|Year : 2011 | Volume
| Issue : 2 | Page : 100-106
Experimental Investigation of Mechanical Properties of MIG Weldments of Aluminum Alloys Plates
MP Garg1, Sarbjit Singh2, Jodh Singh3
1 Department of Mechanical Engineering, MM University, Mullana, India
2 Department of Mechanical Engineering, Punjab Engineering College, Chandigarh, India
3 Department of Mechanical Engineering, DIET, Kharar, Punjab, India
|Date of Web Publication||24-Oct-2011|
M P Garg
Department of Mechanical Engineering, MM University, Mullana
Source of Support: None, Conflict of Interest: None
| Abstract|| |
A study was undertaken to analyze metal inert gas (MIG) welding of aluminum plates keeping in mind the potential capabilities of MIG welding process and selecting aluminum as base metal due to its increased demand in aerospace and automotive industries. Primary welding parameters, viz. welding current, arc voltage, welding speed, were identified and investigations were carried out to study their effect on the tensile strength and yield strength of the weldments by using 2 3 factorial design. A brief introduction to MIG welding processes is included, along with the literature review of research in the field of MIG welding of aluminum.
Keywords: Aluminum alloy 6101, heat affected zone, metal inert gas welding, tensile strength and yield strength
|How to cite this article:|
Garg M P, Singh S, Singh J. Experimental Investigation of Mechanical Properties of MIG Weldments of Aluminum Alloys Plates. J Eng Technol 2011;1:100-6
|How to cite this URL:|
Garg M P, Singh S, Singh J. Experimental Investigation of Mechanical Properties of MIG Weldments of Aluminum Alloys Plates. J Eng Technol [serial online] 2011 [cited 2020 Jan 19];1:100-6. Available from: http://www.onlinejet.net/text.asp?2011/1/2/100/86643
| 1. Introduction and Literature Survey|| |
Welding is a process for joining different metals or materials. American Welding Society (AWS) has defined the welding process as a "materials joining process which produces coalescence of materials by heating them to suitable temperatures with or without the application of pressure or by the application of pressure alone and with or without the use of filler material".
The concept of metal inert gas (MIG) welding was developed in the 1920s, but commercial exploitation did not begin until 1948. With the advancements in the process, the gas metal arc welding (GMAW) is now becoming a more common description because both inert and reactive gases (particularly CO 2 ) are now employed. In the beginning, pre-weld surface cleaning is done to remove surface contamination (dirt, metal particles, oil grease, paint, moisture and oxide coating, etc.) from the base metal by various mechanical and chemical means. Fedoseev et al.  suggested several techniques for surface preparation prior to welding:
- degreasing with organic solvents followed by mechanical scrapping;
- pickling in a 5% NaOH solution followed by a rinse in HNO 3 to remove reaction products;
- pickling such that a passive film is produced;
- pickling followed by mechanical scraping;
- mechanically milling up to 0.5 mm from the surface;
- chemically milling up to 0.3 mm in a 200 g/l alkaline solution and
- vacuum degassing followed by mechanical scraping.
In the welding process, an electric arc is struck between continuously fed electrode of aluminum or aluminum alloys (+) and the job (−). The "spray" mode of metal transfer using argon as the shielding gas and direct current electrode positive (DCEP) power has been found suitable to weld aluminum. DCEP gives good penetration and a cathodic cleaning action at the work surface  .
As suggested by Giachino et al.,  current range for spray arc process ranges from 50 to 600 A, and for pulsed arc MIG welding system it is from 35 to 300 A. Width and depth of weld metal as well as heat affected zone (HAZ) increases with increase in current. Tensile strength decreases with increase in welding current and vice versa. For aluminum, alternating current (AC) is not used in GMAW because of the tendency of the arc to extinguish as the current passes through the zero point. DCEP is used in GMAW as this gives a stable arc, smooth metal transfer, good weld bead, less spatter, higher depth of penetration for a wide range of welding currents and a cathodic cleaning action at the work surface. In case of direct current electrode negative (DCEN), only globular metal transfer is possible, so it is not used in GMAW  . Argon minimum current used for spray transfer with wire electrode of diameter 1.6 mm is 180 A and with wire diameter of 1.2 mm is 135 A  . The metal will be removed by a spray effect approximately above 150 A because of increased e.m.f.  . The maximum amperage at which short-circuit effect can take place is approximately 250 A  . The approximate values of transition currents for aluminum electrodes of different diameters when argon is used as shielding gas are shown in [Table 1].
As explained by Linnert  , welding current is proportional to electrode diameter for a particular electrode material. With increase in electrode extension, a slight decrease in current occurs at which spray transfer develops.
For welding of aluminum arc, Voltage varies between 17 V and 40 V and open circuit voltage (OCV) varies between 50 V and 100 V  . Higher arc voltage results in flatter beads due to increased arc cone which affects the penetration and thus dilution. Dilution is the ratio of area of fused based metal to the total area of weld. Excessive high voltage causes porosity, spatter and undercut. Decrease in arc voltage causes narrow weld bead with deeper penetration. Arc length is proportional to the voltage. Arc length should not be more than the electrode diameter. Magnitude of heat flux is proportional to the arc voltage  .
The speed of movement of the welding torch determines the welding speed. The welding speed range is fairly narrow (0.2-0.3 m/min). Welding speed influences both penetration and weld pool width. High welding speeds increase penetration, thus resulting in higher dilution. Dilution is the ratio of area of fused based metal to the total area of weld  . The melting of base metal first increases and then decreases with increase in weld speed. This happens because initially the arc at every instant acts directly on the base metal and not on the molten pool. However, with further increase in welding speed, less thermal energy gets transmitted to the base metal per unit length of the deposited weld metal. If the speed is further increased, this results in excessive penetration and reduces the heat spread before the weld metal solidifies, thus reducing the transverse shrinkage in butt weld.When welding speed is slow, the filler metal deposition per unit length increases with increase in rate of heat input. On further slowing down, the arc impinges on the molten weld pool, rather than on the base metal, thereby reducing the effective penetration. As a result, the weld pool gets enlarged with decrease in welding speed. The weld pool length becomes larger and the rear of the pool becomes sharper with decrease in welding speed  . As suggested by Quinn  , the reinforced area around the weld, i.e. weld bead, leads to stress concentration effects due to which crack initiation and its growth takes place, which leads to failure of MIG weld. The cracks observed in welds of aluminum alloys are basically hot cracks, which include solidification cracks or hot tears occurring in the weld fusion zone and the liquation cracks found in the HAZ and in the partially melted zone (PMZ). Cracking in aluminum alloy welds is a complex process and is influenced by a combination of mechanical, thermal and metallurgical factors. As investigated by Li et al.,  most aluminum alloys, particularly the heat-treatable alloys, have wide freezing ranges and are thus inherently sensitive to hot cracking. According to Ray  , high cooling rate results in high yield and tensile strength with subsequent loss in ductility due to martensite formation. As investigated by Huang et al.,  liquated grain boundaries (GB) are obviously weak. They can be torn by tensile stresses induced during welding. However, even if hot cracking is avoided during welding, the PMZ can still be susceptible to ductility loss after welding. The filler metal plays an important role in avoiding weld cracking. The primary method for eliminating cracking in aluminum welds is to control weld metal composition through filler addition 13. According to Wahab et al.,  restraining forces arise from the nonuniform thermal field generated by welding. As a result, the plastic strain remains around the weld bead and permanent deformation occurs after welding. Welding speed has little effect on the restraining force, but by increasing heat input, restraining forces increase significantly. There are many problems in welding of aluminum, such as high affinity to combine with oxygen thus forming oxide layer, water vapor dissociation, softening in HAZ in age-hardened alloys, porosity and tunneling. Welding aluminum alloys demands very clean working practice because they are prone to contamination.
A filler metal with melting temperature below that of the base metal greatly reduces the tendency for inter-granular cracking in the HAZ. Such a filler metal minimizes the stresses imposed by the solidification shrinkage of the weld metal until any low melting phase in the HAZ has solidified and developed sufficient strength to resist the stress 15. Weld metal composition known to be sensitive to cracking should be avoided  . In the case of Al-Si and Al-Mg alloy weld metals, sensitivity to cracking is greatest when the weld metal contains about 0.5-2.0% Si or Mg, respectively. Cracking tendencies decrease when the weld metal composition is below or above this range  .
Fusion area increases with increase in heat input. Increasing the heat input increases the restraining force, i.e. both have approximately linear relationship  .
| 2. Design of Experiment|| |
The geometric "+ and −" notation is used to represent high and low levels of factors. Writing the treatment combinations in standard order as (1), a, b, ab, c, ac, bc and abc, the eight runs are listed in planning matrix as in [Table 2].
In the present study, there were a total of eight combinations, and two specimens at each combination were welded. Thus, totally 16 joints (specimens) were welded as per the combinations as shown in the planning matrix [Table 2].
| 3. Experimental Set Up|| |
3.1 Experimental Materials with Specifications
Work Piece: Base metal alloy 6101 in the form of aluminum sheet strip, having thickness 6 mm, length 150 mm, and width 75 mm, was selected. The material has a chemical composition as given in [Table 3].
Mechanical properties are as follows:
Tensile strength = 106 MPa
Yield stress = 91.5 MPa
Hardness = 55 VHN
Fillers material: Commercially available aluminum filler wire of 1.2 mm diameter was used in this work.
Shielding gas: Commercially available argon was used as shielding gas.
Constant speed Set Up: A set up was prepared on the lathe machine to get variable welding speed. The tool post was removed from the carriage and aluminum plates to be welded were placed on it. The torch was held at a fixed position and the carriage was made to slide along bed guide-ways automatically. In this way, two different welding speeds were obtained.
| 4. Results and Discussion|| |
4.1 Radiographic Examination
Four randomly selected specimens were radiographed, and in each specimen no flaw was observed. Observations were recorded as in [Table 4].
Radiographic examination results of the welded specimen are given in [Table 4] which shows that 100% welds seem to be obtained without any defect, i.e. no lack of fusion or porosity in the welded bead. Therefore, from this examination it is concluded that a close to 100% weld efficiency may be obtained and it is verified with the help of mechanical testing.
4.2 Tensile Testing
This test is used to determine the ultimate strength of the welded joint. This test is carried out on Special Purpose Universal Testing Machine (UTM). Specimens were prepared to meet the standard dimensions from the actual run weld samples. These specimens were loaded on the UTM. A gradually increasing load was applied on the specimen till the weld broke. This procedure was repeated for the other specimens and results were recorded. Tensile strength is different for different specimens. [Table 5] shows the design matrix for tensile testing. The results of tensile testing of specimens are as shown in [Table 6].
Specifications of tensile specimens and design matrix are as follows:
Gauge length: 100 mm
Width: 50 mm
Thickness: 6 mm
4.2.1 Results of tensile testing
The variation in yield strength, ultimate strength, and elongation is shown in [Table 7].
Variations in ultimate strength, yield strength and Young's Modulus are plotted in [Figure 1], [Figure 2] and [Figure 3].
Main and interactive effects of primary factors are calculated as below:
Main effect of A (current)
Main effect of B (voltage)
Main effect of C (speed)
Interactive effect of A and B
Interactive effect of A and C
Interactive effect of B and C
Interactive effect of A, B and C
In the present study, the minimum limits of parameters as per the factorial design are welding current 150 A, welding voltage 20 V and welding speed 3 mm/sec. Corresponding yield strength and ultimate strength in the specimen are 81.75 MPa and 85.50 MPa, respectively.
The maximum limits of parameters are welding current 200 A, welding voltage 30 V and welding speed 5 mm/ sec, and the corresponding yield strength and ultimate strength in the specimen are 85.32 MPa and 91.03 MPa, respectively.
By keeping welding current and voltage constant and varying the welding speed, it is found that with minimum speed (specimen-1), yield strength and ultimate strength are 85.05 MPa and 90.70 MPa, respectively. With maximum speed (specimen-5), yield strength and ultimate strength are 84.53 MPa and 88.31 MPa, respectively. It is concluded that with increase in welding speed, there is a slight decrease in yield strength (0.61%) and ultimate strength (2.6%).
By keeping welding current and speed constant and varying the voltage, it is found that with minimum voltage (specimen-2), yield strength and ultimate strength are 85.32 MPa and 91.03 MPa, respectively. With maximum voltage (specimen-4), yield strength and ultimate strength are 81.75 MPa and 85.50 MPa, respectively. It is concluded that with increase in voltage, there is a slight decrease in yield strength (4.1%) and ultimate strength (6.0%).
By keeping welding speed and voltage constant and varying the welding current, it is found that with minimum current (specimen-5), yield strength and ultimate strength are 84.53 MPa and 88.31 MPa, respectively. With maximum current (specimen-6), yield strength and ultimate strength are 83.92 MPa and 87.20 MPa, respectively. It is concluded that with increase in welding current, there is decrease in yield strength (0.7%) and ultimate strength (1.2%).
[Figure 4] shows the effect of various welding parameters and their aliasing effect. It is found that effect of factor (A), i.e. welding current, is significant but in negative direction. It is because with the increase in the welding current, heat input per unit length of the weld increases, which lowers the cooling rate of the weldment. This allows the grains of the fusion zone to have ample time to grow, thus resulting in microstructural coarseness which accounts for lower hardness and ultimate strength values.
Effect of factor (B), i.e. welding voltage, is significant in negative direction, i.e. tensile strength decreases with increase in welding voltage. High arc voltage results in flatter beads due to increased arc cone, and thus more arc spread which affects the ultimate strength adversely.
The factor (C), i.e. welding speed, is also very significant but in the negative direction i.e. ultimate strength decreases with increase in welding speed. It is because increase in welding speed leads to cold weld. With excessive welding speed, there is a substantial drop in thermal energy per unit length of welded joint, resulting in undercutting along the edges of the weld bead because of insufficient deposition of filler metal to fill the path melted by the arc.
The aliasing effects of the factors AB, AC, BC and ABC are also highly significant. Interactive effect of A and B is negative, whereas interactive effects of A and C, B and C, and A, B and C are positive.
The maximum yield strength is 85.32 MPa which is 93.2% that of base metal, and the maximum ultimate tensile strength is 91.03 MPa which is 85.8% that of the base metal. The minimum yield strength is 81.75 MPa which is 89.3% that of the base metal and minimum ultimate strength is 85.5 MPa which is 80.6% that of the base metal.
Following conclusions are drawn from the present work:
- Defect free welds are produced on 6-mm-thick aluminum alloy plates with MIG welding, with a welding current of 150-200 A, welding speed from 3 to 5 mm/sec, and welding voltage from 20 to 30 V.
- The welding samples failed in the region of welding during tensile testing.
- The yield strength and ultimate strength of welding specimens are comparable to those of base metal.
- Mean effects of all the three primary factors, i.e. A (welding current), B (voltage) and C (welding speed), are negative. This means that they affect the ultimate strength adversely when taken individually.
- Interactive effect of A and B is negative. This means that increase in A and B at a time affects the ultimate strength adversely, whereas interactive effect of A and C, B and C, and A, B and C are positive. This means that when both A and C are increased at a time, the ultimate strength will increase. Similarly, when B and C increase simultaneously, the ultimate strength will also increase. Increase in A, B and C at a time will also increase the ultimate strength.
| 6. Nomenclature|| |
| References|| |
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| Authors|| |
M. P. Garg is serving as associate professor in Mechanical Engineering Department of M.M. Engineering College, Mullana (Haryana). His areas of interest include Non conventional manufacturing and CAD/CAM. He is a Life Member of the Indian Society of Technical Education (ISTE) He has more than 10 publications in international, Indian journals and conferences.
Sarbjit Singh is serving as assistant professor in mechanical engg at Punjab Engg College, Chandigarh presently pursuing Ph D from IIT, Roorkee. His areas of interest includes welding, Machining of metal matrix composites. He has published a no of research papers in International and national journals and conferences.
Jodh Singh is as a faculty member in Mechanical Engineering Department, DIET Kharar (Punjab).His areas of interest include welding and manufacturing processes. He has published a no of research papers in national and international conferences.
[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7]