Effect of welding parameters on microstructure, mechanical properties and environment

Welding methods are one of the promising methods to join similar or dissimilar materials. In particular, Tungsten Inert Gas (TIG) welding is widely used in the field of industry. However, correct selected parameters and their effect on the environment are important factors in obtaining a good metal join. The aims of this study to investigate the role of the filler and heat treatment on mechanical properties of the similiar joint 6061 Aluminum, and also discuss the effect of this method on the enviroment based on literature review. Three different fillers consist of ER 4043, ER 4047, ER 5556 and artificial aging with temperatures of 125°C, 155°C, 185°C respectively were conducted using TIG welding. To investigate mechanical properties, the Vickers hardness, tensile test and also microstructure observations were carried out at room temperature. The result shows that the maximum hardness is 91.6 HV and the tensile strength is 180 MPa obtained by using filler ER 5556. On the other hand, artificial aging successfully enhances the mechanical properties of the 6061 aluminum welding.


Introduction
Welding methods are widely used to join varying ferrous or non-ferrous metals in the industry.One of the potential weld technique is tungsten inert gas (TIG), which is a method using a filler rod to fill the weld area and inert gas (helium or argon) to prevent the welding metal from excessive contaminant during melt processes [1].TIG welding results in good mechanical properties like hardness, tensile, bending strength and sluggish corrosion rate [2,3].Varying parameters such as heat input, groove, fillers type, and also post-weld heat treatment (PWHT) are influences the characteristic of welding metal.The TIG has been used to join different aluminum such as 5083, 6061, 7003 and 1050 [4][5][6].TIG welding is one of the welding methods in which the fusion phenomenon occurs during the process.This phenomenon causes microstructure changes and finally influences the strength of material.The role of the filler rod in the microstructure and mechanical properties were studied by many researchers in previous studies [7][8][9][10].A correct selection of filler to be important due to their constituent elements is important.For such, microstructure evolution, precipitates particle formation and defects.Several kinds of filler rod type, such as ER 5356, ER 5556, ER5183, and ER5654 are usually used to join aluminum alloys [4,11].These fillers brought a different effect on mechanical properties.It was reported that welding aluminium 6061 using a 5356 filler shows a better tensile strength.However, the combination of this filler with 6063 low tensile strength is obtained [12].The strength of weld join are strong dependent of the final microstructure.The strength of the weld metal strongly dependent on the final microstructure.In which the equiaxed shape, columnar microstructure microstructure, and Mg2Si particles were observed in the microstructure of aluminum [4,13].Furthermore, microstructure evolution is not only influenced by welding parameters but also the PWHT plays an important role in determining microstructure.The PWHT process, such as artificial aging, encourages the grain growth and formation of precipitate particle [14], this process also causes the evolution of dendrite one to equiaxed grains [15].The welding methods are very widely used for manufacturing in industries, and those quality products and the environmental impact of these methods are necessary to study.The aim of this current research is to investigate the role of the filler rod and PWHT on mechanical properties of the 6061 aluminum weld and the environment of the welding process.

Method
In this current study, the independent variabel consist of three types of filler is ER 4043, ER 4047, and ER 5556.The constituent each of filler as shown in Table 1, 2, and 3.The control variabel of welding process such as current 110 current, 3,6 kj/mm heat input, 14 V of voltage, and argon as shielding gas was carried out to join aluminum 6061by the TIG method, see the composition in Table 4.The PWHT for as weld 6061 aluminum with ER 5556 filler, preceded by the solution treatment at 520 o C for 1 h.The artificial aging with temperature 125 o C, 155 o C, and 185 o C for 8 h holding time act as independent variabels.To investigate the effect of filler and aging temperature on mechanical properties, the tensile strength and hardness test was conducted, they act as dependent variabel in this current study.The standard specimen for tensile strength based on AWS standard D1.2 and Vickers hardness test with 100 gr load.Microstructure evolution observed using an optical microscope.

Results and discussion
3.1.The microstructure of 6061 aluminum welding Figure 1(a-c) shows the microstructure of the HAZ area of three different filler rods introduced to a similar joint of 6061 aluminum using the TIG welding method.It was clearly observed that the different filler rods cause a slight difference in microstructure morphologies.Usually the microstructure consists of aluminum and Si/Mg elements, which are indicated by bright and dark areas 3 respectively.A finer grain and elongated particles are obtained in the HAZ area in comparison with fusion metals and base metals.It is believed that the microstructure evolution correlated with the high temperature given and cooling rate during fusion welding of metal.The pores were formed in the microstructure for three areas which were observed in this current study.The presence of these pores contributes to the decrease in mechanical properties.On the other hand, the varying of filler significantly changes the microstructure in HAZ area.However, a slight difference in the microstructure of the HAZ area with ER 5556, which is that this filler has higher Mg contents.A finer grain and more dark area were obtained in this area, see Figure 1c. Figure 2 shows the microstructure of the HAZ area of the 6061 aluminum weld with different artificial aging temperatures.The evolution of shape and grain size occurs during aging.The optical microscope demonstrates a larger grain formed after aging at different temperatures in comparison with weld material without aging, see Figure 1.The artificial aging treatment, in which the welding metal is heated at medium temperature for 8 hr in the muffle furnace generated the grain growth to be larger.It is believed that the transformation of grains to be larger influences metal strength.However, this process also promoted the Mg2Si intermetallic particles formed between aluminum rich phase as seen in Figure 2. The presence of the intermetallic phase in the matrix of aluminum can act as precipitation strengthening.A combination of both Mg2Si and the soft Al-rich matrix contributed to the mechanical properties improvement.Furthermore, the higher temperature aging generates more oxidation and pores formed in the microstructure, see Figure 2c.Due to temperature influencing the microstructure, it is important to consider the correct selected aging temperature to obtain a good metal welding performance.see Figure 3a.Filler rod 5556 shows the highest hardness for these parameters, probably the increase in hardness correlated with the higher number of Mg elements in the filler rod (4.7-5.5%) which encourages the intemetallic phase formation during the fusion process at high temperatures.The intermetallic phases have a good hardness, but these phases can be detrimental to the ductility of metal.In general, TIG welding using ER 4043 and 4047 filler rods shows near similar hardness for different areas.It may be correlated with their close composition as shown in Table 1 and 2. Furthermore, artificial aging was conducted of 6061 aluminum weld for different temperature, the hardness of this material as shown in Figure 3b.The aging process causes a significant change in the hardness for different areas.Fusion areas which have a higher Mg concentration that originate from the ER 5556 filler rod (Table 3) cause the area to be non-heat treatable, which is denoted by hardness not very different-from the hardness of the fusion area of non artificial aging, see Figure 3a for ER 5556.On the contrary, the base metal area shows an increase in hardness significantly.This phenomenon might be correlated with their aluminum composition to be heat treatable with low magnesium content (< 1%), see Table 4. Figure 4 shows the tensile strength of the 6061 aluminum weld with a different filler.As demonstrated by the blue color in Figure 4, it is clearly observed that a different treatment and parameters process cause a difference in the tensile strength.The filler rod parameters brought a slight effect on tensile strength, which is the strength between 173~181 MPa obtained.The increase is slightly possibly due to their base metal and constituent elements of filler being close to each other.These results correspond to hardness and microstructure observations as shown in Figure 1   On the other hand, the parameters of the aging temperature shows more significant effect on tensile strength improvement, see red color in Figure 4.The higher strength obtained through the aging process than varying filler rods, the increase in strength achieved to 30% than the average of filler rod parameters, which is the maximum strength is 247 MPa, see Figure 4.The aging process causes microstructure evolution, grain growth, and probably intermetallic formation.This evolution could influence the mechanical properties of weld metal.The strength of materials is also strongly influenced by defects formed during the fusion or heat treatment process.Figure 5 demonstrates the fracture morphologies of weld metal with varying filler rod types.Most of the fracture of 6061 aluminum weld occurred in the fusion area, which indicated the weak area of this metal.During the processing, high temperatures resulted from electrical cause the filler rod to melted, this conditions promoted some contaminants and oxide formation dissolve into microstructure.Due to their unstable phases, atomic mobility, and environmental effect at high temperature generated some defects such as porosity and blow hole formed in the weld area, see Figure 5a-b.The composition of each constituent element in area 1 in Figure 6a can be seen in Table 5.The aluminum more than 91 wt.%, and Mg as the main alloy element with 4.47wt%, are fit well with the ER 5556 filler rod (Table 3).On the other hand, a small amount of oxygen (3.59 wt.%) was detected in this area that indicated a contaminant during the melt phase.The environmental effects of welding methods include human health, climate change, ecosystem quality, and also resources [16].Commonly, a welding process usually produces environmental pollutants, more fumes or smoke from the electrode during the welding.The fumes contain many elements, such as metal metallic oxides (M, Fe)3O4, metals, H2S and SO2, which have spherical and irregular structures with particle sizes > 0.3μm were reported [17,18].It is believed that the presence of these contents has negative effects on the environment, particularly human health.Welders over a long period can have over exposure to manganese in their body or other elements, occupational asthma, and lung disorders [19,20].Furthermore, the SMAW shows a better reduction of global warming than TIG through reducing CO2 emissions [21].This condition has encouraged engineers to develop welding methods such as TIG to be used in joining aluminum or other materials, see Figure 7. TIG Welding is one of the methods that are widely used to join aluminum alloys and is also applied in varying manufacturing industries.
Figure 7.The tungsten inert gas (TIG) processing TIG has brought a good effect on a better environment, particularly in ecosystem quality than Metal Inert Gas (MIG), but not in human health Even though a better method is shown by TIG in its effect on the ecosystem, a better skill of the welder is important.The operation of TIG by Apprentice produces more fumes due to more reactive oxygen species that occurred during the process was reported [22].Controlling and reducing fumes or smoke generated during weld processing needs more attention to minimize the potential hazard to the environment and health of welders.A good ventilation room is needed to minimize health risks from these welding processes and also the use of body protection.In addition, the most important hindering negative effect of the welding process is enhanced awareness of the welder [23].Besides the disadvantages, welding is an important process in manufacturing, including the TIG method which has some advantages such as precision welding, control heat input and parameters.

Conclusions
From the data hardness, tensile test, microstructure and literature review in this current study, several points as conclusion can be drawn.The first, heat treatment for such artificial aging shows a significant effect on the tensile strength of 6061 aluminum weld.The second, aging process encourages grain growth and intermetallic formation which are influences the mechanical properties.The third, welding processes such as SMAW, TIG, MIG has negative effects on environmental and human health, further an awareness and designing good ventilation reduce the negative effect of the welding process.

Figure 2 .
Figure 2. The microstructure of HAZ area for different artificial aging temperature; (a) 125 o C, (b) 155 o C, and (c) 185 o C 3.2.The mechanical properties of 6061 aluminum weld Figure 3a-b shows the hardness of three areas of weld metal with different filler rods and artificial aging temperature respectively.The average hardness ±75 HV of the base metal is 6061 aluminum was obtained.This hardness is slightly increased at the HAZ and fusion area for varying filler rods,

Figure 3 .
Figure 3.The hardness of the different area of 6061 aluminum weld with different methods; (a) filler rod and (b) artificial aging temperature and 3 respectively.

Figure 4 .
Figure 4. Tensile strength of 6061 aluminum weld for different filler and artificial aging temperature

Figure 5 .Figure 6
Figure 5. Fracture area tensile test of the 6061 aluminum weld for difference filler rod; (a) ER 4043, (b) ER 4047 (c) ER 5556 Figure 6 SEM image and EDS result of fracture area of weld metal with ER 5556 filler rod, this image revealed that the fracture is brittle.The reducing of ductility may be influenced by the presence of some contaminants (C, O) and the formation of the intermetallic phases, the presence of these elements as shown in Figure 6b.

Figure 6 .
Figure 6.(a) SEM images of the fracture area of 6061 aluminum weld for the ER 5556 filler rod and (b) EDS results of the area number 1 in the SEM image in Figure 6a.

Table 1 .
The composition of the ER 4043 filler rod.

Table 2 .
The composition of the ER 4047 filler rod.

Table 3 .
The composition of the ER 5556 filler rod.

Table 4 .
The composition of the 6061 aluminum.

Table 5 .
The composition of area 1 in the SEM image of aluminum 6061 weld for ER 5556