Essential Guide to Choosing the Right Gas Welding Flux

Flux for Nickel and Nickel Alloy Gas Welding

Gas welding is suitable for the welding of pure Ni, NiCu, and NiCr alloys, but not suitable for welding low-carbon Ni, NiMo, and NiCrMo alloys. In order to improve the deoxidation ability of the welding pool, filler wires containing Mn <3% or Mn 0.06% and Si 0.2% are often used during gas welding. When gas welding nickel and nickel alloys in multiple layers, filler wires containing Cr 20.7% and Ni 74.5% can be used.

The fluxes used for gas welding nickel and nickel alloys are listed in Table 7-6.

Table 7-6 Gas Welding Fluxes for Nickel and Nickel Alloys

Flux compositionsApplications
Anhydrous borax 100%; anhydrous borax 25%, boric acid 75%; borax 30%, boric acid 10%, sodium chloride 10%, potassium chloride 10%.Welding pure Ni
Barium fluoride 60%, calcium fluoride 16%, barium chloride 15%, arabic gum 5%, sodium fluoride 4%.Welding NiCr alloys
A mixture of one part lithium fluoride and two parts calcium fluoride containing 3% hematite, forms a water-based paste as flux, along with the second or third type of solder.Welding NiCr and NiCrFe alloys
Iron casting flux: 65% borax, 35% boric acid.Welding precipitation-hardened NiCr alloys
Lithium fluoride combined with a second or third type of flux forms an aqueous paste flux.Welding NiSi cast alloys

Magnesium and Magnesium Alloy Gas Welding Flux

When gas welding magnesium and its alloys, a flux should be used, which is typically prepared on-site during welding production. The flux for gas welding magnesium and its alloys is primarily composed of fluorides. Fluorides have a lower corrosive effect on magnesium alloys, and some of the gases produced upon decomposition can provide a protective effect.

These gases mainly consist of hydrogen fluoride and fluoride vapor, which play a certain protective role in the weld pool. Additionally, this fluoride gas welding flux also has a reducing and dissolving effect on the oxides of magnesium.

Preparation of magnesium and magnesium alloy gas welding flux: 36% lithium fluoride, 17% calcium fluoride, 20% barium fluoride, 18% magnesium fluoride, and 9% sodium fluoride. The moisture content should not exceed 1%, and other impurities should also not exceed 1%.

After preparing the materials according to the above proportions, the materials are first placed in a furnace for drying. During the drying and heating process, the materials are initially kept at 120°C for 30 minutes, then the temperature is raised to 140°C and kept for 1 hour, followed by heating to 160°C and maintaining that temperature for 30 minutes.

The dried materials are then crushed and sieved through a 100-mesh sieve to produce the finished flux. Care should be taken during the crushing and sieving process to prevent the flux from reabsorbing moisture. Finally, the flux is packed into colored glass bottles and sealed for production use.

When gas welding magnesium and its alloys, CJ401, an aluminum gas welding flux, can also be used. This commercially available flux has good processability and is suitable for welding magnesium alloys. However, the residue of the flux after welding has a greater corrosive effect on the base material, so the residue on both sides of the weld must be thoroughly removed after welding.

During magnesium alloy gas welding, fluxes composed of chlorides and fluorides can also be used. The main formulations are: 54% KCl + 30% CaCl2 + 12% NaCl; 46% KCl + 26% NaCl + 24% LiCl + 4% NaF. Both of these flux formulations can stabilize the welding process and have good processability.

From experience, the former can be used for general gas welding and gas tungsten arc welding, while the latter can only be used for oxyacetylene gas welding. Due to the strong oxidizing nature of these fluxes, when hydrogen is present on the surface of the workpiece, it can cause surface irregularities. The degree of corrosion to the base material is significant, so the residue should be thoroughly removed after welding.

Copper and Copper Alloy Gas Welding Flux

When gas welding copper and copper alloys, CJ301 (i.e., copper gas welding flux) can be used, or the formulation provided in Table 7-7 can be prepared independently.

Table 7-7 Example Formulation of Gas Welding Flux for Copper and Copper Alloys

Serial NumberFlux Composition/%Preparation Method
Dehydrated BoraxBoric AcidSodium PhosphateWood Charcoal PowderSilica SandMagnesium Powder
1100Heat the commercially available borax in a porcelain, refractory clay, graphite crucible, or stainless steel container to 700-750°C for 10-15 minutes to remove moisture, then pour it out, crush, and sift it.
2503515After the components are mixed, grind them finely in a ball mill or mortar.
350152015After the components are mixed, grind them finely in a ball mill or mortar and pestle.
4946Seal the mixture in a graphite crucible, heat it to 1050-1150°C for 5 minutes, then pour it out and grind it.

Flux No. 1 is suitable for gas welding copper and copper alloy thin plates. When using general pure copper wire as the welding wire, it is best to use Flux No. 3 or Flux No. 4.

Usage of copper and copper alloy flux: Before welding, mix the flux with water to form a paste, apply it to the groove of the workpiece and the surface of the welding wire.

Alternatively, wet the welding wire in a solution of water glass (sodium silicate) and water (in a 1:1 ratio), then place it in the flux groove and roll it slightly to uniformly coat the surface of the welding wire with a layer of flux. Subsequently, let it air dry (10-15 minutes). Alternatively, heat the welding wire and roll it in the flux to coat the surface with a layer of flux.

When gas welding brass, fluxes are commonly divided into two types: solid powder fluxes and gas fluxes. The composition of the fluxes can be found in Table 7-8.

Table 7-8: Composition of Brass Gas Welding Flux

TypesFlux Composition/%
BoraxBoric AcidSodium dihydrogen phosphateSodium fluorideMethyl borateFormaldehyde
Solid Powder Flux100
2080
503515
207010
Gas Flux7525

The gas flux for brass consists of a mixture of methyl borate and methanol, which forms a highly volatile solution with a boiling point of 54-56°C. The evaporated gas reacts with oxygen in the flame as follows:

2B(OCH33+9O2=B2O3+6CO2+9H2O (7-8)

The resulting boric anhydride (B2O3) condenses onto the base metal and the welding wire, creating a strong deoxidizing effect during welding. It forms borates with oxides (Cu2O, ZnO) that float on the surface of the molten pool in a thin film state, effectively preventing the evaporation of zinc from the brass.

The uniform delivery of the gas flux into the molten pool improves the stability and quality of the welding process. Consequently, the gas flux has been successfully applied to brass gas welding and brazing. The consumption of gas flux should generally be around 25kg of methyl borate per 1kg of welding wire melted, ensuring both weld quality and effective prevention of zinc evaporation.

When gas welding aluminum and aluminum alloys, flux is added to remove the oxide film and other impurities from the surface, ensuring a smooth welding process and weld quality. The flux for aluminum and aluminum alloys consists of chlorides and fluorides of elements such as K, Na, Ca, and Li, which are crushed and sieved to form a powdered compound in specific proportions.

For example, cryolite (Na3AlF6) can dissolve alumina at 1000°C, and potassium fluoride (KF) can transform refractory aluminum oxide into easily fusible aluminum fluoride (AlCl3, melting point 183°C). This flux has a low melting point, good fluidity, and can improve the fluidity of the molten metal, resulting in well-formed welds.

There are two types of gas welding flux for aluminum and aluminum alloys: fluxes containing Li and fluxes without Li. Potassium chloride in flux containing Li can improve the physical properties of slag, lower its melting point and viscosity, and effectively remove the oxide film, making it suitable for thin plate and all-position welding. However, lithium chloride is expensive and hygroscopic.

Flux without Li has a high melting point, high viscosity, poor fluidity, and is prone to slag inclusion, making it suitable for welding thick and large components. Table 7-9 lists the compositions of these two types of flux, which can be prepared independently or purchased as pre-packaged flux, such as the aluminum gas welding flux labeled CJ401.

Table 7-9: Composition Formulation of Gas Welding Flux for Aluminum and Aluminum Alloys

 IndexFlux Composition/%Characteristics
CryoliteNaFCaF2NaClKClBaCl2LiCl
01(CJ401)7.5~927~3049.5~5213.5~15Melting point approximately 560°C
2835489
34192948
4203050
5454015

The flux for gas welding aluminum and aluminum alloys is highly hygroscopic, therefore it should be tightly sealed in containers to prevent moisture absorption and loss of effectiveness. During welding, the flux should be mixed into a paste with clean or distilled water, then applied to the joint or coated onto the welding wire. It’s best to use the mixed paste immediately and avoid prolonged storage to prevent deterioration.

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