WO2013110214A1

WO2013110214A1 - Method of welding coated materials

Info

Publication number: WO2013110214A1
Application number: PCT/CN2012/070723
Authority: WO
Inventors: David Yang
Original assignee: GM Global Technology Operations LLC
Priority date: 2012-01-27
Filing date: 2012-01-27
Publication date: 2013-08-01

Abstract

A method of welding metal alloys and/or coated metals by fusion welding in a gap-free or narrow-gap lap joint configuration comprises applying a heat source in combination with a vacuum system on an area of engagement of said metals, and forming a molten pool between said metals; creating a negative pressure zone above the molten pool by said vacuum system allowing vaporization of the coating with low boiling temperature; and cooling the molten metal to yield a weld. The vacuum system keeps the keyhole continuously open during welding, which provides a channel for the vapors of the coating to escape, thereby suppressing the weld discrepancies of spatter, blowhole and porosity and resulting in high-quality welds.

Description

A METHOD OF WELDING COATED MATERIALS

FIELD OF INVENTION

[001] This invention is directed to a method of fusion welding in a gap-free or narrow-gap lap joint configuration of metal alloys and/or coated metals wherein the coating or one of the metals in the metal alloy has lower boiling point and/or melting point, resulting with high-quality welds.

BACKGROUND OF THE INVENTION

[002] Coated steels provide an excellent corrosion resistance to guarantee up to 12 years' corrosion protect for the automotive body. It is often necessary to form a linear weld junction between two overlapping steel parts of the autobodies. A laser welding includes directing a high energy laser beam against a surface of one of the parts of the steel and along the weld path, thereby melting a weld slot in the steel, through the steel. As the beam moves further along the path, previously melted metal cools and solidifies to complete the weld.

[003] Many efforts are made in the world trying to achieve welds in the zinc coated steels. American Welding Society set a standard of removing the zinc coating at the interface of metal sheets completely, prior to welding of zinc coated steels. When welding the zinc coated steels in a gap-free or narrow gap lap joint configuration, a highly-pressurized zinc vapor is readily produced at the interface of two metal sheets due to the lower boiling point of zinc (906°C) than the melting point of steel (over 1500 °C). If the highly-pressurized vapor cannot escape from the molten pool, different weld discrepancies will appear in the weld, such as spatter and blowholes, which significantly reduce the mechanical properties of the welds and require the post repair.

[004] Welding processes such as laser welding, form spatters, porosity, blowholes, undercutting and drops (as shown in Figures 1, 2 and 8). The spatter ejected along the laser beam propagation direction prevents the absorption of the workpieces from laser energy and tends to damage the optical lenses. Consequently, only partial penetration can be achieved even in the high power level. In addition, the turbulent molten pool is generated due to the large difference in the velocity and pressure between the vapors of the coating of the steel (i.e. zinc) and the liquid melt across the interface. The instability of the molten pool is manifested by itself in the form of waves being generated on the fluid flow in the presence of swelling and groove. Under these welding conditions, the laser beam is projected on the uneven surface of the molten pool. Due to the uneven energy distribution and block of the laser beam energy by the spatter, the keyhole tends to collapse. Therefore, during the laser welding process the keyhole size and depth is changed and cannot be kept stable. When the keyhole is collapsed or the keyhole depth can't reach the interface, the vapor of the coating is entrapped and expanded inside the molten pool. Once the vapor pressure is beyond the threshold, the liquid metal is ejected SUMMARY OF THE INVENTION

[006] The present invention provides an improved method of welding coated metals in a gap-free or narrow- gap lap joint configuration using a heat source in combination with a vacuum system. Specifically this invention is directed to a method of welding steel coated by zinc. The vacuum system in combination with the heat source is integrated in a co-axial manner or adjacent to each other. During welding, the heat source locally heats up the workpieces, forming a keyhole and creating molten metal pool through the first workpiece and into the overlapping workpieces. The coating, having a lower melting and boiling temperature than the metal substrate, vaporizes while the steel or other metal substrate is being melted by the laser. The vacuum system keeps the keyhole continuously open, which provides a channel for the vapors of the coating (i.e. zinc vapor) to escape.

[007] In one embodiment, this invention is directed to a method of welding metals in a gap-free or narrow-gap lap joint configuration, said method comprising the following steps: a) arranging at least two metals in a gap- free or narrow-gap lap joint configuration, wherein at least one of the metals is coated; b) applying a heat source in combination with a vacuum system on an area of engagement of said metals, and forming a molten pool between said metals; c) creating a negative pressure zone above the molten pool by said vacuum system allowing vaporization of the coating; and d) cooling the molten metal to yield a weld.

[008] In one embodiment, this invention is directed to a method of welding metals in a gap free or narrow-gap lap joint configuration, said method comprising the following steps: a) arranging at least two metals in a gap- free or narrow-gap lap joint configuration, wherein at least one of the metals is a metal alloy and optionally one of the metal alloys is coated; b) applying a heat source in combination with a vacuum system on an area of engagement of said metals and forming a molten pool between said metals; c) creating a negative pressure zone above the molten pool by said vacuum system allowing vaporization of a metal with low boiling temperature; and d) cooling the molten metal to yield a weld.

BRIEF DESCRIPTION OF THE FIGURES

[009] The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the appended figures in which:

[0010] Figure 1 depicts weld discrepancies produced during traditional laser welding process (not of this invention) of 1.2 mm to 1.5 mm lap joint. Figure 1A depicts top view; Figure IB depicts bottom view. Laser power: 3500 W; Focus point: 0.6 mm; Speed 2.4 m/min; Shield gas: pure argon; Flow rate: 30 SCFH (Standard Cubic Feet/ hour); Gap: 0 mm.

[0011] Figure 2 depicts the spatters and blowholes (Figure 2A) and the porosity in the weld (Figure 2B) formed by traditional laser welding (not of this invention) of Figure 1.

[0012] Figure 3 is a schematic presentation of laser welding zinc coated steels in a gap-free or narrow-gap lap joint configuration of this invention. The zinc coated steels are overlapping having a top workpiece of coated steel (300-1) and a bottom workpiece of coated steel (300-3) wherein the zinc coating (300-2) is between the two workpieces. A heat source together with a vacuum system is applied through the welding direction (300-6) forming a keyhole (300-5) and causing vaporization of the zinc coating (300-4). A negative pressure zone (< latm) (300-7) is formed due to the vacuum system, above the welding pool (300-10), generating a counteracting pressure/force (300-8) to reduce or suppress the oscillation of the molten metal pool (300-10). The molten pool is cooled and solidifies (300-9). In addition, the vacuum system can guide the laser-induced plasma and turn the instable laser-induced plasma into stable plasma.

[0013] Figure 4 is a schematic presentation of laser welding metals in a gap-free or narrow-gap lap joint configuration of this invention, wherein the negative pressure zone (< latm) is formed using a side vacuum system (400-12). Vacuum systems (400-12) can also accelerate the escape of low boiling point metal vapor (such as coating) that has been generated during the welding process. The metal sheets are overlapping having a top workpiece (400-1) and a bottom workpiece (400-3). A heat source (400-11) together with a vacuum system (400-12) is applied through the welding direction (400-6) forming a keyhole (400-5) and causing vaporization of low boiling point metal vapor (such as zinc coating) (400-4). A negative pressure zone (< latm) (400-7) is formed due to the vacuum system, above the molten metal pool (400-10) to reduce or suppress the oscillation of the molten metal pool (400-10).

[0014] Figure 5 is a schematic presentation of laser welding zinc coated steels in a gap-free or narrow-gap lap joint configuration of this invention, wherein the negative pressure zone (< latm) (500-7) is formed using a coaxial vacuum system (500-12). Vacuum systems can also accelerate the escape of zinc vapor (500-4) that has been generated during the welding process. The metal sheets are overlapping having a top workpiece (500-1) and a bottom workpiece (500-3). A heat source (500-11) together with a vacuum system (500-12) is applied through the welding direction (500-6) forming a keyhole (500-5) and causing vaporization of low boiling point metal vapor (such as zinc coating) (500-4). A negative pressure zone (< latm) (500-7) is formed due to the vacuum system, above the molten metal pool (500-10) to reduce or suppress the oscillation of the molten metal pool (500-10).

[0015] Figure 6 depicts laser welding zinc coated steels in a gap-free or narrow-gap lap joint configuration of this invention of 1.6 mm to 0.4 mm lap joint. Figure 6A depicts top view. Figure 6B depicts bottom view. No spattering on surface, no blowholes in welding. Laser power: 3200 W; Focus point: 0.3 mm; Speed 3 m min; Shield gas: none; Gap: 0 mm.

[0016] Figure 7 depicts laser welding zinc coated steels in a gap-free or narrow-gap lap joint configuration of this invention of 0.4 mm to 0.4 mm lap joint. Figure 7A depicts top view. Figure 7B depicts bottom view. Laser power: 2000 W; Focus point: 0.3 mm; Speed 3 m/min; Shield gas: none; Gap: 0 mm.

[0017] Figure 8 depicts a conventional laser welding (without the use of a vacuum system) of zinc coated steels in a gap-free or narrow gap lap joint configuration of 0.8 mm to 0.8 mm lap joint. Figure 8A depicts top view. Figure 8B depicts bottom view. Laser power: 3400 W; Focus point: 0.3 mm; Speed 2.4 m/min; Shield gas: none; Gap: 0 mm.

[0018] Figure 9 depicts laser welding zinc coated steels in a gap-free or narrow gap lap joint configuration of this invention of 1.6mm to 1.4 mm lap joint. Figure 9A depicts top view. Figure 9B depicts bottom view. Laser power: 3400 W; Focus point: 0.3 mm; Speed 2.4 m min; Shield gas: none; Gap: 0 mm.

[0019] Figure 10 depicts a microhardness distribution profile of laser welding zinc coated steels in a gap-free lap joint configuration. The X-axis refers to the distance from weld center (mm). The Y-axis refers to the microhardness distribution (Hv).

[0020] It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.

DETAILED DESCRIPTION OF THE INVENTION

[0021] In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, and components have not been described in detail so as not to obscure the present invention.

[0022] In describing and claiming the present method, weld and apparatus the following terminology will be used: the singular forms "a", "an", and "the" include plural referents unless the context clearly dictates otherwise. Amounts, and other numerical data may be presented herein in a range format. It is to be understood that such range format is used merely for convenience and brevity and should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. For example, a thickness of 0.4 to 6 mm should be interpreted to include not only the explicitly recited concentration limits of 0.4 to 6 mm, but also to include individual concentrations such as 1 mm, 2 mm, 3 mm, 4 mm, and subranges such as 0.4 to 2 mm, 2 mm to 6 mm, etc.

[0023] In one embodiment, this invention is directed to a method of welding metals in a gap-free or narrow-gap lap joint configuration, said method comprising the following steps: a) arranging at least two metals in a gap- free or narrow-gap lap joint configuration; b) applying a heat source in combination with a vacuum system on an area of engagement of said metals and forming a molten pool between said metals; c) creating a negative pressure zone above the molten pool by said vacuum system allowing vaporization of a low boiling point metal; and c) cooling the molten metal to yield a weld. In another embodiment, at least one of the metals is a metal alloy. In another embodiment, the metal is not a metal alloy. In another embodiment, at least one of the metals is coated. In another embodiment, the coating has a lower melting temperature and boiling temperature than the metal.

[0024] In one embodiment, this invention is directed to a method of welding metals in a gap-free or narrow-gap lap joint configuration, said method comprising the following steps: a) arranging at least two metals in a gap- free or narrow-gap lap joint configuration, wherein at least one of the metals is coated; b) applying a heat source in combination with a vacuum system on an area of engagement of said metals in a gap-free or narrow- gap lap joint configuration, and forming a molten pool between said metals; c) creating a negative pressure zone above the molten pool by said vacuum system allowing vaporization of the coating; and d) cooling the molten metal to yield a weld. In another embodiment the welding is an arc welding. In another embodiment the welding is a laser welding. In another embodiment, the welding is plasma welding. In another embodiment, at least one of the metals is a metal alloy. In another embodiment, the metal is not a metal alloy. In another embodiment, the coating has lower boiling point than the metal sheet.

[0025] In one embodiment, this invention is directed to a method of welding metals in a gap free or narrow-gap lap joint configuration, said method comprising the following steps: a) arranging at least two metals in a gap- free or narrow-gap lap joint configuration, wherein at least one of the metals is a metal alloy and optionally one or more of the metal alloys is coated; b) applying a heat source in combination with a vacuum system on an area of engagement of said metals and, forming a molten pool between said metals; b) creating a negative pressure zone above the molten pool by said vacuum system allowing vaporization of a metal with low boiling temperature; and c) cooling the molten metal to yield a weld.

[0026] In one embodiment, this invention is directed to a method of laser welding metal in a gap-free or narrow-gap lap joint configuration, said method comprising the following steps: a) arranging at least two metals in a gap-free or narrow-gap lap joint configuration; b) applying a laser beam in combination with a vacuum system on an area of engagement of said metals and forming a molten pool between said metals; c) creating a negative pressure zone above the molten pool by said vacuum system allowing vaporization of the low boiling point metal; and d) cooling the molten metal to yield a weld. In another embodiment, at least one of the metals is a metal alloy. In another embodiment, the metal is not a metal alloy. In another embodiment, at least one of the metals is coated. In another embodiment, the coating has a lower melting temperature and boiling temperature than the metal sheet.

[0027] In one embodiment, this invention is directed to a method of laser welding metal in a gap-free or narrow gap lap joint configuration, said method comprising the following steps: a) arranging at least two metals in a gap-free or narrow-gap lap joint configuration wherein at least one of the metals is coated; b) applying a laser beam in combination with a vacuum system on an area of engagement of said metals, and forming a molten pool between said metals; c) creating a negative pressure zone above the molten pool by said vacuum system allowing vaporization of the coating; and d) cooling the molten metal to yield a weld.

[0028] The present invention provides a welding method in a gap-free or narrow-gap lap joint configuration, wherein at least (i) one of the metals is coated (ii) one of the metals is a metal alloy or (iii) one of the metals is a coated metal alloy wherein the coating has lower melting and boiling point than the metal; and/or one or more metals of the metal alloys have lower melting and boiling point than other metals in the metal alloy. In one embodiment, the welding method uses a heat source (such as laser or power supply) in combination with a vacuum system that produces a linear seam weld free of vapor metals (such as zinc vapor, if coating is zinc). The welding method of this invention suppresses spatters, blowhole, porosity, drop out and undercutting formation for lightweight materials such as the coating of the high strength metal.

[0029] In one embodiment, this invention is directed to a method of stabilizing a molten pool formed in a welding process, said method comprising the following steps: a) arranging at least two metals in a gap-free or narrow-gap lap joint configuration; b) applying a heat source in combination with a vacuum system on an area of engagement of said metals, and forming a molten pool between said metals; c) creating a negative pressure zone above the molten pool by said vacuum system allowing vaporization of the low boiling temperature metal; thereby stabilizing a molten pool. In one embodiment, at least one of the metals is coated. In one embodiment, the coating has a lower melting and boiling temperature than the metal sheet. In one embodiment, the metal is a metal alloy. In another embodiment, the metal alloy includes a low melting point metal which is vaporized by the welding process of this invention.

[0030] In one embodiment, the term "lap joint configuration" refers to two or more metal sheets or metal workpieces which are held together in an overlapping configuration.

[0031] The term "gap-free or narrow gap" refers to tightly clamped workpieces brought into contact. The top and bottom workpieces are clamped. In another embodiment the term "gap-free or narrow gap" refers to a gap of between 0 to 0.2 mm between the workpieces.

[0032] The present invention provides a welding method in a gap-free or narrow-gap lap joint configuration, wherein at least one of the metals is coated using a heat source (such as laser or power supply) in combination with a vacuum system that produces a linear seam weld. The welding method of this invention suppresses spatters, blowhole, porosity, drop out and undercutting formation for lightweight materials such as the coating of the high strength metal.

[0033] In one embodiment, the method of this invention includes applying a heat source in combination with a vacuum system on an area of engagement. In another embodiment, a laser beam in combination with a vacuum system can be applied above, on, or below the top of a metal workpiece, depending on the thickness of the metal members.

[0034] In one embodiment, the method of this invention applies a heat source in combination with a vacuum system on an area of engagement of a metal in a gap-free or narrow-gap lap joint configuration. In another embodiment, the heat source is any source that leads to fusion of a metal. In another embodiment, the heat source is a laser beam, light beam, arc, plasma or an electron beam. In another embodiment, the heat source is a laser beam (laser welding). In another embodiment, the heat source is by power supply (arc welding). In another embodiment, the laser beam is generated from a Fiber, disc, YAG or C(¾ laser beam source. In another embodiment, the laser beam is generated from a Fiber laser beam source. In another embodiment, the laser beam is generated from any laser beam source with different power. In another embodiment, the energy applied on the area of engagement depends on the thickness of the metal sheets. In another embodiment, the energy applied on the area of engagement is between 1 to 40 kilowatts. In another embodiment, the energy applied on the area of engagement is between 1 to 30 kilowatts. In another embodiment, the energy applied on the area of engagement is between 1 to 20 kilowatts. In another embodiment, the energy applied on the area of engagement is between 1 to 4 kilowatts. In another embodiment, the energy applied on the area of engagement is between 2 to 4 kilowatts. In another embodiment, the energy applied on the area of engagement is between 2 to 4 kilowatts. In another embodiment, the energy applied on the area of engagement is between 3 to 4 kilowatts. The distance of the laser beam from the metal members is from few millimeters to meters.

[0035] In one embodiment, the methods of this invention form a molten pool between at least two metals. In another embodiment, the molten pool in formed at least at the depth of the interface between the metals. In another embodiment, the molten pool is formed at least at the depth of the narrow gap between the metals and within the second metal. In another embodiment, the molten pool is formed at the depth of the two welded metals. In another embodiment the molten pool is formed within the welded metals. In another embodiment, the molten pool is formed at least at the depth of the interface between the metals and within the second metal. In another embodiment, the interface includes the narrow gap between the metals. In another embodiment the said heat source cuts a slot between said metals through the thickness of both overlapping parts. In another embodiment the said heat source cuts a slot between said metals through the thickness of both overlapping parts and forms a molten pool therein. [0036] In one embodiment, the vacuum system forms a negative pressure above the molten pool. In another embodiment the pressure is below 1 atm. In another embodiment, the negative pressure or low pressure zone above the molten pool improves the absorption of heat sources' energy. In another embodiment, the negative pressure above the molten pool reduces the oxides formation due to the diluted air. The distance of the vacuum system from the metal members is from few millimeters to meters. In one embodiment, the vacuum system forms a negative pressure between 0.01 to 600 kPa. In another embodiment, the vacuum system forms a negative pressure between 1 to 600 kPa. In another embodiment, the vacuum system forms a negative pressure between 10 to 600 kPa. In another embodiment, the vacuum system forms a negative pressure between 100 to 600 kPa. In another embodiment, the vacuum system forms a negative pressure between 10 to 500 kPa. In another embodiment, the vacuum system forms a negative pressure between 0.01 to 300 kPa. In another embodiment, the vacuum system forms a negative pressure between 1 to 100 kPa. In another embodiment, the vacuum system forms a negative pressure between 1 to 50 kPa.

[0037] In one embodiment, the method of this invention applies a heat source in combination with vacuum system on an area of engagement of a metal in a gap-free or narrow-gap lap joint configuration, wherein at least one of the metals is coated. In another embodiment, laser welding when used to join metal workpieces, forms a "keyhole" of vaporized metal through the molten pool that the laser beam melts. In one embodiment, the keyhole of this invention is stable and open. In one embodiment, the keyhole of this invention does not collapse. The keyhole provides an escape to the vapors of the coating or any other metal with low melting point. In another embodiment, the vacuum system of this invention, prevents the collapse of the keyhole, and keeps the keyhole stable.

[0038] In one embodiment, this invention is directed to a method of welding metal in a gap-free or narrow-gap lap joint configuration. In one embodiment, the methods of this invention include a step comprising arranging at least two metals in a gap-free or narrow-gap lap joint configuration. In another embodiment, the metals are different. In another embodiment, the metals are the same. In another embodiment, the metals are at least two metal units. In another embodiment, the metals are at least two metal sheets. In another embodiment, the metals are at least two metal particles. In another embodiment, the metals are at least two parts of a certain element. In another embodiment, the at least two metals are overlapping, wherein a first metal is on top of a second metal.

[0039] In one embodiment the metal is steel, aluminum alloys, copper alloys, or magnesium alloys. In another embodiment, the metal comprises steel, Al, Mg, Cu, Zn, Sn, Au, Ag or combination thereof. In another embodiment, the metal is steel. In another embodiment, the metal is steel, aluminum alloys, copper alloys or magnesium alloys In another embodiment, the thickness of the metal is 0.1 mm to 6 mm. In another embodiment, the thickness of the metal is 0.1 mm to 3 mm. In another embodiment, the thickness of the metal is 0.1 mm to 2 mm. In another embodiment, the thickness of the metal is 1 mm to 3 mm. In one embodiment, the metal is coated. In another embodiment, at least one metal of the overlapping joints is coated. In another embodiment, both metals of the overlapping joints are coated. In one embodiment, the coating is a material having lower melting point than the metal. In one embodiment, the coating is a material having lower boiling point than the metal. In another embodiment, the coating is zinc coating. In another embodiment, the coating is nickel coating. In another embodiment, the coating is an organic coating. In another embodiment, the metal is steel and the coating is zinc. In another embodiment, the metal is galvannealed (GA), galvanised (GI), or electrogalvanised (EG). In one embodiment, the first metal member is a plate made of steel with a zinc coating. The second metal member is a plate made of steel, which is also coated with zinc. In another embodiment, the metal is an aluminum alloy and the coating is a polymeric type coating containing Ti and Zr. The present teachings are not limited to joints in which both metal members are coated. Overlapping joints in which only one of the two metal members is coated is also used in the method of this invention.

[0040] Figures 3 to 5 provide a schematic presentation of the method of this invention. A weld is formed between a top workpiece and a bottom workpiece wherein the bottom workpiece is coated by zinc coating. The top workpiece may be coated as well by zinc coating. The metal workpieces (sheets) are about 0.1 to 6 mm thickness and have flat portions for joining. Removal of zinc coating layers located at a welding is necessary to prevent weld porosity from forming in the weld.

[0041] As shown in Figures 3, 4 and 5, the method of this invention provides a keyhole 300-5, 400-5 or 500-5, which allows an escape route for zinc vapor 300-4, 400-4 or 500-4 generated during the welding operation. Specifically, the method forms a molten metal pool 300-10, 400-10 or 500-10 between the top and bottom workpiece using a heat source 400-11 or 500-11, which provides a suitable path for venting zinc vapor, and a vacuum system 400-12 and/or 500-12 which accelerate the escape of zinc vapors that has been generated during the welding process. The vacuum system generates a negative pressure to reduce or suppress the oscillation of the molten metal (which is pushed by the zinc vapor). In another embodiment, the vacuum system guides the instable laser-induced plasma and turns it to stable plasma.

[0042] As shown in Figures 3, 4 and 5 a heat source 400-11 or 500-11 such as laser beam is directed on the top of a workpiece (i.e., the surface outboard from the welding surface). The laser beam 400-11 or 500-11 is moved relative to the surface of the top workpiece 400-6 or 500-6 to cut a slot over the path and length of the intended weld seam. A portion of zinc layer is also vaporized. The slot is cut at least completely through the thickness of the top workpiece to expose welding interface. Since the purpose of slot is to provide an escape route for vaporized zinc from welding interface, the depth of slot can suitably extend into, or even through, underlying bottom workpiece depending upon the zinc venting requirement of a particular weld setup. The slot is in a depth so that zinc vaporized at the weld region can be driven from the site.

[0043] In one embodiment, the method of this invention does not include a shielding gas. In another embodiment, a shielding gas is optional. In another embodiment, a shielding gas is argon, helium, nitrogen, air or any noble gas. A shielding gas is used to prevent oxidizing substances from the surrounding atmosphere from getting trapped in the weld. These oxidizing substances are impurities in the air that are typically harmful to the integrity of the weld. The shield gas is directed against the upper surface of the welding region.

[0044] The heat source in combination with a vacuum system is located adjacent to each other 400-12 (Figure 4) or the vacuum system is integrated with the laser bean in a co-axial way 500-12 (Figure 5). The vacuum system accelerates the escape of zinc vapors that are generated during the welding process and generates a negative pressure to reduce or suppress the oscillation of the molten metal (which is pushed by the zinc vapor). The vacuum system also guides the instable laser-induced plasma and turns it to stable plasma.

[0045] The heat source 400-11 or 500-11 may typically have a power output of about one to twenty kilowatts. The beam of the heat source 400-11 or 500-11 is focused to the engagement area to form a slot of the width of the desired weld seam. The high energy pulse cuts the slot and vaporizes the zinc (having a vaporization temperature of about 900°C.) from the weld interface region. This energy level should be high enough to vaporize the zinc and drill the slot but low enough to prevent excessive melting of steel beyond the intended slot width. Figures 4 and 5 present the escape path, indicated by directional arrows 400-4 or 500-4, provided for zinc vapor through the keyhole 400-5 or 500-5 where the bottom of the keyhole 400-5 or 500-5 extends into the bottom workpiece. The flow of the vapor is assisted by the vacuum system.

[0046] The foregoing description is directed, as an example, to joining metal (steel) workpieces with a laser in combination with a vacuum system. However, it should be understood that other fusible metals and/or coatings may be joined using the same process with a proper selection of compatible metals. Thus, other metals or coatings may also be successfully joined with a laser and vacuum system within the guidelines above described.

[0047] In one embodiment, a large shear force is created at the interface of zinc vapor and liquid metal layer during the laser welding zinc coated steels in a gap-free or narrow-gap lap joint configuration. Under the function of this large shear force, the molten pool becomes dramatically instable and the keyhole tends to collapse. When the keyhole is collapsed, a large amount of liquid metal is brought out the molten pool and condensed in the air. Consequently, spatter and blowholes are formed. The use of a vacuum system helps remove the plasma but also provides an external force, as shown in Figure 3. The removal of the laser-induced plasma makes the coupling of laser power into the welded materials more efficiently. Furthermore, when a vacuum system is used during laser welding process, a negative pressure zone is created directly on the top of the molten pool, compared to environmental pressure. The difference in the pressure will generate an external force above the molten pool from the ambient environment. This external force can balance the shear force produced by the zinc vapor. Under these welding conditions, the molten pool maintains stable and the laser power into the weld materials is stable. Therefore, the keyhole is always open for the escape of zinc vapor.

[0048] In one embodiment, the method of this invention provides a high quality weld. In one embodiment, this invention is direcetd to a high quality weld. "High quality" weld refers to a weld which is lacking spatters, blowholes, porosity or combination thereof and is durable. In another embodiment, high quality weld refers to a weld which has minimal spatters, blowholes, porosity, or combination thereof and is durable. In another embodiment, the weld of this invention is hard in the fusion zone with microhardness values of 250 to 350. In another embodiment, high quality weld of this invention is as presented in Figure 6. In another embodiment, the size of the weld can be any size.

[0049] In one embodiment, the method of this invention provides welding apparatus comprising a heat source in combination with a vacuum system. In another embodiment, the heat source is a laser beam from a Fiber, YAG, CO₂, disc or any other laser source. In another embodiment, the vacuum system can be any available vacuum system. In another embodiment, shielding gas is optionally pumped through the apparatus. In one embodiment, the method of this invention uses a welding apparatus comprising a heat source in combination with vacuum system. In another embodiment, the apparatus of this invention is used for welding metals in a gap-free or narrow-gap lap joint configuration, wherein at least one of the metals is coated. In another embodiment, the apparatus of this invention is used for welding metals in a gap-free or narrow-gap lap joint configuration, wherein at least one of the metals is a metal alloy which is optionally coated. In one embodiment, the apparatus is designed to provide a drag force to balance the instable force occurring during different welding process.

[0050] Depending on the applications and different welding processes, this apparatus can be designed with different shapes and sizes.

[0051] The following examples are presented in order to more fully illustrate the preferred embodiments of the invention. They should in no way be construed, however, as limiting the broad scope of the invention.

EXPERIMENTAL DETAILS SECTION

EXAMPLE 1

Laser Welding Process of This Invention

Methods and materials

[0052] The materials used in this study were zinc coated dual phase (DP) 980 and DP590 steels and zinc coated low carbon steels. The zinc coating is hot dipped at the level of 60 gm/m ² per side. The specimens having the dimensions of 200 mm x 85 mm x 1.2 mm and 200mm x 85 mm x 1.5mm, 150 mm x 85mm x 0.8mm, and 150 mm x 85mm x 0.4mm were cut using an abrasive water-jet. The two metal sheets were then tightly clamped together during laser welding process and a zero gap was assumed. The laser welding process was performed using a 4000 W fiber laser. The multi-mode laser beam was brought into the laser welding head by an optical fiber. The laser spot diameter on focus is 0.3 mm. During the laser welding process, the laser beam was focused on the top surface of the two-sheet stack-up. The experimental setup is shown in Fig.3. In addition, the lap joint coupons were sectioned, ground, polished and etched for hardness measurement and examination under the optical microscope.

[0053] A suction (vacuum) device was developed to create a negative pressure zone of about 20 kPa directly above the molten pool. The vacuum system was set in front of laser beam and was integrated with laser system (Figures 4 and 5). Under the effect of the drag force from the suction device, the molten pool was stabilized and the keyhole was kept continuously open, which provided a channel for the zinc vapor to escape.

Results

[0054] Figure 6, 7 and 9 presents welds obtained by laser welding process using a vacuum system. No spatter and blowholes were observed in the welds with full penetration. The conditions for Figure 6 that were used: Laser power: 3200 W; Focus point: 0.3 mm; Speed 3 m/min; Shield gas: none; Gap: 0 mm. The conditions for Figure 7 that were used: Laser power: 2000 W; Focus point: 0.3 mm; Speed 3 m/min; Shield gas: none; Gap: 0 mm. The conditions for Figure 9 that were used: Laser power: 3000 W; Focus point: 0.3 mm; Speed 3 m/min; Shield gas: none; Gap: 0 mm. The vacuum system of about 20 kPa allows consistent stable and open keyhole, which provides a channel for the zinc vapor to escape. Figure 8 shows the welds obtained by conventional laser welding (not including a vacuum system). The conditions for Figure 8 that were used were: Laser power: 3000 W; Focus point: 0.3 mm; Speed 3 m/min; Shield gas: none; Gap: 0 mm. spatters and blowholes are observed in the welds, as shown in Fig.8.

EXAMPLE 2

Microhardness Test

[0055] Microhardness test was conducted across the weld using 200 g load and 10 sec holding time. The microhardness distribution profile is shown in Figure 10. The hardness distribution is not uniform along the weld. The hardness in the fusion zone (the center of the weld) was higher than that in the base materials. Furthermore, the hardness in the heat affected zone was not degraded compared to that of the base materials.

[0056] It will be appreciated by a person skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather, the scope of the invention is defined by the claims that follow:

Claims

WHAT IS CLAIMED:

1. A method of welding metals in a gap-free or narrow-gap lap joint configuration, said method comprising the following steps: a) arranging at least two metals in a gap-free or narrow-gap lap joint configuration, wherein at least one of the metals is coated; b) applying a heat source in combination with a vacuum system on an area of engagement of said metals, and forming a molten pool between said metals; c) creating a negative pressure zone above the molten pool by said vacuum system allowing vaporization of the coating; and d) cooling the molten metal to yield a weld.

2. A method of welding metals in a gap-free or narrow gap lap joint configuration, said method comprising the following steps: a) arranging at least two metals in a gap-free or narrow-gap lap joint configuration, wherein at least one of the metals is a metal alloy and optionally one of the metal alloys is coated; b) applying a heat source in combination with a vacuum system on an area of engagement of said metals, and forming a molten pool between said metals; c) creating a negative pressure zone above the molten pool by said vacuum system allowing vaporization of a metal with low boiling temperature of coating; and d) cooling the molten metal to yield a weld.

3. The method of claims 1 or 2, wherein said metal is steel, aluminum alloys, copper alloys or magnesium alloys.

4. The method of claims 1 or 2, wherein the coating of said metals has a melting and boiling temperature lower than that of the metal.

5. The method of claim 1, wherein said metal is steel and said coating is zinc.

6. The method of claim 1 or 2, wherein said heat source is a laser beam, light beam, arc or plasma.

7. The method of claim 6, wherein said heat source is a laser beam which cuts a slot between said metals through the thickness of both overlapping parts.

8. The method of claims 1 or 2, wherein said method further comprises a formation of a keyhole which allows vaporation of said coating.

9. The method of claims 1 or 2, wherein said method suppresses the weld discrepancies comprising spatter, blowhole and porosity.

10. The method of claims 1 or 2, wherein said heat source comprises an energy level of between one to twenty kilowatts.

11. The method of claim 10, wherein said heat source is a laser beam generated from a Fiber laser beam.

12. The method of claims 1 or 2, wherein said vacuum system is integrated or adjacent to said laser beam.

13. The method of claims 1 or 2, wherein said negative pressure is below 1 atm.

14. The method of claim 13, wherein said negative pressure is between 0.01 to 600 kPa.

15. The method of claims 1 or 2, wherein said narrow gap between said metals is between 0 to 0.2 mm.