through the central part (C) of the electrode (figure 5. II)
  • a nugget (15, 16) formation starts.
  • stage III When current continues to flow (stage III), the nugget volume increases and an indentation, i.e. a depression at the surface of the weld under the electrode surface in contact with the sheet, appears progressively.
  • the transition zone (T) of the electrodes sloped or staircase shaped, comes progressively into contact with the sheet and increases the surface for current flow, making it possible the nugget to grow with limited amount of strain in the steel sheet.
  • the strain rate experienced in the final contact zone 13 is also reduced, which further contributes to restriction or prevention of LME.
  • the central zone diameter dc must not be such as: dc/d Ei _ £ 0.20, otherwise, the intensity may not be sufficient to create a nugget of sufficient diameter, and the current density may be too high, increasing thermal gradient which favours LME.
  • the welding parameters are chosen in order to avoid interfacial failure during shear tensile test of the spot welds.
  • a satisfactory pullout mode is obtained when the nugget diameter d G of the spot weld is higher than 3.5 x t 1/2 , t being the thickness of the thinnest sheets in a stackup weld.
  • this result is achieved by using a current intensity of not less than h, h being the intensity for obtaining a nugget diameter d G of 3.5 x t 1/2 .
  • the intensity is comprised between 0.9 and 1.1 l ex , xp being the intensity at which expulsion of liquid metal starts to be observed in resistance spot welding.
  • xp being the intensity at which expulsion of liquid metal starts to be observed in resistance spot welding.
  • the force applied during welding through the pair of electrodes placed sensibly perpendicular on the steel sheets is comprised between 170 and 750 daN.
  • the inventor has also evidenced that the welding process of the invention is associated to specific geometrical features in the spot weld, making it possible to obtain high resistance to LME cracking.
  • the figure 7 illustrates schematically a resistance spot weld joining two superposed steel sheets.
  • the nugget 26 is the zone that has been melted and thereafter solidified during the welding process.
  • This nugget has a diameter d NG referred as 17 in figure 7.
  • the location P no tch corresponds to the extremity of the nugget intersecting with the planes of the steel sheets and is referred as 25 in this figure.
  • the location P AC 3 corresponds to the position of the isotherm Ac3 in the steel sheet, i.e. to the location 20 of complete transformation into austenite during the heating in the welding cycle. Its position can be determined through polishing of the cross-cut of the weld and further appropriate etching with a reagent such as Nital, by means which are known per se.
  • An indentation zone is present at the surface of the weld which includes a flat central zone 30 and a peripheral zone 29 forming the transition with the steel sheet.
  • the limit P lantation between the central and the peripheral part of the indentation is referred as 24 in figure 7.
  • Figure 8 illustrates the influence of such features, obtained by welding High Formable steels, 1 4mm thick, with Zn coating obtained by electrodeposition or Jet-Vapor Deposition.
  • the steels have a tensile strength (TS) higher than 800 MPa and a total elongation (TEL) such as (TS)x(TEL)>14000MPa%, and a retained austenite surface fraction of 12%.
  • TS tensile strength
  • TEL total elongation
  • Table 1 Compositions of Zn coated steels
  • Steel A is a Carbide-Free-Bainite steel with an electrodeposited Zn coating, 7pm thick. Its tensile strength UTS and total elongation are respectively 1235 MPa and 14.4% (JIS Standard) in the transverse direction. Its microstructure contains 12% of residual austenite, 7% of martensite and 81% of bainite in surface fraction.
  • Steel B is obtained by a Quenching and partitioning treatment and has a Zn coating of 8 pm deposited by Jet Vapor Deposition. Its tensile strength UTS and total elongation are respectively 1131 MPa and 14.1 % (JIS standard) in the transverse direction. Its microstructure contains 12% of residual austenite, 67% bainite, 16% partitioned martensite and 5% of fresh martensite, in surface fraction.
  • a mild steel sheet coated with Zn (Gl coating) has been used for welding together with the steels A or B.
  • Geometrical and welding parameters have been chosen so as to exacerbate the eventual occurrence of Liquid Metal Embrittlement and to evidence clearly the phenomena: by the spot welding of three superposed sheets and the creation of a stackup configuration with increased thickness, the sensitivity to LME cracking is raised.
  • heterogeneous welding is performed by using one steel sheet A or B partially superposed to two sheets of Zn-coated mild steel, 1.75mm thick, with a composition containing: 0.032%C, 0.008%Si, 0.222%Mn, 0.052%AI,
  • Example 1 Mild steel is chosen because its spot welding requires higher current level to get proper welds than the steels having a tensile stress higher than 800MPa. This higher current level induces higher heat input and as a consequence an increase of probability of LME occurrence during the welding of high resistance steels. Thus, the severity of the welding conditions is increased. In the stackups, the welding is performed in such a way that the Zn-coated steel sheet having a tensile strength higher than 800 MPa has one surface in contact with a welding electrode. The eventual cracks are more prone to occur in the indentation zone created by the welding electrode at the sheet surface.
  • the figure 4 shows an example of such crack: 7 and 8 in the figure are mild steel sheets, while 6 is steel A.
  • a LME crack (referenced by 9) has been initiated at the outer part of the indented zone, here referenced as 10.
  • Example 1 Example 1
  • Resistance spot welding of steel A has been performed by using various geometries of copper-alloy water-cooled electrodes. Characteristic features of the geometry of the electrodes (d c , d p , dr, h) are reported in table 2. The surfaces of zones (T) and (P) are flat whereas the zone (C) has an hemispherical shape with a curvature radius of 50mm.
  • Electrode N°15 is a reference electrode with a hemispherical surface of 50mm radius, without protrusion, which comes immediately in full contact with the sheet during the welding cycle.
  • the welding conditions were selected accord to ISO-18278-2 standard, using the following parameters:
  • the value of intensity l exp i.e. the intensity beyond which expulsion appears during welding, has been determined and reported in table 3.
  • welding has been performed by using a current I comprised between 0.9 and 1.1 l exp .
  • the nugget diameter d NG is greater than 3.5 x t 1/2 , t being the thinnest of the two thickest sheets of the assembly.
  • the welding intensity I is higher than h , and lower than l 2 as defined previously.
  • a circular depression, i.e. indentation is present on the surface of the sheets, and corresponds to a deformation of the surface of the spot weld caused by the electrode.
  • the indentation depth (ID) has been measured at its center and reported in table 3.
  • the conditions of the invention make it possible to obtain nugget diameters which are comparable with the one obtained with the reference electrode 15, and which are largely higher than 3.5 x t 1/2 .
  • Electrode n°18 is a reference electrode with a central surface without protrusion.
  • the central surface of all electrodes has hemispherical shape with a curvature radius of 50mm.
  • Test reference 16A refers to the spot weld manufactured from steel A with electrode n°16.
  • Other welding conditions are:
  • the nugget diameter d G is greater than 3.5 x t 1/2 , t being the thinnest of the two thickest sheets of the assembly.
  • the reference electrode does not provide satisfactory results since LME cracks are initiated at the location 13 (see figure 6) due the high strain amount and strain rate in this zone.
  • Replacing electrode 18 by electrode 17, with a central zone and a transition zone including staircase geometry brings an improvement for the steel B since the LME cracks are reduced to such an extent that criteria (C1) and (C2) are fulfilled.
  • this result is not achieved with steel A, the composition of which is more sensitive to LME cracking than steel B.
  • the dihedral angle a (23) between the central and the peripheral zone is too high, which causes the value of (dp-h) to be outside of the range of the invention.
  • the electrode 16 has a transition staircase zone which contributes to the progressiveness of the indentation, together with an angle a less than 30°.
  • resistance spot welding in such conditions corresponds to the invention and generates welds of satisfactory quality with criteria (C1) and (C2) which are met for both steels A and B.
  • coated steel parts manufactured according to the invention can be used with profit for the fabrication of structural or safety parts of vehicles.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Resistance Welding (AREA)

Abstract

L'invention concerne une électrode de soudage par points par résistance ayant une zone de pointe (Z) ayant un diamètre actif dEL permettant un écoulement de courant pendant le soudage par points par résistance, comprenant une zone (1) centrale circulaire (C) dotée d'une surface plane ou hémisphérique, le diamètre de la zone (1) centrale (C) étant dc, une zone (2) circulaire périphérique annulaire (P) concentrique à la zone (1) circulaire (C) ayant une surface plane, la largeur de la zone (2) périphérique annulaire (P) étant dP, la zone (C) centrale faisant saillie par rapport à la zone (2) périphérique (P), la hauteur (5) de saillie entre la zone (1) centrale (C) et la partie externe (22) de la zone périphérique annulaire (P) étant h, une zone (3) de transition circulaire annulaire (T) concentrique à la zone (1) centrale (C), située entre la zone (1) centrale (C) et la zone (2) périphérique (P), la largeur (31) de la zone (3) de transition annulaire (T) étant dT, où dC, dP, dT, h sont tels que : dEL= dC+2dT+2dP, dc>0, dp>0, dT >0, h>0, 0,20 < dc/dEL < 0.60, 0,05 < (dp-h)/ (dEL < 0,25.
PCT/IB2019/053252 2019-04-19 2019-04-19 Procédé de soudage par points par résistance d'aciers à haute résistance revêtus de zinc WO2020212741A1 (fr)

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PCT/IB2020/052903 WO2020212781A1 (fr) 2019-04-19 2020-03-27 Procédé pour souder par points par résistance des aciers à haute résistance revêtus de zinc

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JP7355281B1 (ja) * 2022-06-03 2023-10-03 Ｊｆｅスチール株式会社 溶接継手、溶接部材およびその製造方法、ならびに、抵抗スポット溶接方法
WO2023233704A1 (fr) * 2022-06-03 2023-12-07 Ｊｆｅスチール株式会社 Joint soudé, élément de soudage, son procédé de fabrication et procédé de soudage par points par résistance
WO2023233705A1 (fr) * 2022-06-03 2023-12-07 Ｊｆｅスチール株式会社 Joint soudé, élément de soudage et son procédé de production, et procédé de soudage par points par résistance

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JP7355282B1 (ja) * 2022-06-03 2023-10-03 Ｊｆｅスチール株式会社 溶接継手、溶接部材およびその製造方法、ならびに、抵抗スポット溶接方法
JP7355281B1 (ja) * 2022-06-03 2023-10-03 Ｊｆｅスチール株式会社 溶接継手、溶接部材およびその製造方法、ならびに、抵抗スポット溶接方法
WO2023233704A1 (fr) * 2022-06-03 2023-12-07 Ｊｆｅスチール株式会社 Joint soudé, élément de soudage, son procédé de fabrication et procédé de soudage par points par résistance
WO2023233705A1 (fr) * 2022-06-03 2023-12-07 Ｊｆｅスチール株式会社 Joint soudé, élément de soudage et son procédé de production, et procédé de soudage par points par résistance

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