Influence of AC TIG Current Pulsing on Mechanical ... - IRD India

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Influence of AC TIG Current Pulsing on Mechanical Behaviour of AA 6061-T6 Weldments

1Hemadri Naidu T, ²K. Channakeshavalu

1Research Scholar, K.S. Institute of Technology

2Principal, EWIT Abstract - The prime focus of this research work is to

analyze the improvements in the mechanical properties of AA 6061-T6 with pulsed current Tungsten Inert Gas TIG welding. AA 6061-T6 is a precipitation hardening Aluminium alloy, containing magnesium and silicon as its major alloying elements. It has been used in aerospace applications in construction of coolant tanks, pressure vessels and other storage tanks. The preferred welding process for aluminium alloy is TIG welding due to its comparatively easier applicability and better economy. It has been found that improved mechanical properties in pulsed Current AC TIG welding process over the conventional continuous current process in joining of thin sheets of AA 6061-T6. Pulsed current tungsten inert gas (PCTIG) welding is a variant of TIG welding; cycling of the welding current from high level to low one at a selected regular frequency. High level peak current is selected to get the adequate penetration at bead contour, the background low level is selected to maintain stable arc.

This permits arc energy to be used efficiently to fuse a spot of controlled dimensions in a short time producing the weld as a series of overlapping nuggets and limits the wastage of heat by conducting into the adjacent parent material in a normal constant current welding. In contrast to constant current welding the fact that the heat energy required to melt the base metal is supplied only during peak current pulses for brief intervals of time allows the heat to dissipate into the base material leading to a narrow HAZ. The advantages include improved bead contour, greater tolerance to heat sink variations, lower heat input requirements, reduced residual stresses and distortion.

Metallurgical advantages of pulsed current welding include refinement of fusion zone grain size and substructure, reduced width of HAZ and control of segregation.

Key Terms— Heat input, Pulsed Current, Distortion, TIG Welding, Precipitation, Grain Refinement.

I. INTRODUCTION

1.1 Aluminium

Aluminium and its alloys play very crucial and critical role in the field of engineering material. The predominance of this is attributed to the excellent corrosion properties owing to the tenacious oxide layer, easy to fabricate and high specific strength coupled with best combination of toughness and formability. In the construction of pressure vessels and storage tanks, the weldability play unique role in selection of materials

from the various candidate materials. Aluminium is a soft, durable, lightweight, ductile and malleable metal with an appearance ranging from silvery to dull gray, depending on the surface roughness of the metal.

Aluminium is nonmagnetic and no sparking. It is also insoluble in alcohol, though it can be soluble in water in certain forms. The yield strength of pure Aluminium is 7 to 11 MPa, and its value ranging from 200 to 600 MPa.

Aluminium has about 1/3rd of the density and stiffness of steel. It is also easily machined, cast, drawn and extruded. The AA 6061 is a precipitation hardening aluminium alloy, containing magnesium and silicon as its major alloying elements. It has good mechanical properties and exhibits good weld ability. It is one of the most common alloys of aluminium for general use. It is commonly available in pre-tempered grades such as 6061-O (annealed) and tempered grades such as 6061- T6 (solutionized and artificially aged) and 6061-T651 (solutionized, stress-relieved stretched and artificially aged). The chemical composition of AA 6061-T6 and filler wire 4043 are shown in table 1.

Table 1 Chemical composition (in wt%) of AA 6061-T6 and filler wire 4043.

Material AA 6061 ER 4043

Si 0.65 5.1

Mg 1.1 0.05

Cu 0.3 0.17

Fe 0.5 0.05

Mn 0.1 0.23

Zn 0.12 0.10

Ti 0.1 0.04

Cr 0.1 0.06

Al Balance Balance

1.2 Welding Process

In the domain of joining processes of Aluminium and its alloys, the TIG welding process continues its apex position due to its versatility and flexibility in adaptation. The superior weld quality obtained in TIG elements differentiates the TIG process in comparison with other competing and emerging joining processes.

Even after decades of its inventions, TIG process continued in its marathon race even in aerospace field

where the quality and reliability are vital factors due its inherent advantages. Aluminium fabrication is preferred when compared to other materials fabrication since it offers a considerable mass advantage to the extent of as high as 40% to existing materials. Also repair procedures and methodologies are easily to be adopted and highly recommended for aerospace applications.

The common fusion welding processes for joining of Aluminium and its alloys include TIG welding Metal Inert Gas (MIG) welding, Variable Polarity Plasma Arc (VPPA), welding and high heat density Electron Beam Welding (EBW). Due to its tenacious oxide layer AC power source is predominately used for TIG welding process. During the reverse polarity cycle (RP) the removal oxide takes plays. Otherwise high refractive nature of the aluminium oxide (melting point more than 2000 degree C) and electrical insulating nature makes the weld practically difficult. However AC supply needs high heat input which in turn restricts the weldments in achieving the desired characteristics.

One of the processes which derives both the benefits of AC and DC welding is AC pulsed TIG welding.

Considering the briefly described technological challenges, an area of research work is identified with emphasis on the quality of the research and immediate engineering application and enhancement to the exiting processes. Accordingly research work based on aluminium material and process related to AC TIG welding is selected. The research outcome has strong bearing and usefulness for the development of new aluminium alloy products. A simple TIG welding setup and AC waveform balance as well as pulse modulation is shown in fig .1.1.

Fig. 1.1 TIG welding Setup.

1.3 AC Waveform Balance

A pot can be fitted, when welding in AC mode the positive half cycle cleans the oxides on the aluminium and the negative half cycle produces weld penetration during welding self rectification occurs and causes an imbalance of the waveform, a balance control allows the operator to adjust the amount of time the cleaning or penetration takes in each cycle.

Fig. 1.2 Volt-ampere curve of a typical constant current

power source.

The use of pulsed current greatly extends the control which can be exercised on the process allowing:

 Improved consistency in the under head of unbacked butt welds.

 The ability to overcome differences in heat sink and therefore to join thick to thin material.

 The ability to make cylindrical or circular welds without a build-up of heat and an increase in weld width.

 The ability to produce stable TIG welds at very low level.

 Basically a series of overlapping spot welds, with short cooling periods between such welds.

Fig. 1.3 Pulse modulation.

The heat input (H - J/mm) during the welding process can be calculated by using the eqn (1).

H=V*I/S*60 ... (1) Where,

V=Welding voltage - volts.

I=Welding current - amps.

S=Welding speed - mm/min.

II. STANDARDS FOLLOWED FOR TESTING

ASTM International (ASTM), originally known as the American Society for Testing and Materials, is an international standard organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, and services.

A document that has been developed and established within the consensus principles of the society and that meets the approval requirements of ASTM procedures and regulations.

ASTM standards used

E 8M: Standard test methods for Tension Testing of Metallic Materials [Metric].

E 190: Standard Test method for Bend test of Metallic Materials [Metric].

E 384: Standard Test method for Micro hardness of materials.

III. TEST METHODS

3.1. Tensile Test (Tension testing)

Tensile testing is a fundamental test in which a sample is subjected to uni-axial tension until failure. Tensile properties frequently are included in material specification to ensure quality. The strength of a material often is the primary concern. The strength of interest may be measured in terms of either the stress necessary to cause appreciable plastic deformation or the maximum stress that the material can withstand. These measures of strength are used as safety factors in engineering design. Here, incorporated the material’s ductility to measure of deformation characteristic before it fractures.

3.2 Tensile specimen dimensions

Fig. 3.1 Test specimen.

Table 2 Shows the standard and calculated plate parameters.

Parameters Plate type- Standard values mm

Calculated Values mm G- gage length 200 ± 0.25 112

W- width 40+3/ 40.6 22

R- radius of fillet

13 2.8

L- overall length

450 210

A- Length of 225 126

reduced section B- length of grip section

75 42

C- width of grip section

50 28

T- thickness 2.8

3.3. Bend Tests – It is used to determine physical condition of the weld and weld efficiency. It determines the ductility or the strength of a material by bending the material over a given radius. Following the bend, the sample is inspected for cracks on the outer surface. It also provides insight of the modulus of elasticity and the bending strength or a material.

Bend specimen dimensions

Fig. 3.2 Face Bend.

Fig. 3.3 Root Bend.

Fig. 3.4 Root Bend. Fig. 3.5 Face Bend.

3.4 Hardness Tests - This gives the metals ability to show resistance to indentation which show it’s resistance to wear and abrasion. Hardness may be defined as the resistance to permanent indentation. This gives the metals ability to show resistance to indentation which show it’s resistance to wear and abrasion

3.5 Vickers Hardness (HV) Test - Vickers hardness test method consists of indenting the test material with a diamond indenter, in the form of a right pyramid with a square base and an angle of 136 degrees between opposite faces subjected to a load of 1 to 100 kgf. The full load is normally applied for 10 to 15 seconds. The two diagonals of the indentation left in the surface of the material after removal of the load are measured using a microscope and their average calculated. The area of the sloping surface of the indentation is calculated. The Vickers hardness is the quotient obtained by dividing the kgf load by the square mm area of indentation.

Fig. 3.6 Schematic of the indentation of the vickers hardness testing machine.

HV = 2F Sin(136^◦/2)/d² F= Load in kgf

D= arithmetic mean of the two diagonals, in mm.

When the mean diagonal of the indentation has been determined the Vickers hardness may be calculated from the formula, but is more convenient to use conversion tables. The Vickers hardness should be reported like 800 HV/10, which means a vickers hardness of 800, was obtained using a 10 kgf force. Several different loading settings give practically identical hardness numbers on uniform material, which is much better than the arbitrary changing of scale with the other hardness testing methods. The advantages of the vickers hardness test are that extremely accurate readings can be taken, and just one type of indenter is used for all types of metals and surface treatments. Although thoroughly adaptable and very precise for testing the softest and hardest of materials, under varying loads, the vickers machine is a floor standing unit that is more expensive than the Brinell.

3.6 Microscopic Tests - It’s used to determine the actual structure of the weld and parent metal up to 50,000 times magnification with an electron beam microscope polishing must be of a very high standard.

3.6.1 Microstructure of aluminum alloys

The microstructure takes form when a metal goes from its melted form to a solid metal. This solidification forms a polycrystalline metal with different orientations of the grains. These grains will form the microstructure.

The form of this microstructure depends not only upon the heat transfer but also upon the alloy composition.

The two major growth types of microstructures are the dendritic and the eutectic. Both types of growth are present in almost every casting because of a better cast ability of a near-eutectic or eutectic alloy than that of any other compositions.

The solidification occurs in two stages

 Nucleation

 Growth of the formed nucleus

These two stages will compete deciding whether the final microstructure becomes fine or coarse. Nucleation is the first step but will, depending on the melting condition and alloying elements, hastily be overtaken by the crystal growth, as explained below.

Nucleation in a melt will occur when a cluster of metal atoms is ordered in a discrete crystal lattice at or under the solidification temperature. The cluster has a certain critical size, RC, above which it is stable. This critical size rc is dependent on the temperature. When the nucleus is formed the energy will be lower inside the regular crystal lattice than outside in the melt. Then the nucleus will grow at the expense of the melt due to the driving force, which is the free enthalpy. The lower the energy the more stable is the cluster.

Grain growth is the increase in size of grains in a material at high temperature. This occurs when the recovery and recrystallisation are complete and further reduction in the internal energy can only be achieved by reducing the total area of grain boundary.There are two basic morphologies of growth in aluminum alloys: (i) dendritic (ii) eutectic. During nucleation heat will be liberated to the surroundings by increasing the temperature of the melt .At a temperature just below the solidification temperature growth will take over and nucleation will cease. The phase diagram 3.7 shows that during solidification when aluminium is alloyed with silicon. The lines in the diagram shows, when a phase transformation takes place under equilibrium condition.

Solidification starts when the liquidus line is intersected.

Here the equilibrium exists between the liquid and the solid + liquid. When the solidus line is intersected all liquid will be solidified and phase transformations can only take place by diffusion. Three very common microstructures are found in aluminium alloys are Single-phase microstructure

When the melt has a content of silicon less than 1.65 wt% the microstructure will start to solidify as primary a-dendrites. Primary due to the fact that these grains are formed in the beginning of the solidification. These dendrites consist of solid aluminum with up to 1.65 wt%

silicon in solid solution.

(i) Dendrite (ii) Columnar (iii) Intermetallic particles

Fig. 3.7 Microstructure of aluminium alloy.

The microstructure will be single-phase, only consisting of these a-dendrites grown to a homogeneous grain structure. The size of these grains can be very different dependent on the under cooling and the solidification time. As explained the microstructure will be very fine if the under cooling has been big and hereby caused a high nucleation rate. A fine grained microstructure will give a high strength compared with a coarse grained microstructure.

Two-phase microstructure

The microstructure obtained when the alloy is hypoeutectic with silicon content between 1.65 - 12.6 wt% will be two-phase. Again when the liquidus line is reached, the primary a dendrites starts to solidify. When the eutectic temperature is reached the eutectic phase will begin to solidify between the dendrite arms filling this volume out. The eutectic phase consists of the primary a-aluminium phase and the silicon phase. This will give a two-phase microstructure where the primary aluminium crystals are distributed in the eutectic phase as seen in the figure 3.7. The ratio between the two phases will depend on the content of silicon and also the solidification time. Single-phase microstructure with intermetallic particles this microstructure is found in heat treatable aluminium alloys, such as 2xxx series alloy. The first step is heating the alloy to a temperature just below the solidification temperature. This temperature is held until copper is completely in solid solution with aluminium. By rapidly cooling to a temperature in the range of 30 to 40°C a supersaturated solid solution can be obtained. A supersaturated solid is a solid with a higher content of the alloying element than if the solid was in the equilibrium state. During the ageing period elements from the supersaturated solution begin to precipitate. These intermetallic particles can be found in the grain boundaries as seen in the figure 3.7.

IV. METHODOLOGY

Aluminium AA6061 T6 alloy plate of thickness 2.8mm is selected for study purpose. The plate is prepared for TIG welding by scraping the welding edge of plate by using scrapper and cleaning the surface with acetone solution. The plates are welded by ACTIG welding using filler material 4043.

4.1 Workflow of Research

Table 3 Standard welding parameters used for AC TIG welding with and without pulsation.

Sl. No. Description Values

1 Gas flow rate 14 to 16 ltr/min 2 Filler wire ER.4043, 2.4mm dia 3 Shielding gas 80% Argon 20%

Helium

4 Non

consumable electrode

Tungsten 3.2mm dia

5 Source Alternative Current

(AC) 6 Welding torch Water cooled 7 Type of weld

joint

Closed square grove butt joint

8 Current

frequency

50HZ

Table 4 Variable welding parameters used for AC TIG welding with and without pulsation.

Description AC TIG

welding

Pulsed AC TIG welding Welding current

(I) amps

140-150 125-130 Welding voltage

(V) volts

17-18 17-18

Welding speed (S) mm/min

200 225

Heat input (H) J/mm

761.25 595.00

V. RESULTS AND DISCUSSION

5.1 Test Reports

The two specimens AC TIG and AC TIG with pulsation are prepared for testing as per ASTM standards. For tensile test the ultimate tensile strength has been improved from 218 to 230 N/mm² and 5.3% of improvement for pulsed AC TIG welding. The failure in the welded specimen takes place in the base metal at a certain distance from the weld bead. Hence it shows that weld region is stronger than the base metal There is an improvement in yield stress from 209 to 220 N/mm², there is an improvement for pulsed AC TIG welding of 5%. Percentage of elongation 12.8 to 6.5 has been reduced for pulsed AC TIG welding of 50%. Hardness has been reduced from 96 VHN to 76 VHN for pulsed AC TIG welding of 20.8%. In transverse face and root bend tests the specimen broken into two pieces at HAZ on bending through 180^◦ using a mandrel of diameter 4t.

The bend test results indicate that there are no cracks developed in the weld and hence the results are satisfactory. From micro Vickers hardness test the hardness number is less at weld zone than HAZ.

Hardness number is more in base metal than HAZ. The test weldment region has the least hardness and the strongest part of the welded specimen.

Fig. 5.1 Test Specimen.

Fig. 5.2 Specimen after test.

Fig. 5.2 Comparison of ultimate tensile strength of as welded and AC pulse welded AA 6061.

Fig. 5.2 Comparison of yield stress of as welded and AC pulse welded AA 6061.

Fig. 5.2 Comparison of hardness of as welded and AC pulse welded.

Table 5 Comparison between AC welding and AC pulse welding.

Properties AC TIG welding

AC pulse welding

Remarks

Ultimate tensile stress Mpa

218 230 AC pulse

welding is 5.2% better Yield stress

Mpa

209 220 AC pulse

welding is 5% better

% of elongation

12.8 6.8 AC pulse

welding is 49.2% better Hardness

(VHN)

96 76 AC welding

is 20.8%

better

Microstructure Specimen Micro structure study is done on rectangular welded specimen. When the welding process starts, the molten metal takes the solid grains as its substrate and starts growing. Micro pores of approximately 10microns can be observed in the HAZ

region of the weld. When the temperature graidant is high, columnar structures can be observed and when the temperature graidant is low, dentritic structures can be observed. It consists of fine precipitates of alloying elements uniformly dispersed in the matrix of aluminium sloid solution. In the weld region columnar and dentritic structure observed

Test Result

Fig. 5.3 Microstructure of tested specimen.

VI. CONCLUSION

The pulsation reduces heat input by 21.83%.The reduction in heat input results in improving the mechanical properties. The impact of pulsation is more predominant in yield strength by 5%. This is due to less coarsening of precipitates due to less heat input. The weld microstructure is more with uniformly dispersed intermetallics. The inter dendritic compounds have not formed any net work. The pulsation improves mechanical properties.

ACKNOWLEDGMENT

I would like to express my sincere thanks to Mr.

P.Srinivasa Rao, Scientist, ISRO, Management of ISRO and CPRI, Bangalore and also Advanced metallurgical laboratory.

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IJAETMAS- Journal, vol-3, special-01, April- 2016 Pg. No 292-296.

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