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Influence of arc oscillation frequency on tensile properties and microstructural characteristics of magnetic arc oscillation welded AZ31B magnesium alloy joints
Materials Today: Proceedings xxx (xxxx) xxx
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
Influence of arc oscillation frequency on tensile properties and microstructural characteristics of magnetic arc oscillation welded AZ31B magnesium alloy joints V. Subravel a, N. Alagappan b, N. Babu b,⇑ a b
Dept. of Manufacturing Engineering, Annamalai University, India Dept. of Mechanical Engineering, Annamalai University, India
a r t i c l e
i n f o
Article history: Received 6 May 2019 Received in revised form 29 August 2019 Accepted 1 September 2019 Available online xxxx Keywords: Magnesium alloy Magnetic arc oscillation Arc oscillation frequency Tensile properties Microstructure
a b s t r a c t This investigation aims at assessing the impact of magnetic oscillation frequency on tensile properties and the microstructural pattern of gas tungsten arc welded with AZ31B Magnesium alloy joints. Arc oscillation frequencies ranging from 1 Hz to 3 Hz are used to form various levels of five joints. Unlike the other types joints, the joints having the base of arc oscillation frequency of 2 Hz produced greater tensile properties and it happened because of the presence of finer grains and evenly distributed precipitates in the weld region of the joints. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Materials Engineering and Characterization 2019.
1. Introduction The powerful physical properties such as high specific strength, low density and stiffness combine together to make magnesium (Mg) alloys as a powerful medium in a sort of technical application, specifically in air craft and automotive industries [1,2]. Being a hexagonal close- packed (HCP) metal, magnesium alloys carry low ductility and cold working potential, owing to their inadequate slip systems, certain at room temperature and control the application extensively [3,4]. Hence, consistent welding processes demonstrate a great role in the broad application of magnesium alloys by joining readily – cast fragments into more complex parts [5]. It is found that the gas tungsten arc (GTA) welding technology acts as the chief welding method and it is applied in making magnesium alloys since the GTA holds helpful utility and economy values [6,7]. Fusion zone of magnesium alloy, joined with the gas tungsten arc welded, normally produce coarse grains owing to its thermal circumstances during the process of weld metal solidification. This process, usually releases lesser weld metal mechanical properties and creates reduced resistance to hot cracking. Though, this being greatly advantageous in controlling the solidification structure in the welds. The controlling effect becomes intricate on the high ⇑ Corresponding author. E-mail address: [email protected] (N. Babu).
temperatures and high thermal gradients in the weld region when related to the castings and the epitaxial nature of the growth process. The intensity of a number of weld defects can be minimized if the solidifications are refined [8]. The superior method of welding technique, namely the magnetic arc oscillation helps develop resistance to hot cracking besides producing mechanical properties. Magnetic arc oscillation, when incorporated yields significant level of micro structural refinement with weld region. The MAO is considered to be a superior technique suitable for refining the structure of the grain in the fusion zone of welds. In magnetic arc oscillation technique, the arc column is positioned to oscillate cross wise to the welding direction with a two pole magnetic probe. Arc oscillation develops mechanical stirring in the weld region and breaks down the growing dendrite columns. When the broken dendrites act as the nucleating sites and enhance the cooling rate, the microstructure gets refined [9]. Newly few studies were carried out on effect of magnetic arc oscillation on aluminium alloys and steels. Fusion zone grain refinement in aluminium alloy welds through magnetic arc oscillation and its effect on tensile behavior was studied by Janaki Ram et al. [10]. Sivaprasad et al. predicted the impact of magnetic arc oscillation and current pulsing on microstructure and high temperature tensile strength of alloy 718 (Nickel based precipitation hardenable super alloy) TIG weldments [11]. Effect of mechanical arc oscillation on the grain structure of mild steel weld metal
https://doi.org/10.1016/j.matpr.2019.09.001 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Materials Engineering and Characterization 2019.
Please cite this article as: V. Subravel, N. Alagappan and N. Babu, Influence of arc oscillation frequency on tensile properties and microstructural characteristics of magnetic arc oscillation welded AZ31B magnesium alloy joints, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.001
2
V. Subravel et al. / Materials Today: Proceedings xxx (xxxx) xxx
was examined by Mahajan et al. [12]. Grain refinement in magnetically stirred GTA welds of aluminium alloy was studied by Pearce et al. [13]. The available literature has predominantly focused on the MAO welding on Aluminium alloy and steel only. However, no data is available on the magnetic arc oscillation effect and its parameters of magnesium alloys. Considering this justifiable point in mind, an investigation was engaged to assess the influence of arc oscillation frequency on tensile and microstructure properties of magnetic arc oscillation (MAO) welded AZ31B magnesium alloy joints and the obtained results are put down in this article. 2. Experimental work Employing the machining process the 3 mm rolled AZ31B magnesium plates were cut to the dimensions of 150 150 mm. Tables 1a and 1b records the details of the base material such as chemical composition and mechanical properties. Fig. 1 shows the configuration of square butt joint which can be used for the fabrication of the joints. To eradicate the surface contamination, acetone was used to clean the plates to which engage both mechanical and chemical activities before welding. Mechanical clamps were used to obtain initial joint configuration by retaining the plates in their position. The welding position was match up with the direction of rolling. Magnetic arc oscillation (MAO) welding method was used to form the square butt joints. The MAO equipment, mounted and surrounded with the GTAW torch, on the seam weld is interfaced with the controller. It acts as the monitor on the arch oscillation and amplitude. The Photographs of MAO, equipment and controller unit are shown in Fig. 2(a) and (b). The Argon employed here having a flow 20 l/m consistently acts as a shielding gas. The Welding plates were sized and cut, following the standards prescribed by the ASTM E8M 04 model and changed into sheets form with the dimension of 55 mm and 12.5 mm gauge length and width respectively. The smooth (unnotched) tensile specimens were prepared to evaluate yield strength, tensile strength and elongation of the joints. Using an electro-mechanical controlled universal testing machine (100kN) (Make FIE Blue star, Model UNITEK- 94100) the tensile assessments was conducted. From load displacements diagram, it is confirmed that the offset yield strength was equal to 0.2%. Table 2 evaluates the values of elongation in range in percentile. Fig. 3 shows the dimensions of the tensile specimens. Fig. 4(a) and (b) highlight the photographs of MAO joints and tensile specimens of the welded joints. A vicker microhardness testing machine has been employed to find the level of hardness across the weld cross section with a 0.05 kg load for a 20 s dwell time The required size of the sectioned specimens was polished with various grades of emery paper to reveal the metallographic examinations, A regular reagent a mixture of 10 ml and 4.2 g acetic acid
Fig. 1. Joint configuration.
and picric acid respectively. And 70 ml ethanol and 10 ml distilled water was applied to find the presence of the microstructure of the welded region. The microstructure was analysed using the light optical microscope. Make: MEIJI, Japan; Model: MIL-7100) incorporated with image analyzing software (Metal Vision).
3. Results 3.1. Tensile properties Table 2 shows the results of the transverse properties of the joints made using different arc frequencies. The joint, formed with arc oscillation frequency with a range of 2 Hz exhibits the tensile properties of 248 MPa. higher yield strength of 192 MPa and elongation 7.6%. The ratio which indicates the notch strength measures the tensile strength of the specimen that is notched with maximum load and the extreme strength of the notched one. The notch strength ratio is found less than 1 among all joints. This results indicates the AZ31B magnesium alloy as suspectable to the notches and they belong to the notch brittle materials category. The notch strength ratio of 0.92, found between the unwelded parent metal and the MAO decreases the notch strength ratio of the weld metal. The joint fabricated with arc oscillation frequency of 2 Hz exhibits superior notch tensile ratio of 188 MPa. The ratio between the tensile strength of the weld join and unwelded parent metal is otherwise called joint efficiency. The joint having, the 2 Hz frequency shows a greater level of joint efficiency.
3.2. Macrostructure The macrostructure of the joints, based on dissimilar arc oscillation frequencies are found in Fig. 5. At the lower level of arc frequencies (1 Hz) a partial penetration was observed and the width of bead was found wider. At higher level of arc frequencies (3 Hz) surface defects were observed with thermal and physical disturbance; it happened so because the joints were formed with low level properties. The joint made with application of 2 Hz frequency oscillation, achieved defect free joint with full penetration
Table 1a Chemical composition (wt%) of AZ31B magnesium alloy. Al
Zn
Mn
Ni
Cr
Cu
Mg
2.60
0.67
0.27
0.012
0.008
0.017
Bal
Table 1b Mechanical properties of base metal AZ31B magnesium alloy. 0.2% offset Yield Strength (MPa)
Ultimate tensile strength (MPa)
Elongation in 50 mm gauge length (%)
Reduction in cross section area (%)
Notch tensile strength (MPa)
Notch strength ratio (NSR)
Hardness at 0.05 kg load (Hv)
160
275
14.7
14.3
253
0.92
69
Please cite this article as: V. Subravel, N. Alagappan and N. Babu, Influence of arc oscillation frequency on tensile properties and microstructural characteristics of magnetic arc oscillation welded AZ31B magnesium alloy joints, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.001
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(a) MAO Equipment
(b) Controller unit Fig. 2. Photographs of MAO Unit.
Table 2 Transverse tensile properties of welded joints. Current-75 amps, Volt-18, Welding speed-125 mm/min, Amplitude-0.6 mm. Arc Oscillation Frequency
Yield strength (MPa)
Ultimate tensile strength (MPa)
Elongation in gauge length of 50 mm (%)
Notch tensile strength (MPa)
Notch strength ratio (NSR)
Joint efficiency (%)
1 1.5 2 2.5 3
163 175 192 154 150
206 221 248 193 186
6.8 7.2 7.6 5.2 4.8
161 172 188 159 155
0.78 0.77 0.75 0.82 0.83
75 80 91 70 68
(a) unnotched tensile specimen
(b)notched tensile specimen Fig. 3. Dimensions of tensile specimen.
it occurred so because the joints were fabricated with higher tensile properties.
2 Hz frequency, showed hardness of high level (68 Hv), the joint forged with 1 Hz frequency showed the hardness of lower level (57 Hz) in the weld region.
3.3. Microstructure 3.5. SEM fractograph Fig. 6 shows all the joints with the fusion zones and their microstructures. It is inferred from the microstructure that the arc oscillation frequency exhibit substantial impact on the average grain diameter of welded region is AZ31B magnesium alloy. The arc oscillation frequency of 2 Hz forming the joint, contains finer grains (2 mm in the fusion zone) unlike the other joints. Moreover indication of a a huge number of precipitates that are formed in the fusion zone and those are found in the precipitates were at moderate level; this being the cause for the presence of superior tensile properties. The joints created with the oscillation frequency of 1 Hz shows the coarse grains (40 mm) in the welded region and it occurs because of the inferior tensile properties in the joints. 3.4. Micro-hardness The examination on the micro hardness, shown in Figs. 7a–d was conducted transversely from the centre of the welded portion of the base metal. While the joint, forged by an arc amplitude with
Fig. 8, indicates with the SEM fractographs the specimens of unnotched tensile. Besides the tensile specimen having fractured surface formed with arc oscillation frequency of 2 Hz records presence of a large number of fine and shallow dimples only to project the superior quality of tensile strength and ductility. But the remaining specimens were found to yield any positive result when used with cleavage structure. 3.6. EDS analysis Fig. 9 projects the collection of aluminium and magnesium particles with matrix composition of the weld region. The EDX examination is demonstrated the fact that less amount of 2n is found deposited in the matrix (High temperature causing evaporation) than in the base material and this being the cause for the presence of tensile properties found in the joints, were at the inferior level to the base metal.
Please cite this article as: V. Subravel, N. Alagappan and N. Babu, Influence of arc oscillation frequency on tensile properties and microstructural characteristics of magnetic arc oscillation welded AZ31B magnesium alloy joints, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.001
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(A) Fabricated Joints
Before tensile test
After tensile test (B) Tensile Specimens
Fig. 4. (a) and (b) Photographs of Fabricated joints and tensile specimens.
Oscillation Frequency
Cross – sectional macrostructure
Observations
1
Fusion zone width = 9 mm Fusion zone depth = 1.2 mm Remarks = partial penetration
2
Fusion zone width = 7 mm Fusion zone depth = 3 mm Remarks = Full penetration
3
Fusion zone width = 5 mm Fusion zone depth = 3 mm Remarks =full penetration with surface defects Fig. 5. Effect of arc oscillation frequency on bead geometry.
4. Discussion The effect of arc oscillation frequency ranging from 1 to 3 Hz has been evaluated in this study, It has been identified that the oscillation frequency does a significant part in generating the equiaxed grains. Regarding the oscillation, the frequencies ranging from 1 to 2 Hz. It have effective role in altering the grain structure.
The weld bead morphologies depend on the oscillation frequency. It is observed that, at lower frequency, the bead shape seems to be wide as compared to that at high frequency [15]. This means that there is a definite change in concentration of shrinkage forces, which nearly approaches uniform both at the top and bottom of the weld zone during cooling [16]. Macrosections of weld beads under various oscillation frequencies from 1 to 3 Hz. Side penetra-
Please cite this article as: V. Subravel, N. Alagappan and N. Babu, Influence of arc oscillation frequency on tensile properties and microstructural characteristics of magnetic arc oscillation welded AZ31B magnesium alloy joints, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.001
V. Subravel et al. / Materials Today: Proceedings xxx (xxxx) xxx
BASE METAL
1 Hz
2 Hz
3 Hz
5
Fig. 6. Fusion zone microstructures of AZ31B Mg alloy welds.
Fig. 7a. Effect of Arc Oscillation frequency on Grain Size variation across the fusion zone. Fig. 7c. Effect of Arc Oscillation frequency on Tensile Strength.
Fig. 7b. Effect of Arc Oscillation frequency on Microhardness variation across the fusion zone.
Fig. 7d. Effect of Arc Oscillation frequency of Joint Efficiency variation across the fusion zone.
Please cite this article as: V. Subravel, N. Alagappan and N. Babu, Influence of arc oscillation frequency on tensile properties and microstructural characteristics of magnetic arc oscillation welded AZ31B magnesium alloy joints, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.001
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Fig. 8. SEM factograph of unnotched tensile specimen.
tion slightly increased and the penetration decreased with decreased arc oscillation frequency. [17]. It is inferred that the inferior tensile properties found in the joint made arc oscillation frequency of 1 Hz because of the coarse grain structure. It might be expected that the thermal and mechanical disturbances would be less at lower frequencies [18]. The mean size of weld region found to be 36 mm. The micro-hardness values recorded in the joint made with 1 Hz are found lower than the joint fabricated with 2 Hz in the fusion zone, Lesser hardness values occur because of coarse and elongated grains in fusion zone and furthermore the tensile strength of the joint is found lesser than that of joint made with the oscillation frequency of 2 Hz [19]. During the tensile test, all the specimens have consistently failed at weld region and it is compatible with the hardness profile (Fig. 6). At higher frequencies, i.e greater than optimum, availability of time is less during a half-cycle before the direction of fluid which is found is reversed. The agitated liquid is then able to achieve only a lower velocity, thus reducing the effectiveness of the magnetic field [8], and it drops the amplitude vibrations on molten path and simultaneously stops agitation of the molten path and it results the less refinement on the weld metal. Due to less grain refinement the coarse and elongated grains in the fusion zone (average grain size of fusion zone is about 40 mm), although the
tensile strength of the joint is lower than that joint fabricated with arc amplitude frequency of 2 Hz. When the arc oscillation frequencies was found greater than 2 Hz then tensile properties were reduced. The structure of coarse and elongated grains in weld region and lesser hardness are the reasons for lower tensile strength (188 MPa) of these joints. The MAO frequencies influence the entire weld metal solidification. The force generated by external magnetic field enhances the local Lorentz forces in the weld pool and consequently there occurs in the change direction of maximum thermal gradient. Besides the nature of weld pool gets changed continuously due to arc oscillation, and consequently there change occurs in the direction of maximum thermal gradient at solidifying boundary. Owing to the circumstances an alternative and constructively oriented grains grow over long distances and newer grains over long distances and achieve grains become constructively oriented with respect with respect to the instantaneous direction of greatest thermal gradient leading to grain refinement [12]. According to Kou and Le [11] there are three ways with which the: (a) dendrite fragmentation at the rear end of the weld pool, (b) detachment of partially melted grains from both sides of the weld puddle, and (c) heterogeneous nucleation. The enhanced fluid flow due to MAO frequencies helps in transferring the dendrite
Please cite this article as: V. Subravel, N. Alagappan and N. Babu, Influence of arc oscillation frequency on tensile properties and microstructural characteristics of magnetic arc oscillation welded AZ31B magnesium alloy joints, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.001
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Element
Atomic%
Mg K
94.76
Al K
5.90
Zn K
0.72
(a) 2 Hz Element
Atomic%
Mg K
96.98
Al K
2.44
Mn K
0.20
Zn K
0.78
(b) Base metal Fig. 9. EDS results.
bits, detached grains, and heterogeneous nuclei to the region of constitutional super- cooling just ahead of the solid–liquid interface [10]. Thus, while arc oscillation undoubtedly enhances fluid flow, the penetration of the fluid motion into such a dendrite network might be very difficult. The grain refinement observed in the current investigation is therefore believed to be more due to the other effects of MAO on the weld pool shape, fluid flow, and thermal gradients. The continual change in the weld pool shape due to MAO frequency is considered to be responsible for the reduction in columnar grain size in the interior of the fusion zone [13]. The thermal gradient direction gets changed constantly. So the newer grains consecutively become constructively oriented. Thus, while each grain grows only a small distance, more grains grow resulting in a finer grain structure distributed along the length of the weld. It can be attributed to the presence of finer grains and uniform distribution of precipitates compared to the other joints that are the main reason for the higher hardness in this joint [19]. The greater level of hardness found in the fusion zone is due to the moderately faster cooling rate and the steeper thermal gradients which
develop fine grained microstructure. Grain refinement led to an improvement in fusion zone tensile properties. The maximum tensile strength was achieved for the joint made with a oscillation frequency of 2 Hz. The distribution of higher hardness in the weld metal is also the reason for the higher tensile strength of the joint [17]. The higher fusion zone hardness (68 Hv) was recorded for the joint made using an oscillation frequency of 2 Hz. 5. Conclusions From this investigation, the following important conclusions are derived: The arc oscillation frequency causes impact on the grain size and hardness of fusion zone and subsequently on the tensile properties of GTAW joints of AZ31B magnesium alloy. The five welded joints fabricated with varying arc oscillation frequency range between 1 and 3 Hz and the joint fabricated using an oscillation frequency of 2 Hz, show greater tensile properties than their counterparts.
Please cite this article as: V. Subravel, N. Alagappan and N. Babu, Influence of arc oscillation frequency on tensile properties and microstructural characteristics of magnetic arc oscillation welded AZ31B magnesium alloy joints, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.001
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The formation of finer grains in the fusion zone, higher hardness of fusion zone, and uniformly distributed precipitates in fusion zone are found to be the reasons for the superior tensile properties of the joint fabricated using the arc oscillation frequency of 2 Hz than the other joints.
References [1] B.L. Mordike, T. Ebert, Int. J. Mater. Sc. Eng. A 302 (2001) 37–45. [2] I.J. Polmear, Int. J. Mater. Sci. Technol. 10 (1994) 1–16. [3] M.H. Yoo, S.R. Agne, J.R. Morrisb, K.M. Hob, Int. J. Mater. Sci. Eng. A 319–321 (2001) 87–92. [4] P.G. Patridge, Meter. Rev. 12 (1967) 169. [5] C.T. Chi, C.G. Chao, Int. J. Mater. Proc. Technol. 182 (2007) 369–373. [6] A. Munitz, C. Cotler, A. Stern, G. Kohn, Int. J. Mater. Sci. Eng. A 302 (2001) 68– 73. [7] N. Kishore Babu, C.E. Cross, Mineral. Met. Mater. Soc. ASM Int. (2012). [8] S. Kou, Y. Le, Weld. J. 65 (1986) 305s–313s. [9] S.R. Koteswararao, G. Madhusudhanareddy, Int. J. Mater. Sci. Eng. A (2005) 227–234.
[10] G.D. Janakiram, R. Murugesan, Int. J. Mater. Eng. Perform. 8 (1999) 513–520. [11] K. Sivaprasad, S. Ganesh Sundararaman, Int J. Mater. Sci. Eng. A 448 (2007) 120–127. [12] S. Mahajan, N.S. Biradar, J. Int, Trans Ind. Inst. Met. 65 (2) (2012) 171–177. [13] B.P. Pearce, H.W. Kerrbp, Metall. Trans. B 480 (12b) (1981). [15] J.R. Davis, in: ASM Specialty Handbook, ASM International Materials, Park OH, 1994, pp. 376–419. [16] N.S. Biradar, R. Raman, Int. J. Mater. Eng. Perform. (2012), https://doi.org/ 10.1007/s11665-012-0207-2. [17] Y.H. Kang, S.J. Na, A Weld. J. (2002) 8–13. [18] A. Kumar, P. Shailesh, S. Sundarrajan, Int. J. Mater. Des. 29 (2008) 1904–1913. [19] G. Padmanaban, V. Balasubramanian, Sarin Sundar, Int. J. Mater. Eng. Perform. 19 (2010) 155–165, https://doi.org/10.1007/s11665-009-9389.
Further reading [14] G.M. Reddy, A.A. Gokhale, K. Prasad Rao, Int. J. Mater. Sci. 32 (1997) 4117– 4126. [20] N.S. Biradar, R. Raman, Int. J. Metal. Mater. Transfer 43a (2012) 3179. [21] K. Sivaprasad, S. Ganesh Sundararaman, Mater. Sci. Eng. 428 (2006) 327–331.
Please cite this article as: V. Subravel, N. Alagappan and N. Babu, Influence of arc oscillation frequency on tensile properties and microstructural characteristics of magnetic arc oscillation welded AZ31B magnesium alloy joints, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.001
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
Influence of arc oscillation frequency on tensile properties and microstructural characteristics of magnetic arc oscillation welded AZ31B magnesium alloy joints V. Subravel a, N. Alagappan b, N. Babu b,⇑ a b
Dept. of Manufacturing Engineering, Annamalai University, India Dept. of Mechanical Engineering, Annamalai University, India
a r t i c l e
i n f o
Article history: Received 6 May 2019 Received in revised form 29 August 2019 Accepted 1 September 2019 Available online xxxx Keywords: Magnesium alloy Magnetic arc oscillation Arc oscillation frequency Tensile properties Microstructure
a b s t r a c t This investigation aims at assessing the impact of magnetic oscillation frequency on tensile properties and the microstructural pattern of gas tungsten arc welded with AZ31B Magnesium alloy joints. Arc oscillation frequencies ranging from 1 Hz to 3 Hz are used to form various levels of five joints. Unlike the other types joints, the joints having the base of arc oscillation frequency of 2 Hz produced greater tensile properties and it happened because of the presence of finer grains and evenly distributed precipitates in the weld region of the joints. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Materials Engineering and Characterization 2019.
1. Introduction The powerful physical properties such as high specific strength, low density and stiffness combine together to make magnesium (Mg) alloys as a powerful medium in a sort of technical application, specifically in air craft and automotive industries [1,2]. Being a hexagonal close- packed (HCP) metal, magnesium alloys carry low ductility and cold working potential, owing to their inadequate slip systems, certain at room temperature and control the application extensively [3,4]. Hence, consistent welding processes demonstrate a great role in the broad application of magnesium alloys by joining readily – cast fragments into more complex parts [5]. It is found that the gas tungsten arc (GTA) welding technology acts as the chief welding method and it is applied in making magnesium alloys since the GTA holds helpful utility and economy values [6,7]. Fusion zone of magnesium alloy, joined with the gas tungsten arc welded, normally produce coarse grains owing to its thermal circumstances during the process of weld metal solidification. This process, usually releases lesser weld metal mechanical properties and creates reduced resistance to hot cracking. Though, this being greatly advantageous in controlling the solidification structure in the welds. The controlling effect becomes intricate on the high ⇑ Corresponding author. E-mail address: [email protected] (N. Babu).
temperatures and high thermal gradients in the weld region when related to the castings and the epitaxial nature of the growth process. The intensity of a number of weld defects can be minimized if the solidifications are refined [8]. The superior method of welding technique, namely the magnetic arc oscillation helps develop resistance to hot cracking besides producing mechanical properties. Magnetic arc oscillation, when incorporated yields significant level of micro structural refinement with weld region. The MAO is considered to be a superior technique suitable for refining the structure of the grain in the fusion zone of welds. In magnetic arc oscillation technique, the arc column is positioned to oscillate cross wise to the welding direction with a two pole magnetic probe. Arc oscillation develops mechanical stirring in the weld region and breaks down the growing dendrite columns. When the broken dendrites act as the nucleating sites and enhance the cooling rate, the microstructure gets refined [9]. Newly few studies were carried out on effect of magnetic arc oscillation on aluminium alloys and steels. Fusion zone grain refinement in aluminium alloy welds through magnetic arc oscillation and its effect on tensile behavior was studied by Janaki Ram et al. [10]. Sivaprasad et al. predicted the impact of magnetic arc oscillation and current pulsing on microstructure and high temperature tensile strength of alloy 718 (Nickel based precipitation hardenable super alloy) TIG weldments [11]. Effect of mechanical arc oscillation on the grain structure of mild steel weld metal
https://doi.org/10.1016/j.matpr.2019.09.001 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Materials Engineering and Characterization 2019.
Please cite this article as: V. Subravel, N. Alagappan and N. Babu, Influence of arc oscillation frequency on tensile properties and microstructural characteristics of magnetic arc oscillation welded AZ31B magnesium alloy joints, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.001
2
V. Subravel et al. / Materials Today: Proceedings xxx (xxxx) xxx
was examined by Mahajan et al. [12]. Grain refinement in magnetically stirred GTA welds of aluminium alloy was studied by Pearce et al. [13]. The available literature has predominantly focused on the MAO welding on Aluminium alloy and steel only. However, no data is available on the magnetic arc oscillation effect and its parameters of magnesium alloys. Considering this justifiable point in mind, an investigation was engaged to assess the influence of arc oscillation frequency on tensile and microstructure properties of magnetic arc oscillation (MAO) welded AZ31B magnesium alloy joints and the obtained results are put down in this article. 2. Experimental work Employing the machining process the 3 mm rolled AZ31B magnesium plates were cut to the dimensions of 150 150 mm. Tables 1a and 1b records the details of the base material such as chemical composition and mechanical properties. Fig. 1 shows the configuration of square butt joint which can be used for the fabrication of the joints. To eradicate the surface contamination, acetone was used to clean the plates to which engage both mechanical and chemical activities before welding. Mechanical clamps were used to obtain initial joint configuration by retaining the plates in their position. The welding position was match up with the direction of rolling. Magnetic arc oscillation (MAO) welding method was used to form the square butt joints. The MAO equipment, mounted and surrounded with the GTAW torch, on the seam weld is interfaced with the controller. It acts as the monitor on the arch oscillation and amplitude. The Photographs of MAO, equipment and controller unit are shown in Fig. 2(a) and (b). The Argon employed here having a flow 20 l/m consistently acts as a shielding gas. The Welding plates were sized and cut, following the standards prescribed by the ASTM E8M 04 model and changed into sheets form with the dimension of 55 mm and 12.5 mm gauge length and width respectively. The smooth (unnotched) tensile specimens were prepared to evaluate yield strength, tensile strength and elongation of the joints. Using an electro-mechanical controlled universal testing machine (100kN) (Make FIE Blue star, Model UNITEK- 94100) the tensile assessments was conducted. From load displacements diagram, it is confirmed that the offset yield strength was equal to 0.2%. Table 2 evaluates the values of elongation in range in percentile. Fig. 3 shows the dimensions of the tensile specimens. Fig. 4(a) and (b) highlight the photographs of MAO joints and tensile specimens of the welded joints. A vicker microhardness testing machine has been employed to find the level of hardness across the weld cross section with a 0.05 kg load for a 20 s dwell time The required size of the sectioned specimens was polished with various grades of emery paper to reveal the metallographic examinations, A regular reagent a mixture of 10 ml and 4.2 g acetic acid
Fig. 1. Joint configuration.
and picric acid respectively. And 70 ml ethanol and 10 ml distilled water was applied to find the presence of the microstructure of the welded region. The microstructure was analysed using the light optical microscope. Make: MEIJI, Japan; Model: MIL-7100) incorporated with image analyzing software (Metal Vision).
3. Results 3.1. Tensile properties Table 2 shows the results of the transverse properties of the joints made using different arc frequencies. The joint, formed with arc oscillation frequency with a range of 2 Hz exhibits the tensile properties of 248 MPa. higher yield strength of 192 MPa and elongation 7.6%. The ratio which indicates the notch strength measures the tensile strength of the specimen that is notched with maximum load and the extreme strength of the notched one. The notch strength ratio is found less than 1 among all joints. This results indicates the AZ31B magnesium alloy as suspectable to the notches and they belong to the notch brittle materials category. The notch strength ratio of 0.92, found between the unwelded parent metal and the MAO decreases the notch strength ratio of the weld metal. The joint fabricated with arc oscillation frequency of 2 Hz exhibits superior notch tensile ratio of 188 MPa. The ratio between the tensile strength of the weld join and unwelded parent metal is otherwise called joint efficiency. The joint having, the 2 Hz frequency shows a greater level of joint efficiency.
3.2. Macrostructure The macrostructure of the joints, based on dissimilar arc oscillation frequencies are found in Fig. 5. At the lower level of arc frequencies (1 Hz) a partial penetration was observed and the width of bead was found wider. At higher level of arc frequencies (3 Hz) surface defects were observed with thermal and physical disturbance; it happened so because the joints were formed with low level properties. The joint made with application of 2 Hz frequency oscillation, achieved defect free joint with full penetration
Table 1a Chemical composition (wt%) of AZ31B magnesium alloy. Al
Zn
Mn
Ni
Cr
Cu
Mg
2.60
0.67
0.27
0.012
0.008
0.017
Bal
Table 1b Mechanical properties of base metal AZ31B magnesium alloy. 0.2% offset Yield Strength (MPa)
Ultimate tensile strength (MPa)
Elongation in 50 mm gauge length (%)
Reduction in cross section area (%)
Notch tensile strength (MPa)
Notch strength ratio (NSR)
Hardness at 0.05 kg load (Hv)
160
275
14.7
14.3
253
0.92
69
Please cite this article as: V. Subravel, N. Alagappan and N. Babu, Influence of arc oscillation frequency on tensile properties and microstructural characteristics of magnetic arc oscillation welded AZ31B magnesium alloy joints, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.001
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(a) MAO Equipment
(b) Controller unit Fig. 2. Photographs of MAO Unit.
Table 2 Transverse tensile properties of welded joints. Current-75 amps, Volt-18, Welding speed-125 mm/min, Amplitude-0.6 mm. Arc Oscillation Frequency
Yield strength (MPa)
Ultimate tensile strength (MPa)
Elongation in gauge length of 50 mm (%)
Notch tensile strength (MPa)
Notch strength ratio (NSR)
Joint efficiency (%)
1 1.5 2 2.5 3
163 175 192 154 150
206 221 248 193 186
6.8 7.2 7.6 5.2 4.8
161 172 188 159 155
0.78 0.77 0.75 0.82 0.83
75 80 91 70 68
(a) unnotched tensile specimen
(b)notched tensile specimen Fig. 3. Dimensions of tensile specimen.
it occurred so because the joints were fabricated with higher tensile properties.
2 Hz frequency, showed hardness of high level (68 Hv), the joint forged with 1 Hz frequency showed the hardness of lower level (57 Hz) in the weld region.
3.3. Microstructure 3.5. SEM fractograph Fig. 6 shows all the joints with the fusion zones and their microstructures. It is inferred from the microstructure that the arc oscillation frequency exhibit substantial impact on the average grain diameter of welded region is AZ31B magnesium alloy. The arc oscillation frequency of 2 Hz forming the joint, contains finer grains (2 mm in the fusion zone) unlike the other joints. Moreover indication of a a huge number of precipitates that are formed in the fusion zone and those are found in the precipitates were at moderate level; this being the cause for the presence of superior tensile properties. The joints created with the oscillation frequency of 1 Hz shows the coarse grains (40 mm) in the welded region and it occurs because of the inferior tensile properties in the joints. 3.4. Micro-hardness The examination on the micro hardness, shown in Figs. 7a–d was conducted transversely from the centre of the welded portion of the base metal. While the joint, forged by an arc amplitude with
Fig. 8, indicates with the SEM fractographs the specimens of unnotched tensile. Besides the tensile specimen having fractured surface formed with arc oscillation frequency of 2 Hz records presence of a large number of fine and shallow dimples only to project the superior quality of tensile strength and ductility. But the remaining specimens were found to yield any positive result when used with cleavage structure. 3.6. EDS analysis Fig. 9 projects the collection of aluminium and magnesium particles with matrix composition of the weld region. The EDX examination is demonstrated the fact that less amount of 2n is found deposited in the matrix (High temperature causing evaporation) than in the base material and this being the cause for the presence of tensile properties found in the joints, were at the inferior level to the base metal.
Please cite this article as: V. Subravel, N. Alagappan and N. Babu, Influence of arc oscillation frequency on tensile properties and microstructural characteristics of magnetic arc oscillation welded AZ31B magnesium alloy joints, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.001
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(A) Fabricated Joints
Before tensile test
After tensile test (B) Tensile Specimens
Fig. 4. (a) and (b) Photographs of Fabricated joints and tensile specimens.
Oscillation Frequency
Cross – sectional macrostructure
Observations
1
Fusion zone width = 9 mm Fusion zone depth = 1.2 mm Remarks = partial penetration
2
Fusion zone width = 7 mm Fusion zone depth = 3 mm Remarks = Full penetration
3
Fusion zone width = 5 mm Fusion zone depth = 3 mm Remarks =full penetration with surface defects Fig. 5. Effect of arc oscillation frequency on bead geometry.
4. Discussion The effect of arc oscillation frequency ranging from 1 to 3 Hz has been evaluated in this study, It has been identified that the oscillation frequency does a significant part in generating the equiaxed grains. Regarding the oscillation, the frequencies ranging from 1 to 2 Hz. It have effective role in altering the grain structure.
The weld bead morphologies depend on the oscillation frequency. It is observed that, at lower frequency, the bead shape seems to be wide as compared to that at high frequency [15]. This means that there is a definite change in concentration of shrinkage forces, which nearly approaches uniform both at the top and bottom of the weld zone during cooling [16]. Macrosections of weld beads under various oscillation frequencies from 1 to 3 Hz. Side penetra-
Please cite this article as: V. Subravel, N. Alagappan and N. Babu, Influence of arc oscillation frequency on tensile properties and microstructural characteristics of magnetic arc oscillation welded AZ31B magnesium alloy joints, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.001
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BASE METAL
1 Hz
2 Hz
3 Hz
5
Fig. 6. Fusion zone microstructures of AZ31B Mg alloy welds.
Fig. 7a. Effect of Arc Oscillation frequency on Grain Size variation across the fusion zone. Fig. 7c. Effect of Arc Oscillation frequency on Tensile Strength.
Fig. 7b. Effect of Arc Oscillation frequency on Microhardness variation across the fusion zone.
Fig. 7d. Effect of Arc Oscillation frequency of Joint Efficiency variation across the fusion zone.
Please cite this article as: V. Subravel, N. Alagappan and N. Babu, Influence of arc oscillation frequency on tensile properties and microstructural characteristics of magnetic arc oscillation welded AZ31B magnesium alloy joints, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.001
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Fig. 8. SEM factograph of unnotched tensile specimen.
tion slightly increased and the penetration decreased with decreased arc oscillation frequency. [17]. It is inferred that the inferior tensile properties found in the joint made arc oscillation frequency of 1 Hz because of the coarse grain structure. It might be expected that the thermal and mechanical disturbances would be less at lower frequencies [18]. The mean size of weld region found to be 36 mm. The micro-hardness values recorded in the joint made with 1 Hz are found lower than the joint fabricated with 2 Hz in the fusion zone, Lesser hardness values occur because of coarse and elongated grains in fusion zone and furthermore the tensile strength of the joint is found lesser than that of joint made with the oscillation frequency of 2 Hz [19]. During the tensile test, all the specimens have consistently failed at weld region and it is compatible with the hardness profile (Fig. 6). At higher frequencies, i.e greater than optimum, availability of time is less during a half-cycle before the direction of fluid which is found is reversed. The agitated liquid is then able to achieve only a lower velocity, thus reducing the effectiveness of the magnetic field [8], and it drops the amplitude vibrations on molten path and simultaneously stops agitation of the molten path and it results the less refinement on the weld metal. Due to less grain refinement the coarse and elongated grains in the fusion zone (average grain size of fusion zone is about 40 mm), although the
tensile strength of the joint is lower than that joint fabricated with arc amplitude frequency of 2 Hz. When the arc oscillation frequencies was found greater than 2 Hz then tensile properties were reduced. The structure of coarse and elongated grains in weld region and lesser hardness are the reasons for lower tensile strength (188 MPa) of these joints. The MAO frequencies influence the entire weld metal solidification. The force generated by external magnetic field enhances the local Lorentz forces in the weld pool and consequently there occurs in the change direction of maximum thermal gradient. Besides the nature of weld pool gets changed continuously due to arc oscillation, and consequently there change occurs in the direction of maximum thermal gradient at solidifying boundary. Owing to the circumstances an alternative and constructively oriented grains grow over long distances and newer grains over long distances and achieve grains become constructively oriented with respect with respect to the instantaneous direction of greatest thermal gradient leading to grain refinement [12]. According to Kou and Le [11] there are three ways with which the: (a) dendrite fragmentation at the rear end of the weld pool, (b) detachment of partially melted grains from both sides of the weld puddle, and (c) heterogeneous nucleation. The enhanced fluid flow due to MAO frequencies helps in transferring the dendrite
Please cite this article as: V. Subravel, N. Alagappan and N. Babu, Influence of arc oscillation frequency on tensile properties and microstructural characteristics of magnetic arc oscillation welded AZ31B magnesium alloy joints, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.001
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Element
Atomic%
Mg K
94.76
Al K
5.90
Zn K
0.72
(a) 2 Hz Element
Atomic%
Mg K
96.98
Al K
2.44
Mn K
0.20
Zn K
0.78
(b) Base metal Fig. 9. EDS results.
bits, detached grains, and heterogeneous nuclei to the region of constitutional super- cooling just ahead of the solid–liquid interface [10]. Thus, while arc oscillation undoubtedly enhances fluid flow, the penetration of the fluid motion into such a dendrite network might be very difficult. The grain refinement observed in the current investigation is therefore believed to be more due to the other effects of MAO on the weld pool shape, fluid flow, and thermal gradients. The continual change in the weld pool shape due to MAO frequency is considered to be responsible for the reduction in columnar grain size in the interior of the fusion zone [13]. The thermal gradient direction gets changed constantly. So the newer grains consecutively become constructively oriented. Thus, while each grain grows only a small distance, more grains grow resulting in a finer grain structure distributed along the length of the weld. It can be attributed to the presence of finer grains and uniform distribution of precipitates compared to the other joints that are the main reason for the higher hardness in this joint [19]. The greater level of hardness found in the fusion zone is due to the moderately faster cooling rate and the steeper thermal gradients which
develop fine grained microstructure. Grain refinement led to an improvement in fusion zone tensile properties. The maximum tensile strength was achieved for the joint made with a oscillation frequency of 2 Hz. The distribution of higher hardness in the weld metal is also the reason for the higher tensile strength of the joint [17]. The higher fusion zone hardness (68 Hv) was recorded for the joint made using an oscillation frequency of 2 Hz. 5. Conclusions From this investigation, the following important conclusions are derived: The arc oscillation frequency causes impact on the grain size and hardness of fusion zone and subsequently on the tensile properties of GTAW joints of AZ31B magnesium alloy. The five welded joints fabricated with varying arc oscillation frequency range between 1 and 3 Hz and the joint fabricated using an oscillation frequency of 2 Hz, show greater tensile properties than their counterparts.
Please cite this article as: V. Subravel, N. Alagappan and N. Babu, Influence of arc oscillation frequency on tensile properties and microstructural characteristics of magnetic arc oscillation welded AZ31B magnesium alloy joints, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.001
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The formation of finer grains in the fusion zone, higher hardness of fusion zone, and uniformly distributed precipitates in fusion zone are found to be the reasons for the superior tensile properties of the joint fabricated using the arc oscillation frequency of 2 Hz than the other joints.
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Please cite this article as: V. Subravel, N. Alagappan and N. Babu, Influence of arc oscillation frequency on tensile properties and microstructural characteristics of magnetic arc oscillation welded AZ31B magnesium alloy joints, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.001