Method for laser welding

Abstract

The present disclosure relates to methods for joining first steel blank to a second steel blank to form a combined blank, comprising: positioning the first and second steel blanks such that the first steel blank at least partially overlaps the second steel blank in an overlap region, and laser welding the first steel blank to the second steel blank with one or more weld seams in the overlap region. An absolute welding speed of the laser welding is 10 – 30 mm / sec, and the first steel blank and / or the second steel blank have a corrosion protective coating comprising aluminium. The present disclosure further relates to methods for press hardening the combined blank.

Description

METHOD FOR LASER WELDING

[0001] The present application claims the benefit of EP24382395.2 filed on April 16th, 2024.

[0002] The present disclosure relates to methods for welding, and more particularly relates to remote laser welding and lap joint welding. The present disclosure further relates to methods for joining blanks or components in a lap joint.BACKGROUND

[0003] In the automotive industry, the development and implementation of lightweight materials or components is becoming more important in order to satisfy criteria for manufacturing lighter vehicles. The demand for weight reduction is especially driven by the goal of reduction of CO2 emissions. Additionally, the growing concern regarding occupant safety also leads to the adoption of materials which improve the integrity and the energy absorption of the vehicle during a crash.

[0004] Press hardening, also known as Hot Forming Die Quenching (HFDQ) or “hot stamping” typically uses boron steel sheets to create stamped components with Ultra-high Strength Steel (UHSS) properties, with tensile strengths of e.g., 1.500 MPa or 2.000 MPa or even more. The increase in strength allows for a thinner gauge material to be used, which results in weight savings over conventionally cold stamped mild steel components. Throughout the present disclosure UHSS may be regarded as a steel having an ultimate tensile strength of 1.000 MPa or more, particularly after a press hardening process.

[0005] In a HFDQ process, a blank to be hot formed may be heated to a predetermined temperature e.g., austenization temperature or higher (and particularly between Ac3 and an evaporation temperature of e.g. a coating of the blank). A furnace system may be used for this purpose. By heating the blank, the strength of the blank is decreased, and deformability increases i.e. to facilitate the hot stamping process. This is known as a direct hot stamping process. In an indirect hot stamping process, a blank is subjected to a forming operation at room temperature and is subsequently heated and quenched.

[0006] There are several known Ultra High Strength steels (UHSS) for hot stamping and hardening. The blank to be hot formed may be made e.g., of a boron steel, which may be coated or uncoated, such as Usibor® 1500 (22MnB5) or Usibor® 2000 (37MnB4 or 37MnB5) commercially available from ArcelorMittal.

[0007] Typical vehicle components that may be manufactured using the HFDQ process include door beams, bumper beams, cross / side members, A / B pillar reinforcements, front and rear rails, seat crossmembers and others.

[0008] UHSS may exhibit tensile strengths as high as 1.500 MPa, or even 2.000 MPa or more, particularly after a press hardening operation. Once hardened, a UHSS may have a martensitic microstructure. This microstructure enables an increased maximum tensile strength and yield strength per weight unit.

[0009] In addition to the Ultra High Strength Steels mentioned before, more ductile steels may also be used in parts of the structural skeleton requiring energy absorption. These steels may be used in hot stamping processes but will not obtain a martensitic microstructure in the process. Ductibor ® 1000 is an example of a suitable, more ductile steel.

[0010] Prior to a press hardening process, several blanks may be joined to each other to form a Tailor Welded Blank (TWB) comprising different thicknesses or different materials. The idea of the use of TWB is that a resulting component can be optimized in terms of weight and provide tailored strength and stiffness where needed. A TWB is generally formed by edge-to-edge butt welding of the different blanks. TWB may be made e.g. of several steel blanks, with different compositions and / or different thickness.

[0011] In other cases, blanks may be joined to each other in a lap weld joint: one blank partially or completely overlaps another blank. A combined blank may have a thickness tailored to provide increased strength and stiffness in specific portions of the blank and the component obtained after forming. The combined blank may be subjected to e.g. a hot stamping or cold stamping process.

[0012] A patchwork blank is an example of a combined blank formed with a lap weld joint. Another example is a combined blank to form e.g., a unitary door ring such as disclosed in W02020 / 002335. Such a combined blank may also be called an Overlap Patch Blank.

[0013] In these cases, resistance spot welding (or simply “spot welding”) is generally used to join the individual blanks to each other. A disadvantage of spot welding is that the process is relatively slow compared to e.g., laser welding. It has further been found that in hotstamping processes that are carried out after joining the blanks to each other, some of the spot welds do not have sufficient shear strength (depending on how much deformation takes place during stamping).

[0014] Steel blanks, and particularly boron steel blanks, may have a corrosion protective coating, specifically an AlSi coating or Zinc based coating. Particularly the aluminium of an aluminium-silicon coating may be problematic if mixed in the weld pool. The presence of aluminium may promote the creation of ferrite and perlite in a subsequent hot stamping process and thereby can affect the creation of a martensitic microstructure which is to provide high strength and stiffness. This problem is generally avoided or reduced in the case of spot welding: the pressure applied to the blanks in the case forces the coating to the edge of the weld zone as melting takes place. In the case of remote laser welding however, it has been found that aluminium concentrations in the weld zone occur, leading to weak spots in the resulting product after hot stamping.

[0015] One known solution is to remove part of the coating or the whole coating through ablation prior to positioning the blanks on top of each other. This however adds a step to a manufacturing process, thereby increasing production time and cost. Other solutions include the use of an alternative coating for the steel blanks, or e.g. welding with a filler material (but this is hardly possible in the case of a lap joint).

[0016] The present disclosure provides examples of methods of laser welding which can overcome at least some of the aforementioned drawbacks.SUMMARY

[0017] In a first aspect, a method for joining a first steel blank to a second steel blank to form a combined blank is provided. The method comprises positioning the first and second steel blanks such that the first steel blank at least partially overlaps the second steel blank in an overlap region, and laser welding the first steel blank to the second steel blank with one or more weld seams in the overlap region. An absolute welding speed of the laser welding is 10 - 30 mm / sec, and the first steel blank and / or the second steel blank have a corrosion protective coating comprising aluminium.

[0018] In accordance with this aspect, a method for joining blanks in a lap weld joint is provided which does not require the removal of the corrosion protective coating and still can avoid aluminium concentrations in the weld zone. It has surprisingly been found that by lowering a weld speed to 10 - 30 mm / sec (as opposed to around 50, 60 or 70 mm / sec whichis usual in the art), the coating enters the weld zone to a much lesser extent. Without wishing to be bound to a specific theory, it is believed that the lower weld speed allows the coating to be melted prior to melting of the blanks and thereby allows the coating to move to the edges and outside of the weld zone. With little or no aluminium in the weld zone, the formation of martensite can be achieved in a subsequent hardening process.

[0019] This result can be obtained without the need for ablation or other treatment to remove (part of) the corrosion protective coating. The use of remote laser welding, allows for a faster welding process compared to resistance spot welding, as well as a reduced plant layout, simpler fixtures and reduced costs.

[0020] The term “absolute welding speed” is herein used to indicate the absolute value of the welding speed vector. Welding patterns are known in which a weld spot wobbles or oscillates, e.g. transverse to the weld path. In such a case, the absolute welding speed can vary within an oscillation and would have to be determined as the resultant taking both a component of welding speed along the weld path, as well as a transverse component into account.

[0021] Throughout the present disclosure, the use of filler material may be avoided i.e. laser welding does not include the use of a filler material i.e. the welds are formed entirely by melting parts of the base steel blanks, without the addition of filler materials to the base metals.

[0022] The blanks may be joined in the overlap region with one or more weld seams that result from remote laser welding. Remote laser welding is a welding process that utilizes laser technology to join materials together from a distance. In traditional welding processes, the welder is in close proximity to the workpiece, manipulating the welding equipment directly. Remote laser welding, on the other hand, involves using a laser beam to perform the welding operation from a remote location, often with the help of advanced robotic systems.

[0023] Different types of lasers may be used in examples of the present disclosure, e.g. a CO2 laser or a solid state laser. Different types of solid state laser may be used including an Nd: YAG laser, a fibre laser or a disc laser.ln some examples, the first steel blank and the second steel blank both have a corrosion protective coating comprising aluminium.

[0024] In some examples, the corrosion protective coating comprises 80% or more by weight of aluminium. Particularly AlSi coatings are known comprising about 90% (by weight) of aluminium and about 10% of silicon. In some cases, magnesium is added to the coating.

[0025] In examples, the laser welding may be carried out with a single spot. In accordance with the present disclosure, there is no need for multiple spots, or complicated movement patterns which have been taught in the prior art to reduce the coating in the weld zone.

[0026] In some examples, a laser power for the laser welding may be 1 - 8kW, specifically 1.5 - 6kW, and more specifically 1.5 - 3kW. A suitable laser power may be determined by the skilled person in accordance with circumstances: an increased thickness of blanks leads to an increase in required laser power to melt the blanks. In general, the laser power may be smaller than taught in the prior art, since the laser speed is also lower: the lower speed leads to more heat being introduced in a given area. In a non-limiting example, a laser power of 2 - 2,4 kW may be used to join steel blanks which in the overlap region have a thickness of about 3 mm (e.g. one blank with a thickness of 1 .2 mm and another blank with a thickness of 1 .8 mm or two blanks with a thickness of about 1 .5 mm).

[0027] In some examples, the absolute welding speed of the laser welding may be 20 - 30 mm / sec, specifically 20 - 25 mm / sec.

[0028] In some examples, one or more of the weld seams may be substantially rectilinear. Particularly, multiple rectilinear weld seams may be arranged along the edges of the overlap region. By welding along the edges, the combined blank performs more like a single blank (during and after stamping).

[0029] In some examples, the weld seams extend entirely through a thickness of the second steel blank. In other examples, the weld seams extend only partially through a thickness of the second steel blank. The latter possibility may be preferred for components which will be visible in the end product.

[0030] In some examples, the resulting combined blank may be cold stamped to form a cold stamped product and the cold stamped product may be heated to above an austenization temperature and subsequently quenched. In other examples, the combined blank may instead be subjected to a process hot stamping, i.e. heating and subsequent forming and quenching at the same time. Both direct and indirect press hardening processes are thus covered by the present disclosure.BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Non-limiting examples of the present disclosure will be described in the following, with reference to the appended figures, in which:Figures 1 A - 1 D schematically illustrate a lap joint welding;Figure 2A schematically illustrates an example of manufacturing an automotive component; andFigures 2B - 2F illustrate the technical effects brough about in examples of the present disclosure;Figures 3A - 3C schematically illustrate examples of blanks and components which may be manufactured with the method of figure 2A; andFigure 4 illustrates a substantially G-shaped weld which may be used in embodiments of the present disclosure.

[0032] The figures refer to example implementations and may only be used as an aid for understanding the claimed subject matter, not for limiting it in any sense.DETAILED DESCRIPTION OF EXAMPLES

[0033] In these figures, the same reference signs have been used to designate matching elements.

[0034] Figures 1A - 1 D schematically illustrate examples of lap joint welding. Figure 1A illustrates a cross-sectional view, whereas figure 1 B illustrates a top view. A first steel blank 10 is positioned on top of second steel blank 12 such that an overlapping region 14 is formed. In some cases, the overlap may be partial (as illustrated in figure 1), and in other cases, the overlap may be complete. In the case of a complete overlap the first component is entirely positioned on top of the second component. This may be the case of a patchwork blank, wherein a local reinforcement is formed by a small “patch” blank being arranged on top of another blank.

[0035] The steel blanks may be of press hardenable steel such as boron steel e.g., 22MnB5, 37MB5 or 38MnB5. One or both steel blanks may include a protective coating such as an aluminium-silicon coating.

[0036] As illustrated in figure 1 B, the first and second components may traditionally be joined to each other through a plurality of spot welds in the overlapping region.

[0037] In accordance with the present disclosure, as illustrated in figure 1 C, the first and second components may be joined to each other through a plurality of laser welds.

[0038] In examples, the blanks may have a thickness of e.g., 0,8 - 3 mm, specifically 0,8 - 2 mm. The blanks may have the same thickness or different ones. It will be clear that more than two blanks may be joined to each other in the same fashion.

[0039] In the example of figure 1 C, along each of the edges of the overlap region, rectilinear weld seams 16A - 16D are arranged to join the first blank to the second blank. Remote laser welding may be used for these weld seams. If one or both steel blanks have a protective coating, one or two layers of coating are arranged between the steel blanks. In order to avoid the coating in the weld zone, remote laser welding of these weld seams is carried out with a welding speed of 10 - 30 mm / sec, specifically 15 - 25 mm / sec. In other examples, welding may be carried out at 20 - 30 mm / sec, more specifically 20 - 25 mm / sec.

[0040] Similar weld seams are shown in figure 1 D, where along each edge of the overlap region, shorter straight welds (“weld stitches”) are arranged, to join the first blank to the second blank.

[0041] Figure 2A illustrates a flowchart of an example of a method 30 for manufacturing a component e.g., for a structural framework of a vehicle. The component may be e.g. a B- pillar, a door ring, a roof ring, or a bumper reinforcement or bumper assembly, or other.

[0042] Method 30 comprises, at block 32, the manufacture of multiple blanks. The blanks may be cut from a steel coil in one example. Method 30 then further comprises a method for joining a first blank to a second blank comprising, at block 34, positioning the first and second blanks such that the first blank at least partially overlaps the second blank in an overlap region. The method further comprises, at block 36, laser welding the first blank to the second blank with one or more weld seams in the overlap region, at an absolute welding speed of 10 - 30 mm / sec.

[0043] In specific examples, the absolute welding speed may be substantially constant, i.e. vary less than 5 mm / sec along a weld seam. In specific examples, the absolute welding speed may vary but always remain below the threshold of 30 mm / sec.

[0044] Method 30 in this example further comprises subjecting the resulting combined blank to a hot stamping operation at block 38. The hot stamping may be direct hot stamping or indirect hot stamping.

[0045] Figures 2B and figure 2C show cross-sectional view of a weld zone of a weld seam of overlapping steel blanks, before a hot stamping operation. In both figure 2B and figure 2C a blank with a thickness of 1 ,8 mm is superposed on a blank with a thickness of 1 ,2 mm. Both blanks are made of 22MnB5 steel with an AlSi coating (about 90% aluminium, about 10% silicon). In both figures 2B and 2C, the weld was carried out using remote laser welding, using a ND:Yag laser with a laser power of 2,1 kWand a single spot. In both cases, the weld seam was straight i.e. rectilinear.

[0046] The laser speed in the case of figure 2B was 65 mm / sec and the laser speed in the case of figure 2C was 20 mm / sec. In the cross-sectional view of figure 2B, at the edges of the weld zone, in between the first and second blanks, an amount of aluminium may be spotted. In figure 2C, it may be seen that a larger amount of aluminium accumulates at the edge and outside the weld zone. There is thus less aluminium in the weld zone which favours martensite formation in the later hot stamping process.

[0047] It may further be seen that the weld zone in figure 2C is markedly wider than in the case of figure 2B. This is caused by the lower speed of the laser during welding. Even though potentially more coating is melted in this case, the coating does not enter the weld zone or enters the weld zone to a lesser extent.

[0048] Figures 2D - 2F illustrate further experiments of welding carried out on superposed blanks, the lower blank having a thickness of 1 ,2 mm and the upper blank having a thickness of 1 ,6 mm. Both blanks are again made of 22MnB5 and have an AlSi coating of about 90% (by weight) of aluminium and 10% (by weight) of silicon. After the welding process using remote laser welding, the resulting combined blank has been heated to above Ac3 temperature, and for sufficient time to fully austenize the combined blank and has been subsequently quenched at a cooling rate above a critical cooling rate.

[0049] Five samples were tested in this case: samples 10 - 13 were welded at a welding speed of about 20 mm / sec, whereas sample 14 was welded at a welding speed of about 50 mm / sec. Figures 2E and 2F show a cross-sectional view of samples 12 and 14 respectively, wherein again it may be seen that the weld zone is much narrower in the case of welding with a higher weld speed.

[0050] After subjecting the combined blank to austenization and quenching, the hardness was measured throughout the thickness of the blank and in the weld zone. The results are shown in figure 2D. Only the sample 14 which was welded at a higher welding speed, has a noticeable decrease in hardness in the weld zone. Hardness is a direct indicator of ultimate tensile strength. In this sample, the weakest point of the assembly is in the weld zone, whereas this is not the case for all of the other samples.

[0051] This can easily be explained by the presence of aluminium in the weld zone, which leads to the formation of perlite and ferrite rather than martensite after heating and quenching.

[0052] Figures 3A - 3C show examples of combined blanks comprising areas with different thickness. In the examples shown in these figures, the combined blanks are formedby a plurality of blanks. In examples, individual blanks may have a thickness of 0,8 - 2 mm, for example 1 ,2 mm. If both blanks have a thickness of 1 ,2 mm, the thickness may be 2,4 mm in the area of overlap.

[0053] Specifically, larger blanks comprising length and width of e.g., 1 - 2 meters or more, may be manufactured by joining individual blanks to each other prior to a forming operation. In some examples, the blanks with overlapping regions may comprise blanks which after forming may be at least one of a unitary roof ring of a vehicle, a unitary rear ring of a vehicle, unitary door ring of a vehicle, a unitary firewall panel of a vehicle, a frame for the protection of a battery box of a vehicle and a unitary bumper beam assembly of a vehicle.

[0054] Figure 3A shows an example of a blank before being deformed to form a unitary roof ring of a vehicle. As shown in figure 3A, the unitary roof ring may be made from four blanks, a first blank 310, a second blank 320, a third blank 330 and a fourth blank 340, wherein the first and second blanks 310, 320 may be longitudinal beam blanks, and the third and fourth blanks 330, 340 may be crossbeam blanks. The longitudinal beam blanks may be joined to the front cross beam blank and to the rear cross beam blank, forming a substantially closed ring shape. The blanks may be joined to each other e.g., through laser welding or spot welding.

[0055] The blanks may be joined with each other by forming one or more overlapping regions 350 formed by partially overlapping the blanks with each other. That is, one blank is only partially positioned over another blank and the blanks are then joined to each other. An overlapping region thus acquires an increased thickness as compared to the remainder of the blanks. Such an increase in thickness can be used to tailor mechanical properties as needed and provide local reinforcements, e.g., in areas where increased strength and / or stiffness are required.

[0056] Also shown in figure 3A, a patch blank 370 may be joined to at least one of the plurality of the blanks that form combined blank 300. A patch blank may be regarded herein as a blank that entirely overlaps another blank, i.e. , a patch blank may be positioned entirely within a perimeter of another blank. The patch blank may be joined to the other blank by welding, e.g., spot welding or remote laser welding. The resulting combination of “basic” blank and patch blank may sometimes be referred to as “patchwork blank”.

[0057] A patch blank 370 may be added as a reinforcement in order to increase strength of a specific area of the blank 300. The overlapping region formed by overlapping a patch blank 370 with another blank comprises increased thickness as compared to the remainderareas of the blank. In some examples, preheating one or more preselected areas of the blank may comprise preheating an area of the blank comprising a patch blank.

[0058] The plurality of blanks 310 - 340 may comprise different thicknesses and / or different materials.

[0059] In any of the overlapping regions 350, 370 disclosed in the example of figure 3A, remote laser welding as taught herein may be used to join the blanks to each other. It should be clear that in further examples, some of the overlapping regions may be joined with a plurality of spot welds, and other overlapping regions may be joined using remote laser welding. The choice for one type of weld or another may depend e.g. on the size and shape of the overlapping region.

[0060] In some examples the blank or the hereinbefore described blanks may be made from ultra-high strength steels (LIHSS). Boron steel, e.g., 22MnB5, or other steel compositions mentioned or referred to before may be suitable LIHSS. These blanks, e.g., boron steel blanks, may comprise an aluminium silicon coating or zinc coating.

[0061] llsibor® 1500P is an example of a 22MnB5 steel. The composition of llsibor® is summarized below in weight percentages (rest is iron (Fe) and impurities):Maximum carbon (C) (%): 0.25Maximum silicon (Si) (%): 0.4Maximum manganese (Mn) (%): 1.4Maximum phosphorus (P) (%): 0.03Maximum sulphur (S) (%): 0.01Aluminium (Al) (%): 0.01 - 0.1Maximum titanium (Ti) (%): 0.05Maximum niobium (Nb) (%): 0.01Maximum copper (Cu) (%): 0.20Maximum boron (B) (%): 0.005Maximum chromium (Cr) (%): 0.35

[0062] llsibor® 1500P may have a yield strength of e.g., 1.100 MPa, and an ultimate tensile strength of 1.500 MPa after a hot stamping operation.

[0063] llsibor® 2000 is an example of a 37MnB5 steel, which is another boron steel with even higher strength. The yield strength of llsibor® 2000 may be 1.400 MPa or more, and the ultimate tensile strength may be above 1.800 MPa. The composition of Usibor® 2000 is summarized below in weight percentages (rest is iron (Fe) and impurities):Maximum carbon (C) (%): 0.36Maximum silicon (Si) (%): 0.8Maximum manganese (Mn) (%): 0.8Maximum phosphorus (P) (%): 0.03Maximum sulphur (S) (%): 0.01Aluminium (Al) (%): 0.01 - 0.06Maximum titanium (Ti) (%): 0.07Maximum niobium (Nb) (%): 0.07Maximum copper (Cu) (%): 0.20Maximum boron (B) (%): 0.005Maximum chromium (Cr) (%): 0.50Maximum molybdenum (Mb) (%): 0.50

[0064] MBW-K® 1900 is a manganese-boron steel 34MnB4 from ThyssenKrupp™ which may have an ultimate tensile strength of 1900 MPa. As offered commercially today, the steel is uncoated. In example of the present disclosure one blank of uncoated steel may overlap at least partially with another blank that does have a corrosion protective coating.

[0065] The chemical composition of MBW-K® 1900 is summarised below in weight in percentages (rest is iron (Fe) and impurities):Maximum carbon (C) (%): 0.38Maximum silicon (Si) (%): 0.40Maximum manganese (Mn) (%): 1.40Maximum phosphorus (P) (%): 0.025Maximum sulphur (S) (%): 0.010Minimum aluminium (Al) (%): 0.015Maximum chromium and molybdenum (Cr + Mo) (%): 0.50Maximum titanium (Ti) (%): 0.13Maximum boron (B) (%): 0.005

[0066] The plurality of blanks that form the combined blank may comprise different material and / or thicknesses. For example, blanks of press hardenable manganese boron steels like llsibor® or MBW-K® 1900 (e.g., llsibor® 1500 and / or llsibor® 2000) may be used in the blanks forming the combined blank. Using these types of materials in hot forming and subsequent quenching processes leads to a predominantly martensitic structure. One or more of the blanks may be made from a different material, e.g., Ductibor® 1000.

[0067] Ductibor® 1000 is another material used in hot stamping for increasing the elongation when compared to Usibor® 1500 and Usibor® 2000. Also Ductibor® 1000 may be provided with an AlSi coating.

[0068] The yield strength of Ductibor® 1000 may be 800 MPa or more, and the ultimate tensile strength of 1000 MPa or more. The composition of Ductibor® 1000 is summarized below in weight percentages (rest is iron (Fe) and impurities):Maximum carbon (C) (%): 0.10Maximum silicon (Si) (%): 0.6Maximum manganese (Mn) (%): 1.8Maximum phosphorus (P) (%): 0.03Maximum sulphur (S) (%): 0.01Aluminium (Al) (%): 0.01 - 0.1Maximum titanium (Ti) (%): 0.05Maximum niobium (Nb) (%): 0.10Maximum copper (Cu) (%): 0.20Maximum boron (B) (%): 0.005Maximum chromium (Cr) (%): 0.20

[0069] Other steels which are suitable for hot stamping and provide for a lower ultimate tensile strength, but improved ductility include e.g. Ductibor ® 500 which has a tensile strength of 550 MPa or higher, and a yield strength of at least 330 or 350 MPa. The composition of Ductibor® 500 is summarized below in weight percentages (rest is iron (Fe) and impurities):Maximum carbon (C) (%): 0.1Maximum silicon (Si) (%): 0.5Maximum manganese (Mn) (%): 1.7Maximum phosphorus (P) (%): 0.03Maximum sulphur (S) (%): 0.025Aluminium (Al) (%): 0.015 - 0.2Maximum titanium (Ti) (%): 0.09Maximum niobium (Nb) (%): 0.10Maximum copper (Cu) (%): 0.20Maximum boron (B) (%): 0.001Maximum chromium (Cr) (%): 0.20

[0070] In order to improve the ductility and energy absorption in specific areas of a component, it is known to introduce softer regions within the same component. This improves ductility locally while maintaining the required high strength overall. By locally tailoring the microstructure and mechanical properties of certain structural components such that they comprise regions with very high strength (very hard regions), i.e. regions with high ultimate tensile strength and high yield strength and regions with increased ductility (softer regions), i.e. regions with lower ultimate tensile strength and lower yield strength and increased elongation before break, it may be possible to improve their overall energy absorption and maintain their structural integrity during a crash situation and also reduce their overall weight. Such soft zones may also advantageously change the kinematic behaviour in case of a collapse of a component under an impact.

[0071] One way of obtaining such softer zones is by the inclusion in the combined blank of sub-blanks made of e.g. Ductibor® 1000 or 500, or similar materials.

[0072] Other known methods of creating regions with increased ductility ("softzones" or "soft zones") in structural components of vehicles include the provision of tools comprising a pair of complementary upper and lower die units, each of the units having separate die elements (steel blocks).

[0073] The die elements may be designed to work at different temperatures, in order to have different cooling rates in different zones of the part being formed during the quenching process, and thereby resulting in different material properties in the final product e.g. softareas which will generally have a lower ultimate tensile strength and a lower yield strength, but allow for more elongation before breaking. E.g., one die element may be cooled in order to quench the corresponding area of the component being manufactured at high cooling rates and to thereby reduce the temperature of the component rapidly and obtain a hard martensitic microstructure. Another neighbouring die element may be heated in order to ensure that the corresponding portion of the component being manufactured cools down at a lower cooling rate, in order to obtain a softer microstructure, including e.g., bainite, ferrite and / or perlite. Such an area of the component may remain at higher temperatures than the rest of the component when it leaves the die.

[0074] Alternative methods for creating regions with increased ductility include differential heating prior to stamping, and / or local heat treatments (using e.g., laser, IR heating or induction heating) after a stamping operation.

[0075] Other examples of automotive components that may be manufactured using embodiments of the present disclosure are illustrated in figures 3B and 3C. Figure 3B illustrates a reinforcement of a B-pillar. The reinforcement of a B-pillar may be made by hot stamping a combined blank comprising a main blank 40 and a patch 42 positioned on top of the main blank. As schematically illustrated in figure 3B, a plurality of straight laser seams 44 along the edges of the overlap region may be used to join the main blank 40 to the patch 42. The resulting combined blank may be heated to above an austenization temperature and subsequently deformed and quenched. The same materials or similar materials as those mentioned with respect to figure 3A may be used in the example of figure 3B.

[0076] Figure 3C illustrates a combined blank 50 for the manufacture of a unitary door ring. The unitary door ring comprises a rocker portion, a hinge pillar portion, an A-pillar portion and a B-pillar portion. The term “portion” is herein used, because in the resulting unitary door ring, there are no separate B-pillar, A-pillar, hinge pillar and rocker, since there is only a single structure.

[0077] The combined blank 50 in this example comprises a blank 51 corresponding to a rocker portion, a blank 52 corresponding substantially to a hinge pillar, a blank 53 corresponding substantially to the A-pillar, a blank 54 corresponding to an upper region of the B-pillar and a blank 55 corresponding to a lower region of the B-pillar.

[0078] In this example, each of the blanks 51 , 52, 53, 54, and 55 are arranged to be partially overlapping, i.e. an overlapping region 56 is formed between the blank 55 of the lower B- pillar and blank 51 corresponding to the rocker; an overlapping region 57 is formed between the blank 51 corresponding to the rocker, and the blank 52 corresponding to the hinge pillar;an overlapping region 58 is formed between the blank 52 corresponding to the hinge pillar and the blank 53 corresponding to the A-pillar; an overlapping region 59 between blank 53 corresponding to the A-pillar and blank 54 corresponding to the B-pillar; and an overlapping region 60 formed between the blank 54 of the upper B-pillar and the blank 55 of the lower B-pillar.

[0079] The overlapping regions in the A-pillar, the B-pillar portion and the rocker portion in this example are joined to each other using laser welds. In the case of overlapping region 59, a single continuous weld which substantially follows the outer edge of the overlapping region is used. In overlapping region 60, a plurality of straight weld seams 44 are used, and in overlapping region 56, shorter laser stitches are used. All these laser welds may be carried out with an absolute welding speed of 10 - 30 mm / sec.

[0080] The example of the present disclosure may also be combined with further weld techniques.

[0081] Spot welds may be used in one or more of the overlapping regions to join the blanks to each other. Also, G-shaped welds, such as described in co-pending European patent application 24 382 220.2 may also be used in one or more of the overlapping regions.

[0082] The substantially G-shaped welds may also be created with remote laser welding, particularly with an absolute weld speed of 10 - 30 mm / sec. Such a G-shaped weld is further illustrated in figure 4. The weld seam comprises a substantially circular portion and a substantially straight portion at an end of the substantially circular portion. The substantially circular portion has a radius and corresponding to a circular arc around a circle centre 24 with a central angle of 270° - 350°, more specifically 320 - 345°. The substantially straight portion extends radially to the circle centre.

[0083] The central angle of a circular arc is defined as the angle formed at the centre of a circle by two radii that define the arc.

[0084] In some examples, like in the example of figure 4, the substantially straight portion includes a portion that extends beyond the circle centre. The substantially straight portion may extend beyond the circle centre by e.g., 10 - 50% of the radius of the substantially circular portion.

[0085] In some examples, the G-shaped weld seam incudes a rounded transition between the substantially circular portion and the substantially straight portion. The rounded transition may have a transition radius of 10 - 50%, specifically 15 - 30% of the radius of the substantially circular portion.

[0086] G-shaped welds of different dimensions may be provided. With reference to figure 4, some dimensions may be illustrated. A diameter E of the substantially circular portion may be e.g., 2 - 50 mm, specifically 4 - 20 mm, and more specifically 6 - 14 mm. The radius B of the substantially circular portion may be e.g., 1 - 25 mm, specifically 2 - 10 mm, and more specifically 3 - 7 mm.

[0087] The rounded transition may have a transition radius RF of 10 - 50%, specifically 15 - 30% of the radius of the substantially circular portion 22, i.e., the transition radius may be e.g. 0.1 - 12.5 mm, specifically 0.5 - 1.5 mm.

[0088] A distance C between an end of the substantially circular portion and the straight portion 26 may be 0.5 - 15 mm, specifically 1 - 10 mm, more specifically about 1.25 mm. A distance D between the start of the transition portion 28 and the substantially straight portion 26 may be 0.5 - 15 mm, more specifically about 1.25 mm.

[0089] The combined blank may be subjected to a hot stamping process, and again, the same or similar materials may be used as mentioned before for this process. And similarly, as before, the individual blanks may have different thicknesses and / or may be made of different materials.

[0090] Although only a number of examples have been disclosed herein, other alternatives, modifications, uses and / or equivalents thereof are possible. Furthermore, all possible combinations of the described examples are also covered. Thus, the scope of the present disclosure should not be limited by particular examples but should be determined only by a fair reading of the claims that follow.

Claims

CLAIMS1 . A method for joining a first steel blank to a second steel blank to form a combined blank, comprising: positioning the first and second steel blanks such that the first steel blank at least partially overlaps the second steel blank in an overlap region, laser welding the first steel blank to the second steel blank with one or more weld seams in the overlap region and without using a filler material, wherein an absolute welding speed of the laser welding is 10 - 30 mm / sec, and wherein the first steel blank and / or the second steel blank have a corrosion protective coating comprising aluminium.

2. The method of claim 1 , wherein the first steel blank and the second steel blank both have a corrosion protective coating comprising aluminium.

3. The method of claim 1 or 2, wherein the corrosion protective coating comprises 80% or more by weight of aluminium.

4. The method of claim 3, wherein the corrosion protective coating comprises about 90% aluminium and about 10% silicon, by weight.

5. The method of any of claims 1 - 4, wherein a thickness of the first and second steel blanks is 0,8 - 3 mm, specifically 0,8 - 2 mm.

6. The method of any of claims 1 - 5, wherein the laser welding is laser welding with a single spot.

7. The method of any of claims 1 - 6, wherein a laser power for the laser welding is 1 - 8 kW, specifically 1 ,5 - 6 kW, more specifically 1.5 - 3kW.

8. The method of any of claims 1 - 7, wherein the laser welding is performed with a solid state laser, or with a CO2 laser.

9. The method of any of claims 1 - 8, wherein one or more of the weld seams are substantially rectilinear.

10. The method of any of claims 1 - 9, wherein the weld seams extend entirely through a thickness of the second steel blank.

11. The method of any of claims 1 - 9, wherein the weld seams extend only partially through a thickness of the second steel blank.

12. The method of any of claims 1 - 11 , wherein the first and / or the second steel blank are made or press hardenable steel.

13. The method of claim 12, wherein the first and / or the second steel blank are made of 22MnB5, 37MnB5 or 38MnB5.

14. The method of claim 12 or 13, and further comprising cold stamping the combined blank to form a cold stamped product and heating the cold stamped product to above an austenization temperature and subsequent quenching.

15. The method of claim 12 or 13, and further comprising hot stamping the combined blank.

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