Comparison between gas flame welding and shielded gas welding in case of aluminium alloys

Although the assembly by welding of aluminum alloys is a common assembly process, it is done in quite difficult conditions, being forced to take into account a number of shortcomings, such as: high thermal conductivity which translates into losses of high heat in the welding area; formation of aluminum oxide films that melt at high temperatures, well above the melting temperature of the alloy; large expansion coefficients that result in the appearance of large deformations, the appearance of cracks in the joint at high temperatures due to loss of mechanical strength of the alloy, but also the appearance of pores, inclusions of aluminum oxide and corrosion in various aggressive environments. Despite all these shortcomings that occur in welding, aluminum alloys can be joined indestructibly by welding. The present paper aims, is to make a comparative study, based on experimental data, of two welding assembly processes, one with gas flame and another with shielded inert gas environments-WIG, for aluminum alloys of group 6000. The base material will be presented, the preparation of the basic material for welding through the two mentioned welding procedures and the steps taken to make the welded joints. The comparative study will include the parameters of welding regimes, non-destructive defectoscopy methods with which certain discontinuities can be identified and the destructive mechanical tests. The experimental results will be able to lead to pertinent conclusions on the possibilities of aluminum alloys welding assembly, using those two welding processes mentioned above, the results being easily generalized in the industrial fields.


Introduction
The production of non-ferrous metals and alloys is growing worldwide and occupies an important place in the construction of welded structures.Of these non-ferrous alloys, aluminum alloys are widespread.
Depending on the alloying elements and heat treatment, aluminum grades can exhibit a wide variety of properties, from good appearance, ease of manufacture, corrosion resistance, high power-toweight ratio, good weldability and high crack resistance.The appropriate selection of aluminum grade depends on the required application and working conditions [1].
Non-ferrous metals and alloys differ from steel by their composition and by their characteristic properties.The welding behavior is also appreciated depending on the melting and boiling temperature, the thermal and electrical conductivity, the mechanical characteristics at high and low 1303 (2024) 012032 IOP Publishing doi:10.1088/1757-899X/1303/1/012032 2 temperatures, the gas dissolution capacity, etc. Welding these aluminum alloys requires highly concentrated heat sources to compensate for conduction losses [2].
There are a multitude of aluminum alloys that can be assembled in non-removable assemblies by electric arc welding and gas flame welding.In both cases there are advantages and disadvantages.In this sense, in this paper we will analyze for a certain aluminum alloy from the 6000 series, the possibility of welding with electric arc, in a shielded gas environment (Argon) -the WIG process, with as well as the possibility of welding with gas flame.In this sense, will be presented the basic material, the non-removable assembly procedures with their specific particularities, the filler materials used, as well as the mechanical fracture tests.The purpose of the research is to determine which of the two non-removable assembly processes is more suitable for use.The topic is of interest because both assembly processes referred to above are widely used in the industrial environment.

Base material
Aluminum is characterized by a series of specific properties such as: specific weight of 2.7g / cm 3 , melting point of 658 C, boiling point of 2500 C, has a high thermal conductivity, low electrical resistivity, good plasticity, high resistance to low temperatures and high corrosion resistance.Instead, aluminum has low mechanical strength σr <100N / mm 2 , high coefficient of expansion and low modulus of elasticity.
Aluminum alloys are divided into two major categories: deformable and cast.The deformable ones are used in untreated state or hardened by dispersed precipitation.Most of the aluminum constructions are non-heat treated, but lately, joints from hardened alloys have also appeared through precipitation treatments.
Aluminum alloys in the 6xxx series (6061, 6063) contain silicon and magnesium in approximate proportions necessary for the formation of magnesium silicide (Mg2Si), thus making them heattreatable.Although not as strong as most 2xxx and 7xxx series alloys, 6xxx series aluminum alloys have good toughness, weldability, machinability and relatively good corrosion resistance, with medium strength.
Two plates of dimensions 2x150x150 mm were used for the butt joint, for each process-WIG and oxy gas welding, as can be seen in figures 1 and 2.
In this paper was used as a base material for the butt assembled samples, material type AlMgSi0.

Characteristics of welding and brazing of aluminium alloys
The characteristic element that appears when welding aluminum is the Al2O3 aluminum oxide film, which is formed due to the high affinity of aluminum for oxygen.This film has a high melting point of 2050 ̊ C and stop the base metal from melting, often remaining in the form of non-metallic inclusions in the weld seam.This aluminum oxide must be removed either due to the bombardment effects in the electrical arc welding, with positive ions when fed in DC with reverse polarity or by dissolution with the help of fluxes in case of the oxy gas welding.The rupture of the oxide film is generally obtained for small thicknesses, for large thicknesses being necessary mechanical or chemical removal measures.
Another problem appeared when welding aluminum is the appearance of gases in the weld seam, near the melting line of the base metal.The main cause of the appearance of pores in welded aluminum seams is considered to be hydrogen.Removing the pores from the weld seam is one of the most important concerns in establishing welding technology.For this it is necessary to perform a good cleaning of oxides and greases on the surfaces of the parts subjected to welding [4].
An important difficulty that occurs when welding aluminum alloys is the phenomenon of hot cracking.Cracking is generally caused by the silicon content.Quantities up to 0.6% Si can cause cracking.The presence of iron has an inverse influence.A content of 0.7% Fe leads to increased crack resistance.In the same sense, the additions of 5-6% Mg also influence.
Welding of aluminum alloys is much more difficult due to the occurrence of cracking phenomena, especially those that contain alloying elements.This can be combated due to aging treatments and precipitation of intermetallic phases [5].
The filler metal used to the welding of aluminum and its alloys is chosen from the condition of not forming in the metal bath fragile products such as Mg2Si, CuAl2, MnAl6 and Al-Fe-Si.They can penetrate the weld seam and substantially reduce plasticity.
The wire used for welding must have a mechanical strength at least equal to that of the annealed base material.
An important condition when choosing the wire for welding aluminum is the solidification interval.The lower it is, the lower is the risk of cracking.The solidification interval is recommended to be below 50 ̊ C.
Welding wires are chosen with the same composition as the base metal if the heat treatment is done after welding, or with a higher mechanical strength if no subsequent heat treatment is applied.These wires contain alloying elements based on Zr, Cr and Ti.It is recommended that the wires be clean for welding and the coatings and fluxes well dried.
In this work, two non-removable assembly processes were used, one by WIG welding and another by welding with a gas flame.

WIG welding
The welding process in shilded gas environments has many advantages: easy handling of the gun used for welding, good arc control, narrow molten welding bath, spray-free spring electrical arc welding and clean welding seam.
The AC current was used for WIG welding process, to cope with the high melting temperature of the oxide layer on the surface of the molten metal bath.By using the AC current, the aim is to break the oxide layer and remove it by means of ions that pass from the electrode to the part.AC welding is associated with a continuous change in the polarity of the infusible tungsten electrode.There are two phases (half waves), a positive phase and a negative phase.The positive phase causes the aluminum oxide layer on the surface of the material to break (the so-called cleaning effect).At the same time a cap is formed on the tip of the tungsten electrode.The size of this cap depends on the duration of the positive phase.It will be taken into account that too large a cap leads to the formation of a diffuse electric arc welding with low penetration of the weld.The negative phase cools the tungsten electrode on the one hand and reaches the required penetration on the other hand.It is important to choose correctly the ratio (balance) between the positive phase (cleaning effect and the size of the cap) and the negative phase (depth of penetration to the weld).In this sense, it is necessary to adjust the AC balance.The default (zero) balance adjustment is 65% and this ratio refers to the negative half-wave part.Rupture of the oxide film can generally be done for small thicknesses.For large thicknesses, additional measures are required to remove the oxide film mechanically or chemically.
The shielding gas welding machine is a Stahlwerk TIG 200 AC / DC inverter.The device has a high frequency and voltage generator that allows the ignition of the arc without electrical contact at an output current up to 200 A. The IGBT technology used by the machine is based on powerful IGBT transistors, which have the advantages of high blocking voltage and robustness. .The flow of the shielding gas when the button is pressed on the gun, before closing the welding circuit, protects the infusible electrode from excessive wear and protects the welding seam from oxidation.The shielded gas flow released after welding protects the infusible electrode from excessive wear and protects the welding molten bath from oxidation.The operating modes of the control can be determined via the 2T / 4T switch.This mode provides an extended control over the current flow.The smooth slope of the arc extinguishing prevents the appearance of end craters at the end of the welding.After the button on the gun has been released, the device switches to an automatic lowering phase.Downslope only works in 4T mode.
The filler material chosen is the Al-4043-AlSi5 rod according to EN ISO 18273 (2004) or ER 4043 according to AWS-A-5.10.The filler material was chosen with a chemical composition similar to the base material, possibly with a surplus of 1-2% Mg, to compensate for combustion losses.The shielded gas used for welding was high purity Argon.The gas flow is 15 l / min.
The parameters of the welding regime are presented in table 1 After welding the sample, 6 specimens with a width of 25 mm were cut from it, as seen in figure 3. Subsequently, these specimens will be tested for fracture, by static traction

Figure 3. WIG welded samples
It should be mentioned that, before cutting the test pieces, in order to detect possible discontinuities, a visual and penetranting liquid tests was done for the welding seam obtained by the WIG welding process.Following the non-destructive investigations, no notable defects were highlighted, as can be seen in figure 4.

Oxy-gas flame welding
In the gas flame welding of aluminum and its alloys, the parameters from table 2 were used and a series of technological recommendations were taken into account.The preparation of the sheets to be welded is done by cleaning, degreasing and pickling the sheets on a width of 30 to 40 mm, on both sides of the joint.Degreasing is done with a solution of 3 ... 5% sodium hydroxide and 1 ... 3% sodium silicate, followed by a wash with hot water and a pickling with a solution of 10% nitric acid, again following a wash with water. .It will then be preheated to 300 ... 350 ̊ C which helps to transmit the heat well, during welding.To prevent the parts from cooling during welding, they are placed on thermally insulating material (asbestos or refractory bricks).
For a thickness of 2 mm, an I-joint will be used, with a very small gap between the sheets.Regarding the welding regime, several elements were respected.Due to the lack of finding a suitable filler material, thin fillets cut from the base material were used as filler material.The main difficulty that appears when welding, consists in the formation on the surface of the molten metal bath of an aluminum oxide film, very difficult to fuse.To remove this drawback, a stripping flux with the following composition is used: 28% sodium chloride, 50% potassium chloride, 14% lithium chloride and 8% sodium fluoride.The flux will be applied both on the edges to be welded and on the filler material.
When welding, a light fuel flame with a volumetric ratio between oxygen and acetylene from 0.95 to 1 will be used, which helps to prevent the formation of Al2O3 aluminum oxide.
Regarding the power of the flame, bottled acetylene and a clean, soot-free burner were used.The burner was chosen with a larger number than the one used for welding steel of the same thickness, because aluminum alloys have a high thermal conductivity.
Taking into account the fact that our application involves butt welding with thicknesses below 5 mm, we choose the welding process to the left.At the beginning, until the molten metal bath is formed, the burner is kept almost in a vertical position, after which it is tilted at an angle of 45… .60 .The filler material is held with the same inclination.
After welding the sample, 6 specimens with a width of 25 mm were drawn and cut, as seen in figure 5, which will then be tested for breaking by static traction.

Figure 5. Oxy gas samples
It should be mentioned that, before cutting the test pieces, in order to detect possible discontinuities, a visual and a penetrating liquids control was made of the welding seam.Following the non-destructive investigations, an irregular geometric shape of the welding bead was highlighted, some isolated pores and some short longitudinal cracks, as can be seen from figure 6.

Mechanical tests
As shown above, after cutting, there were 6 specimens marked with WS1-WS6 for WIG welding and with OD1-OD6 for gas flame welding.
All specimens were tested for tensile tests traction, on the LVF 1000 test machine.The load was executed until the specimens fractured.
The fracturing force was recorded for each test piece.Some fractured specimens are shown in figures 7 and 8. Figure 8. Oxy-gas welded samples.

Results and interpretation
The recorded values of the fracturing forces for the specimens obtained from the WIG welded sample are presented in table 3. The recorded values of the fracturing forces for the specimens obtained from the welded sample with gas flame are presented in table 4. The graphical representation of the values of the fracturing forces depending on the type of test piece is presented in figure 9.
Analyzing the samples examined by the optical-visual and penetrating liquids described in points 4 and 5, we find that in the case of WIG welding, the welding seam has a normal appearance, has a regular geometric shape and no discontinuities that can be highlighted.with that methods.
On the other hand, also examining by optical-visual methods and non-destructive defectoscopy with penetrating liquids, we find that the welding seam made by the oxy-gas flame welding process showed an irregular geometric shape, some isolated pores and some longitudinal short cracks.

Figure 9. Fracturing forces
Regarding the results of the static tensile tests, as can be seen in Figure 9, the forces at which the WS1; WS2; WS3; WS4; WS5 and WS6 specimens fractured, welded by the WIG process, are about 25% higher than the forces at which the OD1; OD2; OD3; OD4; OD5 and OD6 specimens fractured welded by gas flame welding process.

Conclusions
The comparative study was performed between the WIG welding process and the oxy-acetylene flame welding process using different filler materials, but close in chemical composition to the base material.It is mentioned that in the case of welding with oxyacetylene flame, rods of filler material made of the basic material were used.
It was observed that in the case of WIG welding, the specimens investigated by the optical-visual method and with penetrating liquids did not show visible defects that would influence the mechanical fracture tensile tests.
On the other hand, in the case of specimens obtained by gas flame welding, both in the optical visual control and in the one with penetrating liquids, the irregular shape of the welding seam and some surface pores and cracks along the welding bead were observed.These highlighted discontinuities act as local stress concentrators and cause a decrease in breaking forces at static tensile tests.It is found that the fracture tensile tests forces of WIG welded specimens, are approximately 25% higher than the fracture forces of oxyacetylene welded specimens.
The explanation for the appearance of porosity defects and longitudinal cracks in samples welded with oxyacetylene flame could be given by the use of an inappropriate preheating temperature or a stripping flux that does not properly remove oxides from the bath.As a result of not removing the oxides from the bath, problems occur because their melting temperature is very high, ie above 2200 ̊ C and cannot be removed, because the base material melts at about 650C.

Figure 4 .
Figure 4. Non destructive tests with penetrant liquids for the WIG welding

Figure 6 .
Figure 6.Non destructive tests for the Oxy-gas welding

Table 2
Oxy-gas welding parameters

Table 3
Fracturing forces for WIG welded samples

Table 4
Fracturing forces for oxy-gas welded samples