MIG arc brazing of the hottest coated plates

2022-08-14
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MIG arc brazing of coated plates

preface

with the increasing application of coated plates (such as galvanized plates) in industry in recent years, galvanized plates, together with aluminum and magnesium, have been paid more and more attention as the most promising materials

however, how to effectively weld galvanized sheet metal has always troubled engineers and technicians, because the melting point of zinc is about 420 ℃, the volatilization temperature is 908 ℃, and the high-temperature arc of ordinary fusion welding (about 3000 ~ 4000 ℃) will inevitably promote a large amount of zinc evaporation, which will lead to various welding defects and destroy the corrosion resistance of zinc layer. Now, these problems have become a thing of the past, thanks to the innovative technology developed by frontius - MIG arc brazing

mig arc brazing also belongs to gas metal arc welding (MAG) in a sense. It uses copper based welding wire with low melting point to replace carbon steel welding wire. When welding, the heat input is low, the base metal will not melt, and the evaporation of zinc is minimized, which improves the corrosion resistance of the weld (copper based weld is also corrosion resistant). Thus, better and more effective welding of galvanized sheet is ensured

mig arc brazing has achieved great success in practical industrial applications. Many automobile manufacturing industries are increasingly using copper based welding wires to produce important parts. Recently, MIG brazing is also used to weld uncoated plates (such as alloy steel, non alloy steel, stainless steel, copper), which also achieves good results

this paper comprehensively expounds MIG arc brazing from the aspects of welding arc, welding machine, welding wire, wire feeder, welding gun inclination and the comparison of welding results

overview

with the development of modern industry, the application of corrosion-resistant coated plates is becoming more and more extensive. Among many methods of steel corrosion prevention, galvanizing is a very effective and cheap method

of course, it is also possible to galvanize the surface of mechanical parts after processing to prevent corrosion. However, this requires that the workpiece must be strictly cleaned and carefully packed before galvanizing, and another feasible method is to manufacture the workpiece with pretreated (i.e. Galvanized) plates. These pretreated plates can be produced by electroplating or hot-dip plating. The thickness of zinc coating is 1 ~ 20 μ M depends on the production process and method adopted by Shandong Huayu, which is in the critical stage of new and old kinetic energy conversion and product transformation and upgrading

a large number of galvanized sheets are used in automobile manufacturing, refrigerator, construction, ventilation and heating facilities, furniture manufacturing and other fields. Galvanizing has become an important anti-corrosion method for steel, not only because zinc can form a dense protective layer on the surface of steel, but also because zinc has cathodic protection effect. When the galvanized layer is damaged, it can still prevent the corrosion of iron base metal through cathodic protection effect, which can extend to the area without protective layer of 1 ~ 2mm, Therefore, galvanizing can effectively protect the micro cracks caused by the incision and cold processing of the plate and the zinc burning area near the weld to prevent rusting from here

physical properties of metal materials table material melting point boiling point

zinc 420 ℃ 908 ℃

steel 1300 ℃ --- cusi3 950 ℃ -

the melting point of zinc is about 420 ℃, and the volatilization temperature is 908 ℃, which is not conducive to welding. When the arc is just ignited, it volatilizes. The volatilization and oxidation of zinc will lead to pores, lack of fusion and cracks, and even affect the arc stability. Therefore, the best way to weld galvanized sheet is to reduce heat input

a feasible method is to use brazing materials to weld galvanized plates. The most commonly used welding wires are copper silicon alloy (such as cusi3) and copper aluminum alloy. Due to the high content of copper in these welding wires, the melting point is relatively reduced (about 1000 ℃ ~ 1080 ℃, depending on the alloy composition). At this time, the base metal has not been melted, and such joints are actually brazed joints

generally, argon is the recommended shielding gas. However, experiments have proved that cusi3 can also be used as a shielding gas with a small amount of oxygen or carbon dioxide, so that the stability of the arc is better

MIG arc brazing for filler metal has the following advantages:

1) no corrosion of weld

power supply: tps4000 welding wire: cusi3 φ 1.0mm

protective gas: 100% argon

welding speed: 40mm/min

welding current: 112a

welding voltage: 15.2V

wire feeding speed: 7.4m/min

2) little splash

3) less burning loss of zinc coating

4) low heat input

5) easy mechanical processing of welds

6) cathodic protection in the near seam area

the following copper alloys can be used as welding wires for MIG arc brazing galvanized sheet:

● a202m:sg-cusi3

(material No. 2.1461)

● a207m:sg-cusi2mn

(material No. 2.522) non standard

● a2115/8m:sg-cual8

(material No. 2.0921)

in practical work, cusi3 welding wire is the most widely used. Its advantages are low weld hardness and easy machining after welding. The flowability of welding wire largely depends on the silicon content. The higher the silicon content, the better the flowability of molten pool, which can meet the requirements of small gap joints

a207m welding wire can also be used to weld galvanized sheet. The 1% Mn content in the welding wire is mainly to improve the hardness of the weld, and the weld processing is relatively difficult after welding. This kind of welding wire is mainly used in occasions that do not need to be treated after welding

cual8 welding wire is mainly used to weld aluminum clad steel. In addition, many other copper alloys can also be used as brazing wires:

● a216m:sg-cual8ni2

● a200sm:sg CuSn

● a203/6m:sg-cusn6

short circuit transition and spray transition

thick zinc coating (above 15um) will produce a large amount of zinc vapor during welding, affecting the stability of welding, so it is best to use short circuit transition or spray transition of short arc, and short arc length can make the arc more stable. For the above reasons, there are strict requirements for welding power supply and its control characteristics

pulse arc

when welding in argon rich shielding gas environment, short-circuit transition can be controlled by selecting appropriate base value and peak current parameters. When the optimal parameters are selected, just one metal droplet is separated from the welding wire by each pulse. Fronius company has found that there must be different pulse waveforms for different filler metals and shielding gases

in order to minimize the volatilization of galvanizing, low-energy MIG arc brazing must be used, which requires the power supply to provide a particularly stable arc at low power. If there is a very sensitive arc length feedback control, it can maintain a stable short arc at a very low base current

in a word, low heat input can reduce the volatilization of zinc, thus reducing the porosity, improving the joint strength, and also conducive to the machining of welds (such as grinding)

when the dry elongation of the welding wire (the length of the welding wire between the conductive nozzle and the arc) changes, there should be no splash, which requires that during the process of the dry elongation change, the metal transition form should be controlled as "one droplet per pulse"

cooperative operation

in order to obtain a good weld, the welding power supply is required to provide very flexible parameter selection, so that many types of welding wires have a good transition form during pulse welding, which requires a large number of continuously adjustable parameters (about 30 parameters). However, so many parameter adjustments make the operation of the power supply very difficult, and only a few experts can freely use it

therefore, it is required to pre program the welding parameters required by various welding wires and their corresponding shielding gas, so that the cooperative (single) operation can be achieved, so that the welding machine is easy to use for each welder.

in fact, for various base metals, the selection of the best parameters required by welding wires and shielding gas has been made for users during the equipment manufacturing, and 5.5 discs have been placed in the cup punching tool to align with the punch die, These empirical results are stored in EPROM (storage module) to form a database. Users only need to directly look at the measured experimental data and select the filling material on the welding power supply

wire feeder

the brazing wire is very soft, which has special requirements for the wire feeding system. There should be no damage during wire feeding, and the contact pressure should not be too large. Installing a suitable wire feeding wheel with four-wheel drive can provide sufficient wire feeding force. Usually, a smooth semi-circular groove wire feeding wheel is used. Another key point is that the wire guide tube can be fed accurately and smoothly, and the wire guide tube must be flexible (Teflon or graphite fiber tube). Frontius four-wheel wire feeder adopts smooth U-shaped groove wire feeder and soft and wear-resistant graphite fiber wire feeder

welding gun inclination

the welding gun "pushes forward" (the forward direction is opposite to the inclination) to braze the thin plate. The arc at the base current will preheat the zinc coating in front to the volatilization temperature, and the heat brought by the droplet transition will volatilize the zinc coating. Little zinc vapor enters the molten pool and continues to be discharged during the solidification process. Therefore, there are few or even no residual pores in the weld. When the welding gun is "pulled back" (the forward direction is the same as the inclination direction), the preheating effect cannot reach the volatilization temperature of zinc, which means that a large amount of zinc vapor will diffuse into the molten pool. Although the inclination of the welding gun has a "back heating" effect, which can prolong the solidification time of the molten pool, it is not enough to make a large amount of zinc vapor escape from the weld, and the influence of the escaping zinc vapor on the stability of the arc is greater than that of "pushing forward" welding

improve safety

traditional MAG welding (i.e. CO2 gas shielded welding) produces a large amount of zinc vapor, which damages the health of operators. With MIG arc brazing welding, the heat input is low, and the welding smoke and spatter are greatly reduced, which can effectively improve the working environment

diameter of filling material mm transition form protective gas tensile strength n/mm2

sg2 0.8 short circuit transition ar+18%co2 320.9*

cusi3 1.0 pulse particle transition ar 309.5*

tensile strength test

the following is the static tensile strength obtained from a group of different filling materials and different transition forms (free short path transition and pulse arc transition)

test:

before the test, the sample underwent X-ray flaw detection, As expected, the sample welded with SG2, whether free short-circuit transition or pulse transition, contains a large number of pores, because of a large amount of evaporation of zinc. The sample welded with cusi3 has no pores, because the brazing temperature is low, zinc volatilizes less, pores are less, and the tensile strength is improved

mig brazing application range

mig arc brazing can be used to weld low-alloy steel, non alloy steel and stainless steel. Its main purpose is to weld steel plates with coating on the surface

the low melting point of the welding wire used and the low heat input during welding reduce the volatilization of zinc in the near seam area of the workpiece and the back of the weld

and MIG brazing, like MAG welding, can weld various types of joints and all position welding. Even in the case of vertical down, vertical up and overhead welding, satisfactory results can be obtained, and the welding speed can also reach the level of MAG welding (100cm/min)

nowadays, MIG arc brazing has been widely used in automobiles and supporting structures. Even the important scientific testing instrument steel with high seismic strength and internal and external structural defects (such as bicycle support) also uses MIG brazing welding method

this is because the MAG welding with short-circuit transition produces a convex weld, which reduces the tensile strength of the weld; On the other hand, the traditional brazing will cause obvious deformation of the pipe. MIG arc brazing overcomes the shortcomings of the above two methods, with low workpiece input and concave weld

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