MXPA99011143A - Welding by points with plasma arch in bodyworks, for automobile - Google Patents

Abstract

The present invention relates to a method of spot welding on one side in a construction of metallic bodies for automobiles with vaporisable coatings or ingredients, the construction having overlapping metal sheets, the method consists in: (a) striking a plasma column on a selected point on one side of the construction, the plasma column being created by making a plasma gas at a predetermined flow velocity through an electric arc created by a predetermined electric current path, (b) protecting the plasma column in an inert gas containing at least 5-30% by volume of oxygen, melting the plasma column protected at least the metal of a sheet at the point while the oxygen increases the fluidity and wettability of the molten metal and reduces its surface tension allowing that any vaporizer of the ingredients escape quietly through the molten metal, and (c) stop the shock of the plasma arc to allow the molten metal to solidify and complete the weld by point

Description

POINT WELDING WITH PLASMA BOW IN CAR BODYWORKS This invention relates to welding technology in automotive body constructions, and more particularly, to welding in car body constructions, from one side, while achieving sound welds of steels containing vaporizable ingredients, such as plates covered with zinc, and avoiding shortening the life of the torch. Resistance welding, by points, is used more frequently for car body constructions; the number of points welded by resistance for a car body is usually greater than 4000 (as can be seen in Figure IA). Resistance welding, by points, requires access from both sides of the joint; this limits the design options, and often requires access holes for the joint to be welded 50 in order to complete the weld (as shown in Figure IB and Figure IC). As a result, the size and weight of the parts must be increased to compensate for the lost material and the reduction in stiffness caused by the access holes. Welding on one side of the body can overcome these disadvantages. Welding on one side has been done by the use of laser or plasma, each of which creates a metal melting bath on the most adjacent plate to be welded, whose bath then conducts heat to the next adjacent plate to melt a slightly smaller point; The cooling of the melted, amalgamated points creates the welded joint between these. Plasma arc welding on one side is more precious because it involves less investment and operating costs, it is easier to operate and maintain and does not require a secure enclosure. It is difficult to create effective spot welding from one side of the material that contains vaporizable ingredients, such as zinc-coated steel such as electrogalvanized or hot-dipped steel. Zinc will boil at temperatures well below the melting point of the steel (such as 900 W compared to 15,000 to melt the steel). The zinc vapor must escape through the interface of the molten bath by welding, unless , there is a space between the plates and the interface, as normally a space is not present and, if present, needs careful control in production that is difficult to achieve. Therefore, zinc vapors usually do not have a convenient escape route and explode abruptly to create an outlet through the molten bath. There is no convenient access for vaporized zinc to escape since the molten steel has very high surface tension, low fluidity and low wettability; these characteristics prevent any migration of vapors. An explosion of the molten steel bath leaves behind gaps in the welded joint which is a serious defect and this exploding zinc vapor can cause contaminants to adhere to the nozzle of the welding gun and reduce its operating life. This invention provides an improved method for spot welding on one side of steels with vaporisable coatings or ingredients, and at the same time employs a plasma arc to perform the welding. According to the present invention, there is provided a method of spot welding on one side in a bodywork construction for metallic automobiles with vaporisable coatings or ingredients, the construction having overlapping metallic layers, the method consists in: (a) striking a plasma column at a selected point on the construction side, the plasma column being created by passing a plasma gas at a predetermined flow rate through an electric arc created by a predetermined path of the electric current; (b) protect the plasma column in an inert gas containing at least 5-30% by volume of oxygen, the • protected plasma column melting at least one layer metal at the point while oxygen increases the fluidity and wettability of the molten metal and reduces its surface tension allowing any vaporization of the ingredients to calmly escape through the molten metal; and (c) stop the plasma arc striking allowing the molten metal to solidify and complete spot welding. Advantageously, the arc current can be controlled to be initially low, for a period to allow the plasma column to push a molten bath radially outwardly and through the next most adjacent layer; the current must then be increased during the remainder of the welding cycle while reducing the flow rate of the plasma. It is also advantageous to reduce the arc current during the last phase of the welding cycle to eliminate any vaporized, residual ingredients in the welded joint; after the plasma column has ceased it is preferable to replace a reducing gas with oxygen containing protective gas after completing the welding cycle.

The invention will now be described in greater detail, by way of example, with reference to the accompanying drawings, in which: Figure IA is a perspective view of a common bodywork construction of the prior art illustrating the number of joints that must be soldered. Figure IB is an amplified view of the portions of the interior of the construction in Figure IA illustrating the need for access holes to complete the weld. Figure 1C is a schematic illustration of how to perform resistance welding by the prior art using access holes. Figure 2A is a perspective view of a car body construction that is in the process of spot welding with a plasma arc, on one side, according to this invention; Figure 2B is a schematic, diagrammatic, amplified diagram of a part of the body construction of Figure 2A showing the relationship of a plasma arc gun with respect to the superimposed plate of the construction that no longer requires cutting the access holes to facilitate welding. Figure 2C is a sectional view, in elevation of a plasma arc gun, schematic being used in the heating process of galvanized steel, the illustration shows the disruptive effect of boiling zinc when certain characteristics of this are not used. invention; Figures 3A-3C are a series of diagrammatic views of elevation cuts illustrating how plasma arc welding, in accordance with this invention, effects welding without disruption by vaporized ingredients; Figure 4 is a view of a central, amplified section of a nozzle used in plasma arc welding. Figure 5 is a schematic illustration of the controls for the welding torch of this invention, which allows the regulation and administration of plasma and protective gases in a selective manner; Figure 6 is a timing diagram illustrating an example of how plasma and protective gases can be controlled by solenoid valves and driven in time relation to the current variation of the arc to provide different conditions during the phases prior to welding , during welding and after welding; and Figure 7 is a micrograph (lOOx) of a section of a joint welded by points made in accordance with this invention.

As shown in Figures 2A-2B, the robot 10 is programmed to move the plasma welding head 11 to a position of approximately 10 mm from the target for spot welding 12 in a construction 13 composed of overlapping plates, panels or layers. 14, 15. The head 11 advances to make contact with the most adjacent layer 14 and applies sufficient force to put the unit 13 of the layers in contact with each other (50-200 pounds). The sequence of the required welding is selected by a robot control program stored in an interface unit 16 of a welding console 18. This unit may have the ability to select at least 9 different combinations of welding current and welding time., to adapt different welding conditions. Upon receipt of the signal "end of welding", the head 11 retracts and the robot is free to move to the next target for "spot welding." The power supply 17 for the welding head converts the incoming AC power supply to a supply for DC controlled welding Using the frequency inverter principle, a fast, controlled response is provided for a smooth, controlled welding arc The functions of the welding console 18 include the following: control and verification of plasma and gas flow protector, start of the pilot arc, interpretation of the control of the electric current between the plasma interface and the power supply, and verification of the coolant The console also includes a welding sequencer 20 with solid state control modules for precise control of the variables within a welding operation The characteristics of the interface unit 16 include the selection of the robot of the channel functions and press, manual programming of the channel valves by means of keypad, selected channel screen and actual pressure, the control interface of the plasma equipment to / from the robot and the line controller, and lights indicating the state. The pulse function, which adds a controlled pulse to the output current, is particularly used when welding zinc coated material. The welding sequencer controls the sequencing and synchronization functions for the plasma spot welding program. A microprocessor-based unit of the sequencer 20 provides the user with a selection of less; its function includes all the operations of the welding sequence, offering greater flexibility and includes a disk controller for the storage of welding programs and real results of the welding • operation. As shown in Figure 4, the welding head 11 may consist of a torch integrated in a specially designed pneumatic cylinder unit. Movement of the cylinder unit brings the end of the plasma head into contact, called the protective rate 23 with (or near) the top plate or panel 14. The head 11 may also include proximity switches for the detection of movement of the cylinder unit, a pneumatic valve for the operation of the cylinder and a terminal box to control the signaling. The welding head is installed on a plate for the gun change; This allows the gun to be exchanged within a certain time of the cycle. The air and electric control services pass through the change plate. Plasma services include water inlet, water outlet, electric power, protective gas - and plasma gases, all permanently connected to the gun. The change of the nozzle of the plasma torch can be initiated by means of a pressure button that interrupts the plasma system, drains, water and brings the robot to a convenient position. The welding system uses its own refrigerant recirculator 31. The use of deionized water is required to avoid the establishment of hydrolysis in the plasma head. Feeding the welding services from the welding console to the plasma head is easier with the units mounted on the robot. These welding services can not be routed through a system of rapid change, therefore, a tracking system monitors the services between the console and the head of the robot. The plasma and protective gases are supplied from a local multiple system in the line. The auto start and end of the movement procedures are controlled with the line controllers programmed for the correct sequencing of the gases, water, pilot arc and controls of the main arc. The flows of plasma gas, protective gas and water are constantly monitored, with auto-off if a fault is detected. If the main arc can not be started or the welding signal term is not provided, the system tries to perform another welding at the same position. If the retry fails, then a flag of the fault is emitted, thus avoiding lost solders. The plasma arc welding system of this invention needs one-side access only for welding. This is highly advantageous because not only is robotic repositioning much easier and faster, there is no need for access holes in the construction where the beams or channels contain a joint area (see Figure 2B); the construction no longer needs an increase in size and weight to compensate for the lost material and the reduction of stiffness caused by these access holes.

As shown schematically in Figure 2C, the plasma torch nozzle is composed of a central electrode (anode) 32 which is designed to be oriented normal to the surfaces of the unit (connected as a cathode) that is going to be welded The arc is generated between the electrode 32 and the workpiece or unit 33. The plasma arc is constrained by a nozzle unit that extends downwardly around the electrode and extends a slight distance beyond the tip 34. of the electrode. The plasma is created by passing a plasma gas 35 down between the electrode 32 and the interior of the part of the nozzle 36_ whose gas, when passing through the electric arc established from the electrode, is ionized sufficiently to conduct an electric current. and thus become a plasma. By using a nozzle with artificial atmosphere 36, a high intensity column 37 plasma beam is produced. The result is a stable and controllable high temperature plasma. The gun or torch further possesses a protective cover 38 which is spaced apart from the outer surface of the nozzle part 36 and has a terminal portion which preferably contacts the unit 33 during the welding cycle. The space between the protective layer and the nozzle provides a channel 39 through which the shielding or protective gas passes to protect against rusting the molten metal bath (which is being generated by the welding cycle). The electrode 32 is preferably constructed of tungsten and is secured within the spplette behind the mouth of the nozzle and is thus protected from external impurities that would normally attack the surface at very high temperature of the electrodes. This arrangement increases the electrode life over other arc welding processes to such an extent. Unlike TIG or MIG welding processes, plasma arc welding does not require the use of high frequency arc initiation. The ignition of the plasma arc is achieved by a pilot arc that remains at all times. The high frequency is only required for the initial ignition of the pilot arc. The fact that only high frequency is used once per working day avoids potential problems of high frequency interference with respect to the electronic hardware that controls the robot or other aspects of the welding system. The protective layer 38 is constructed to withstand a combination of heat and indentation. The layer can be easily replaced as a consumable article. The water cooling of the gun or torch can be provided by channels 42 (shown in Figure 4). The size of the gun or welding head 11 is smaller. Most robots for welding car body constructions have a weight limit for the welding gun (or torch) that will not exceed 100 kg; Many parts of the body construction have box channels or shapes that need a resistance welding gun with a size that would have to exceed the weight limit of the robot. Vaporizable materials that are present as a coating or present within the laminated steel itself may cause explosions boiling at a temperature below the melting point of the workpiece material. For example (as shown in Figure 2C) the zinc boils at 900 ° C while the steel, which is going to be welded, melts at a temperature of 1500 ° C. Therefore, when the molten bath 43 is created in the sheet or upper plate 14 (while spot welding is performed) the zinc vaporizes and tries to escape with much difficulty through the interface or the molten bath. The vapor pressure of zinc accumulates in large packages or bubbles 44 and then finally leave the molten bath in an explosion. This explosion leaves behind a gap in the solder joint, a serious effect and the exploded molten metal tends to adhere to the nozzle or electrode of the plasma torch seriously reducing the life of the torch or nozzle. An important feature of this invention is the addition of oxygen during a preselected part or throughout the welding cycle to reduce the surface tension of the molten steel or the material being welded and increase the fluidity and wettability of the molten materials. As shown in Figures 3A-3C, plasma arc welding is obtained by burning through the plate or upper plate 14 to form a molten metal bath of the solder 46 from one side of the unit. This eliminates the restriction of other welding processes that would require a precise torch-to-joint ratio; Although • the torch touches the unit, it is not necessary for the robot to make contact with the unit, which avoids variations in the height of the thickness that are so necessary in resistance welding. The system of this invention can weld multiple sheets or thicknesses, even up to 4 panels, which is not tolerable with resistance welding. Accordingly, as shown in Figure 3A, the top sheet is heated by the plasma column 37 and the molten metal bath 46 is created. The molten bath becomes larger and is pushed down by the plasma or jet gas 35 to make contact with the plate of the bottom plate 15. To allow migration of the zinc vapor through the molten bath, suitable oxygen must be present in the shield gas 40 to reduce the surface tension of the molten metal bath and increase its fluidity and wettability (characteristics that allow the zinc vapor to escape while the molten metal bath remains adhered to the panel or plate). There is no formation of zinc vapor bubbles and thus there is no explosive condition. Instead, many very small bubbles form in the molten metal cloth and can migrate outwards like a fine mist of the weld material coming out of the molten metal bath. As shown in Figure 3A and 3B, the lower plate is heated by conduction (at 48) from the bath of molten metal which has flowed in its manner in contact therewith. The heat accumulates at a smaller point within the lower plate 15 and melts. The molten metal bath of the weld 46 is made slightly larger in the upper plate 14, while the lower plate is progressively melted through in 49. As shown in Figure 3C, when a sufficient point 50 melts in the bottom plate, plasma arc and column are interrupted and the molten metal is allowed to cool to form a solid, welded joint 51. Although oxygen has reduced the surface tension in the molten metal bath, very fine bubbles may allow a slight accumulation of small splash on the nozzle which, over a long period of time, may adversely affect the operation of the nozzle. Returning to Figures 5 and 6, the gas and current controls are used to remedy this problem. To minimize splashing of small bubbles, it is possible to observe certain parameters at the beginning or reflux before the welding cycle. These parameters include the use of a strong plasma gas flow (approximately 20 liters / minutes) in a time of approximately 0.5 seconds through the plasma channel, as well as through the protective gas channel. At the same time, a small arc current (approximately 10 amps) is used during this short period of 0.5 seconds. When the welding cycle is started, the plasma gas flow is maintained at 20 liters per minute and the arc current is increased to 20 amps for 0.5 seconds. The strong flow of plasma gas forces the molten metal from the molten metal bath to flow outward and thus carry any of the splashes out of the nozzle; The small arc current also reduces the amount of small bubble splashes. During the first 0.2 seconds of the welding cycle, none of the oxygenated protective gas enters the nozzle. After 0.2 seconds of the welding cycle, the flow rate of the protective gas Ar + H2 is advantageously reduced and then at 0.5 seconds in the welding cycle, the flow velocity of the plasma gas is further reduced to approximately 1.5 liters per minute after that. At the same time, the oxygenated protective gas is introduced through the protective gas channel at a flow rate of approximately 10 liters per minute. The arc current at the same time increases to 35 amps. In this way the molten metal bath is prevented from being blown out and the increase in arc current gets the proper size for the molten metal bath. To avoid some small bubbles of zinc vapor left in the solder joint, the arc current is not suddenly interrupted, but rather it is reduced to the 20 amps level for a period of approximately 0.25 seconds before completing the welding cycle. This gives rise to an excellent appearance of welding without blowholes. To perform these time-controlled functions of the plasma and protective gas with different compositions and flow rates and different electric current, Figure 5 illustrates the solenoid valves used to perform these functions. Figure 6 also represents an example of how a program can be used to make these considerations of different functions.

Spot welding of plasma requires more time than resistance welding, having a point diameter usually 7 mm compared to 4 mm for spot welding by resistance. However, the complete welding cycle time for plasma arc welding is larger than for a resistance spot welding, equivalent, with time approximately 2.25 seconds compared to 1.1 seconds for resistance welding (an increase of double time). The impact of slightly longer welding times is displaced by the faster position of the torch since less manipulation is required and the gun is not retracted. Some of the molten metal may be oxidized during the plasma arc welding sequence of this invention; this is controlled to a tolerable level by controlling the duration of use of the protective gas during the welding cycle. A reducing protective gas containing hydrogen is admitted through the protective gas channel initially used for approximately 0.2 seconds of the welding cycle and again after completion of the welding cycle. The use of oxygenated protective gas during the critical periods of welding provides much larger consumable electrode light, start of viable arc, controlled penetration, high welding speed and is highly repetitive in automated soldering. The excellent welding created by the practice of this invention is shown in Figure 7. This is a photomicrograph of a cut of a spot weld by electrogalvanized steel plasma. There is a narrow oxide layer 60 on the upper surface of the spot welding, and the interface area 61 is free of oxides; the integrity of the weld with tensile strength of more than 304 kgf (2990 N) is maintained. If the welding time is set to more than 2 seconds, the resistance of the spot welding by plasma will be equal to or better than the spot welds.

Claims (7)

1. A method of spot welding on one side in a construction of metal bodywork for automobiles with vaporisable coatings or ingredients, the construction having overlapping metal sheets, the method consists in: (a) striking a plasma column on a selected point of a construction side, the plasma column being created by passing a plasma gas at a predetermined flow rate through an electric arc created by a predetermined electric current path; : (b) protect the plasma column in an inert gas containing at least 5-30% by volume of oxygen, melting the plasma column protected at least the metal of a sheet at the point while the oxygen increases the fluidity and wettability of the molten metal and reduces its surface tension allowing any vaporization of the ingredients to escape quietly through the molten metal; and (c) stop the plasma arc striking to allow the molten metal to solidify and complete spot welding. The method as recited in claim 1, wherein the plasma arc current is initially low for a period of about 0.5 seconds to allow the plasma arc column to push the molten metal bath radially outward and therefore through the first sheet and at the same time the current of the arc increases while the flow velocity of the plasma is reduced during the rest of the plasma welding. The method as recited in claim 2, wherein the arc current is reduced during the last 0.25 seconds of the shock of the plasma column. 4. The method as recited in claim 1, wherein after the plasma arc column has ceased, a protective, reducing gas is replaced by oxygen. The method as recited in claim 1, wherein the protective gas is selected from argon and helium. The method as recited in claim 1, wherein argon is selected as the main protective gas containing oxygen in a volume amount of 25-35%. The method as recited in claim 1, wherein helium is selected as the protective gas containing oxygen in a volume amount of 10-25%.