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Effects of Zn powder on alloying during friction surfacing of Al–Mg alloy
Journal Pre-proof Effects of Zn powder on alloying during friction surfacing of Al–Mg alloy Seyedeh Marjan Bararpour, Hamed Jamshidi Aval, Roohollah Jamaati PII:
S0925-8388(19)34069-1
DOI:
https://doi.org/10.1016/j.jallcom.2019.152823
Reference:
JALCOM 152823
To appear in:
Journal of Alloys and Compounds
Received Date: 9 July 2019 Revised Date:
25 October 2019
Accepted Date: 27 October 2019
Please cite this article as: S.M. Bararpour, H.J. Aval, R. Jamaati, Effects of Zn powder on alloying during friction surfacing of Al–Mg alloy, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/ j.jallcom.2019.152823. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Effects of Zn powder on alloying during friction surfacing of Al-Mg alloy Seyedeh Marjan Bararpour1, Hamed Jamshidi Aval1*, Roohollah Jamaati 1 1
Department of Materials Engineering, Babol Noshirvani University of Technology, Shariati Avenue, Babol, Iran, Post Code: 47148-71167 Tel.: +98 11 35501808; fax: +98 11 35501802; Email: [email protected]
Abstract: The present study investigated the effect of zinc powder on the microstructure and mechanical properties of Al-Mg alloy friction surfaced on an AA5052 aluminum alloy substrate. The zinc powder was applied to the coating through insertion of the hole in the Al-Mg aluminum alloy consumable rod. The addition of Zn powder greatly increases the thermal stability of the coating and reduces kinetic of grain growth. Considering a linear approximation between the grain size and the heat treatment time, the grain growth rate in the coating containing Zn and Zn free is approximately 0.55 µm/hr and 1.46 µm/hr, respectively. Based on the XRD results in pre heat treatment condition of sample containing Zn powder, in addition to the iron-rich particles and Mg2Si, the MgZn and Mg7Zn3 particles formed. The longtime solid solution and aging treatment result in the AlMg4Zn11 precipitates in the coted sample. After 8 hours heat treatment, the coating hardness reaches 70.14 BHN, which shows an increase of 14% as compared to before heat treatment. Keywords: Friction surfacing; Al-Mg alloy; Zinc powder; Microstructural characterization; Mechanical properties. 1. Introduction Friction surfacing is a type of solid-state coating process based on severe plastic deformation of consumable materials and their deposition under axial load. The friction surfacing process consists of four steps. In the first step, force is applied to a consumable rod that rotates at a constant speed. Then, a viscoplastic layer is produced with frictional heat between the interface of the rod and a substrate. In the next step, a bond is formed using the diffusion process, leading to the creation of a shearing interface. Finally, the viscoplastic layer is continuously deposited by the translation motion of the consumable rod. One of the advantages of this method is the production of fine and homogeneous microstructure that offers good corrosion and abrasive properties. Also, since this process is carried out in solid state, it allows the coating of materials 1
that cannot be processed by fusion methods. A wide range of materials are coated by this process [1]. Improving the local properties of the material through changes in the microstructure and chemical composition is one of the topics of interest to various researchers. In many of these investigations, this is done through the application of severe plastic deformation known as mechanical alloying. Various methods are available for this purpose, and various metal systems have been studied to date for the purpose of whether plastic deformation can lead to the formation of metal composites or super saturated solid solutions. First attempts for mechanical alloying were made through metal powder ball milling. At first, ball milling of copper base alloy were considered [2-4]. However, a wide range of different materials combinations was studied by this method and it was found that there is a possibility of alloying during this process in various metal systems [5]. Through high pressure torsion (HPT) it is possible to apply sever plastic deformation to metallic materials. It was found that using the HPT method, it is possible to create a solid solution even in systems that are in an immiscible equilibrium state [6]. Accumulative roll bonding (ARB) is another form of severe plastic deformation that results in the production of a solid mixture of metals. In addition to conventional methods for mechanical alloying, the friction stir processing (FSP) is another form of severe plastic deformation that has been proposed for the possibility of mechanical alloying with this method. The most researches has mainly been done on mechanical alloying of aluminum matrix by other elements. In this case, two major groups of metal materials, including elements with high solubility in aluminum such as Zn, and Cu, and elements with limited solubility such as Ni, and Ti have been studied. In the case of the recent group, according to literatures [7-10], the FSP process mainly results in the formation of intermetallic compounds. In the first group elements, mechanical alloying generates a solid solution during the process. Yadav et al.[11] investigated the formation of an Al-Zn solid solution using the FSP process. The zinc was incorporated by a multi-pass FSP process to dissolve and form a super-saturated solid solution in pure aluminum. They found that hardness in the processing zone was significantly improved compared to the base metal. The solid solution formed in the first pass, partially decomposed in the second pass and contributed to the formation of Al-Zn precipitates.
2
Aside from the abovementioned methods, the friction surfacing process is highly capable of fabricating new materials through the formation of intermetallic compounds and alloying in situ. Bararpour et al. [12] recently indicated that the severe plastic deformation provided during the friction surfacing process can be considered as a tool for in-situ alloying during the process. In this research, it is tried to investigate the effect of zinc powder and different heat treatment conditions (homogenizing and artificial aging) on the microstructure and mechanical properties of Al-Mg alloy friction surfaced on an AA5052 aluminum alloy substrate. The Zn powder applied to the consumable rod through drilling of holes in the AA5083 aluminum rod cross section. 2. Experimental procedure In the present study, the chemical composition of the Al-Mg consumable rod is 4.00% Mg, 0.06% Cu, 0.51% Mn, 0.13% Si, 0.12% Fe, Al balance, in wt%. Moreover, the chemical composition of the AA5052 substrate is 2.21% Mg, 0.04% Cu, 0.08% Mn, 0.09% Si, 0.21% Fe, Al balance, in wt%. AA5052 substrate was cut in dimensions of 150×150×2 mm3. The consumable rod was 20 mm in diameter and 100 mm in length. The axial feeding rate of 125 mm/min, the rotational speed of 800 rev/min, and the transverse speed of 125 mm/min were used in the friction surfacing experiments. In order to apply zinc powder to the Al-Mg consumable rod, four 2.5 mm diameter holes drilled with a 30 mm depth in the consumable rod cross section. The holes were drilled at a 3 mm radial distance from the center and at an angle of 90º relative to each other. According to Figure 1 the Zn powders with average size 2.5 µm were inserted inside the holes. Friction surfacing of zinc free consumable rod was also performed. According to the preliminary studies in the rotational speed (ɷ) range of 600-1000 rpm, the traverse speed (vx) 75150 mm / min and the axial feeding rate (vz) 100-200mm / min, the maximum coating efficiency achieved at ɷ , vx , and vz 800 rpm, 125 mm / min, and 125 mm / min, respectively. Therefore, all coating in this study performed with the last cited friction surfacing parameters. After coating to determine the effect of homogenizing and aging treatment on the microstructure and mechanical properties of the coated samples, homogenizing at 375 °C and the artificial aging at 180 °C were carried out for 4, 8 and 12 hours. In order to investigate the microstructure, the coated samples were cut transversely and the microstructure was studied after grinding/polishing operations and etching by Poulton's etchant
3
using optical and electron microscopes. The grain sizes were determined based on ASTM: E11213 and the general intercept procedure. To identify phases in the microstructure, X-ray diffraction (XRD) analysis was used. The hardness of the coated samples was measured using the Brinel hardness instrument with a 15 kg load and a 10 s dwell time. In order to evaluate the strength of the coating, a shear punch test was used in accordance with the procedure reported in the reference [13]. 3. Results and Discussion Figure 3 shows the microstructure of the Al-Mg consumable rod as well as the friction surfaced samples containing zinc and zinc free. The average grain size in the consumable rod is 250µm. The microstructure of the consumable rod according to the EDAX analysis contains Mg2Si, and iron-rich particles. The average particle size of Mg2Si, and iron-rich in the consumable rod is 4.5µm, and 1.5µm, respectively. The coating microstructure in both Zn containing and Zn free samples containing very fine grains compared to the microstructure of the consumable rod. The equiaxed fine-grained microstructures in the coatings are due to dynamic recrystallization during the friction surfacing process and the severe plastic deformation applied to the consumable rod. Although the exact mechanism of recrystallization during the coating process is not clear, dynamic recovery (DRV), discontinuous dynamic recrystallization (DDRX), continuous dynamic recrystallization (CDRX), and geometric dynamic recrystallization (GDRX) [14, 15] may occur depending on the nature of the process and properties of aluminum alloy. The stacking fault energy is an important parameter that influences the microstructural evolutions in aluminum alloys at high temperature deformation through the effect of cross-slip and dislocation climb. Although the addition of alloying elements reduces the stacking fault energy of aluminum, the addition of zinc with a stacking fault energy of 220 mJ / m2 will not significantly alter the stacking fault energy of 188 mJ / m2 aluminum during alloying [16, 17]. Therefore, it can be expected that the addition of zinc will not have a significant change in the recrystallization mechanism and the formation of equiaxed microstructures during the friction surfacing process. It should be noted that although both Zn containing and Zn free samples have equiaxed fine grains, the average grain size of the coating containing Zn (6.2 µm) is smaller than that without Zn (7.5 µm). The smaller grain size in the Zn containing coating can be attributed to the effect of solid solution pinning, which reduces the driving force of the grain boundary migration [12, 18,
4
19] during dynamic recrystallization. Another reason to explain smaller grain size in the Zn containing coating is that the presence of Zn powders in the consumable rod cross section changes the frictional conditions and creates less frictional heat during coating. Therefore, it is expected that lower heat during the process will result in less grain growth in the Zn containing sample. However, in Zn free sample, the integrity of the consumable rod cross-section and the stability of friction conditions result in more frictional heat generation. Figure 3 shows the microstructure of the sample containing Zn powder after homogenizing and solid solution heat treatment in 350ºC for 4, 8 and 12 hours, followed by aging for 4, 8, and 12 hours at a temperature of 180ºC. Also, the microstructure of Zn free sample for comparison after solid solution and aging heat treatment for 12 hours is shown in Figure 3 (d). As can be seen, with increasing solid solution and aging time, the grain size increases in the Zn containing sample. The grain size of the coating in the samples containing Zn powder after 4, 8 and 12 hours of solid solution and aging treatment is 8.1, 11.2 and 12.5 µm, respectively. Considering a linear approximation between the grain size and the heat treatment time, the grain growth rate in the coating containing Zn powder is approximately 0.55 µm/hr. However, in Zn free sample after 12 hours of heat treatment, the grain size of the coating is 25.4 µm. Again, given the linear approximation between the coating grain size and the duration of the heat treatment, it is observed that the growth rate of grain in the coating without powder is approximately 1.46 µm/hr. As it can be seen, the addition of Zn powder greatly increases the thermal stability of the coating and reduces kinetic of grain growth. Undoubtedly, as shown in Figure 3 (a-c) and Figure 4 (a-c), the presence of pinning particles on the grain boundary plays a key role in creating the thermal stability characteristic. In order to more accurately analyze precipitates and particles formed during heat treatment, the SEM image and distribution of the main elements of Zn, Si and Mg in the coating containing Zn powder, is shown in Figure 5. In the pre heat treatment condition of the coating containing Zn powder, it is observed that the Zn distribution is completely homogeneous. After 4 hours of heat treatment, Zn-rich particles accumulate in some places, which are mostly grain boundaries. By increasing the heat treatment time to 8 hours, the non-uniformity in the distribution of Zn-containing particles reached its maximum. As can be seen, these particles accumulate mainly on the grain boundaries, and a small part of them are placed inside the grain. After 12 hours, it is observed that the concentration of Zn particles in the grain boundaries is reduced and the distribution of these particles in the whole structure becomes 5
more uniform. By increasing the heat treatment time, the amount of accumulation of Zn containing particles reduced. In other words, by carrying out heat treatment up to 8 hours, the Zn is mainly accumulated on the grain boundaries due to the higher energy level of grain boundaries and, during the aging treatment, causes the formation of more Zn containing particles in this zone. In order to identify the particles and precipitates formed during the heat treatment, XRD analysis was performed on coatings with and without Zn powder. Figure 6 shows the results of the XRD analysis and Figure 7 shows the particles and precipitates in the SEM image of microstructure. Based on XRD analysis in Zn free sample in both pre and post heat treatment condition, only Mg2Si and iron-rich particles are detected. SEM images in Figure 7 (c and d) confirm XRD results. According to the EDAX analysis, very small amounts of Si particles are detectable in the microstructure of Zn free sample. As can be seen, the size, type and morphology of iron-rich particles do not differ significantly in pre and post heat treatment conditions. However, Mg2Si particles after heat treatment become coarser than before heat treatment. Based on the XRD results in pre heat treatment condition of sample containing Zn powder, in addition to the ironrich particles and Mg2Si, the MgZn and Mg7Zn3 particles formed. As seen in the SEM image (Figure 7 (a)), the Zn rich particles are in the range of 100-250 nm and uniformly distributed in the aluminum matrix. After the heat treatment for 4 and 8 hours, the nature of the particles containing Zn is not changed (according to XRD results (Figure 6)). However, based on the SEM images (Figure 7 (b and e)), the particle size of the zinc is larger and, as can be seen in Figure 7 (e), after 8 hours of heat treatment, the high density of the Zn containing precipitates is formed at the grain boundary. The major precipitates in the grain boundary based on EDAX analysis are Mg7Zn3. In addition to the MgZn and Mg7Zn3 precipitates, a small amount of AlMg4Zn11 precipitates is observed in SEM image. The reason for not identifying these precipitates in the XRD analysis is probably due to the small amount of these precipitates in the microstructure. AlMg4Zn11 precipitates are observed in the process of precipitation of Al-Mg-Zn when prolonged heat treatment is performed [20]. Due to the long duration of homogenizing and aging heat treatment, formation of this precipitate is expected. Based on XRD analysis, by increasing the heat treatment time, in addition to the MgZn and Mg7Zn3 particles, AlMg4Zn11 precipitates also form in the microstructure. According to Figure 7(f), the important point in the coated sample heat treated after 12 hours is decreasing of Mg7Zn3 precipitates concentration in the grain 6
boundaries and the formation of Zn containing particles in a homogeneous and uniform manner throughout the microstructure. In Figure 8, the results of the hardness of the coating in various conditions of heat treatment are shown. The hardness of the Zn free and Zn containing samples in the pre heat treatment condition indicates that the hardness is 60.62 and 61.30 BHN, respectively. As previously mentioned, the presence of smaller grains and the high percentage of MgZn and Mg7Zn3 particles in the zinc containing sample can result the higher hardness in this sample. After heat treatment in the sample containing Zn, it is observed that with increasing heat treatment time up to 4 hours, the hardness of the coating is 53.23 BHN, which is less than the pre-heat treatment condition. The reason for this can be due to the growth of the grain, as well as the coarsening and inhomogeneous distribution of MgZn and Mg7Zn3 particles. After 8 hours heat treatment, the coating hardness reaches 70.14 BHN, which shows an increase of 14% as compared to before heat treatment. Due to the fact that the grain size in this specimen is greater than that before the heat treatment, it is possible to attribute the reason for increasing the hardness to the formation of thick Mg7Zn3 interconnected precipitates along the grain boundary (grain boundary reinforcement with intermetallic phase), as well as the formation of AlMg4Zn11 precipitates. After 12 hours heat treatment, the hardness of the coating is reduced to 69.67 BHN. Although AlMg4Zn11 precipitates have been formed more in this sample, this can be attributed to the grain growth as well as the discontinuity of the Mg7Zn3 precipitates at the grain boundary. Also, it can be seen that due to grain growth in the Zn free sample after 12 hours heat treatment hardness reduced to 56.9 BHN. The results of the shear punch test are shown in Figure 9. Also, the strength- normalized displacement curve of coated sample is shown in the Figure 10. Before the heat treatment, the strength of the Zn containing and Zn free samples is 212.07 MPa and 178.68 MPa, respectively. However, the consumable rod strength is 168.02 MPa. The strengthening by precipitates and grain boundary are the main mechanism for increasing the strength of the Zn containing sample. After heat treatment for 4 hours, the Zn containing sample strength is reduced to 169.47 MPa. Grain growth as well as non-homogeneous precipitates distribution in this sample can lead to a decrease in strength. By increasing the heat treatment time to 8 hours and the formation of zinc containing intermetallic compounds in the grain boundaries, it will increase the strength to
7
213.83 MPa. On the other hand, it should be noted that the higher strength in this sample is accompanied by a reduction in the elongation of the coating. Again, with the increase in the heat treatment time, the coating strength reduced to 183.07 MPa, but the elongation of coating increased. The reduction in the interconnected intermetallic compounds containing Zn in the grain boundaries can result in decreasing of strength after 12 hours heat treatment. The strength of the Zn free sample after 12 hours heat treatment is equal to 162.01 MPa. It is observed that the variation in the strength of the coating is similar to the hardness variation. This means that with an increase in the heat treatment time, the strength and hardness of Zn containing coting first reaches its maximum after 8 hours, but after 12 hours decreases. 4. Conclusions This study investigated the effect of zinc powder and different heat treatment conditions (homogenizing and artificial aging) on the microstructure and mechanical properties of Al-Mg alloy friction surfaced on an AA5052 aluminum alloy substrate. The main conclusions can be summarized as follows: -
Considering a linear approximation between the grain size and the heat treatment time, the grain growth rate in the coating containing Zn and Zn free is approximately 0.55 µm/hr and 1.46 µm/hr, respectively. The addition of Zn powder greatly increases the thermal stability of the coating and reduces kinetic of grain growth.
-
By increasing the heat treatment time, the amount of accumulation of Zn containing particles reduced. In other words, by carrying out heat treatment up to 8 hours, the Zn is mainly accumulated on the grain boundaries due to the higher energy level of grain boundaries and, during the aging treatment, causes the formation of more Zn containing particles in this zone.
-
Based on the XRD results, after longtime solid solution and aging treatment in addition to the Mg2Si, MgZn, Mg7Zn3, and iron rich particles the AlMg4Zn11 precipitates formed.
-
After 8 hours of heat treatment, the high density of the Zn containing precipitates is formed at the grain boundary. The major precipitates in the grain boundary based on EDAX analysis are Mg7Zn3.
-
It is observed that the variation in the strength of the coating is similar to the hardness variation. This means that with an increase in the heat treatment time, the strength and
8
hardness of Zn containing coting first reaches its maximum after 8 hours, but after 12 hours decreases.
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[18] K. Wada, K. Takeshima, T. Uesugi, Y. Takigawa, K. Higashi, Effects of Solute Fe, Zn and Mg on Recrystallization in Aluminum, MATERIALS TRANSACTIONS, 57 (2016) 329-334. [19] D. Chandrasekaran, Grain size and solid solution strengthening in metals, in, Materialvetenskap, 2003. [20] L. Blaz, M. Sugamata, G. Wloch, J. Sobota, A. Kula, Structure and consolidation of rapidly solidified Meso 10 alloy flakes, Journal of Alloys and Compounds, 506 (2010) 179-187.
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Figure caption Figure 1- SEM image of zinc powders used in this work. Figure 2- The microstructure of a) consumable rod, b) coated sample with Zn powder, c) coated sample without Zn powder, d) EDAX analysis of Mg2Si, and e) EDAX analysis of iron-rich particles. Figure 3- Optical microscopy images of samples after homogenization and solid solution treatment; a) Zn containing sample after 4h, b) Zn containing sample after 8h, c) Zn containing sample after 12 h, and d) Zn free sample after 12h. Figure 4- The SEM image and the distribution of pinning particles in heat treated coated samples; a) after 4 hours, b) after 8 hours, and c) after 12 hours. Figure 5- The SEM image and the distribution of the main elements in the sample containing Zn powder in the pre and post heat treatment conditions; a) coated sample without heat treatment, b) after 4 hours, c) after 8 hours, and d) after 12 hours. Figure 6- XRD plots for coated samples before and after heat treatment. Figure 7- The SEM image of coated sample; a) Zn containing sample before heat treatment, b) Zn containing sample after 4 hours heat treatment, c) Zn free sample before heat treatment, d) Zn free sample after 12 hours heat treatment, e) Zn containing sample after 8 hours heat treatment, and f) Zn containing sample after 12hr heat treatment. Figure 8- The hardness value for coated samples. Figure 9- Shear strength of consumable rod and coated samples. Figure 10- The strength-normalized displacement curve of Zn containing and Zn free samples.
Figure 1- SEM image of zinc powders used in this work.
(a)
(b)
(c)
(d)
(e) Figure 2- The microstructure of a) consumable rod, b) coated sample with Zn powder, c) coated sample without Zn powder, d) EDAX analysis of Mg2Si, and e) EDAX analysis of iron-rich particles.
Pinning particles
Pinning particles
(a)
(b)
(c)
(d)
Pinning particles
Figure 3- Optical microscopy images of samples after homogenization and solid solution treatment; a) Zn containing sample after 4h, b) Zn containing sample after 8h, c) Zn containing sample after 12 h, and d) Zn free sample after 12h.
Pinning particles
Pinning particles
(a)
(b) Pinning particles
(c) Figure 4- The SEM image and the distribution of pinning particles in heat treated coated samples; a) after 4 hours, b) after 8 hours, and c) after 12 hours.
(a)
(b)
(c)
(d)
Figure 5- The SEM image and the distribution of the main elements in the sample containing Zn powder in the pre and post heat treatment conditions; a) coated sample without heat treatment, b) after 4 hours, c) after 8 hours, and d) after 12 hours.
Figure 6- XRD plots for coated samples before and after heat treatment.
Mg7Zn3
MgZn Mg2Si
MgZn Mg2Si
Al6(Fe,Mn)
Al6(Fe,Mn)
(a)
Mg7Zn3
(b) Al6(Fe,Mn)
Mg2Si Si Al6(Fe,Mn) Mg2Si Si
(c)
(d)
MgZn
AlMg4Zn11
AlMg4Zn11
Mg7Zn3 MgZn
Mg7Zn3
(e)
(f)
Figure 7- The SEM image of coated sample; a) Zn containing sample before heat treatment, b) Zn containing sample after 4 hours heat treatment, c) Zn free sample before heat treatment, d) Zn free sample after 12 hours heat treatment, e) Zn containing sample after 8 hours heat treatment, and f) Zn containing sample after 12hr heat treatment.
80 70
Brinell hardness
60 50 Without Zn
40
With Zn 30 20 10 0 As recived
After aging 4h
After aging 8h
Figure 8- The hardness value for coated samples.
After aging 12h
250
Shear strength (MPa)
200
150 Without Zn 100
With Zn
50
0 As recived
After aging 4h
After aging 8h
After aging AA5083 12h consumable rod
Figure 9- Shear strength of consumable rod and coated samples.
250
With Zn- after 4hr aging With Zn- after 8hr aging
Strength (MPa)
200
With Zn- after 12hr aging With Zn- Pre Heat treatment Without Zn
150
100
50
0 0
0.2
0.4
0.6
0.8
1
1.2
Normalized displacment
Figure 10- The strength-normalized displacement curve of Zn containing and Zn free samples.
Fine grained (Al-Mg) / Znp coating was fabricated on the AA5052 substrate.
Alloying during friction surfacing process caused precipitation hardening behavior.
The addition of Zn powder greatly increases the thermal stability of the coating and reduces kinetic of grain growth.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0925-8388(19)34069-1
DOI:
https://doi.org/10.1016/j.jallcom.2019.152823
Reference:
JALCOM 152823
To appear in:
Journal of Alloys and Compounds
Received Date: 9 July 2019 Revised Date:
25 October 2019
Accepted Date: 27 October 2019
Please cite this article as: S.M. Bararpour, H.J. Aval, R. Jamaati, Effects of Zn powder on alloying during friction surfacing of Al–Mg alloy, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/ j.jallcom.2019.152823. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Effects of Zn powder on alloying during friction surfacing of Al-Mg alloy Seyedeh Marjan Bararpour1, Hamed Jamshidi Aval1*, Roohollah Jamaati 1 1
Department of Materials Engineering, Babol Noshirvani University of Technology, Shariati Avenue, Babol, Iran, Post Code: 47148-71167 Tel.: +98 11 35501808; fax: +98 11 35501802; Email: [email protected]
Abstract: The present study investigated the effect of zinc powder on the microstructure and mechanical properties of Al-Mg alloy friction surfaced on an AA5052 aluminum alloy substrate. The zinc powder was applied to the coating through insertion of the hole in the Al-Mg aluminum alloy consumable rod. The addition of Zn powder greatly increases the thermal stability of the coating and reduces kinetic of grain growth. Considering a linear approximation between the grain size and the heat treatment time, the grain growth rate in the coating containing Zn and Zn free is approximately 0.55 µm/hr and 1.46 µm/hr, respectively. Based on the XRD results in pre heat treatment condition of sample containing Zn powder, in addition to the iron-rich particles and Mg2Si, the MgZn and Mg7Zn3 particles formed. The longtime solid solution and aging treatment result in the AlMg4Zn11 precipitates in the coted sample. After 8 hours heat treatment, the coating hardness reaches 70.14 BHN, which shows an increase of 14% as compared to before heat treatment. Keywords: Friction surfacing; Al-Mg alloy; Zinc powder; Microstructural characterization; Mechanical properties. 1. Introduction Friction surfacing is a type of solid-state coating process based on severe plastic deformation of consumable materials and their deposition under axial load. The friction surfacing process consists of four steps. In the first step, force is applied to a consumable rod that rotates at a constant speed. Then, a viscoplastic layer is produced with frictional heat between the interface of the rod and a substrate. In the next step, a bond is formed using the diffusion process, leading to the creation of a shearing interface. Finally, the viscoplastic layer is continuously deposited by the translation motion of the consumable rod. One of the advantages of this method is the production of fine and homogeneous microstructure that offers good corrosion and abrasive properties. Also, since this process is carried out in solid state, it allows the coating of materials 1
that cannot be processed by fusion methods. A wide range of materials are coated by this process [1]. Improving the local properties of the material through changes in the microstructure and chemical composition is one of the topics of interest to various researchers. In many of these investigations, this is done through the application of severe plastic deformation known as mechanical alloying. Various methods are available for this purpose, and various metal systems have been studied to date for the purpose of whether plastic deformation can lead to the formation of metal composites or super saturated solid solutions. First attempts for mechanical alloying were made through metal powder ball milling. At first, ball milling of copper base alloy were considered [2-4]. However, a wide range of different materials combinations was studied by this method and it was found that there is a possibility of alloying during this process in various metal systems [5]. Through high pressure torsion (HPT) it is possible to apply sever plastic deformation to metallic materials. It was found that using the HPT method, it is possible to create a solid solution even in systems that are in an immiscible equilibrium state [6]. Accumulative roll bonding (ARB) is another form of severe plastic deformation that results in the production of a solid mixture of metals. In addition to conventional methods for mechanical alloying, the friction stir processing (FSP) is another form of severe plastic deformation that has been proposed for the possibility of mechanical alloying with this method. The most researches has mainly been done on mechanical alloying of aluminum matrix by other elements. In this case, two major groups of metal materials, including elements with high solubility in aluminum such as Zn, and Cu, and elements with limited solubility such as Ni, and Ti have been studied. In the case of the recent group, according to literatures [7-10], the FSP process mainly results in the formation of intermetallic compounds. In the first group elements, mechanical alloying generates a solid solution during the process. Yadav et al.[11] investigated the formation of an Al-Zn solid solution using the FSP process. The zinc was incorporated by a multi-pass FSP process to dissolve and form a super-saturated solid solution in pure aluminum. They found that hardness in the processing zone was significantly improved compared to the base metal. The solid solution formed in the first pass, partially decomposed in the second pass and contributed to the formation of Al-Zn precipitates.
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Aside from the abovementioned methods, the friction surfacing process is highly capable of fabricating new materials through the formation of intermetallic compounds and alloying in situ. Bararpour et al. [12] recently indicated that the severe plastic deformation provided during the friction surfacing process can be considered as a tool for in-situ alloying during the process. In this research, it is tried to investigate the effect of zinc powder and different heat treatment conditions (homogenizing and artificial aging) on the microstructure and mechanical properties of Al-Mg alloy friction surfaced on an AA5052 aluminum alloy substrate. The Zn powder applied to the consumable rod through drilling of holes in the AA5083 aluminum rod cross section. 2. Experimental procedure In the present study, the chemical composition of the Al-Mg consumable rod is 4.00% Mg, 0.06% Cu, 0.51% Mn, 0.13% Si, 0.12% Fe, Al balance, in wt%. Moreover, the chemical composition of the AA5052 substrate is 2.21% Mg, 0.04% Cu, 0.08% Mn, 0.09% Si, 0.21% Fe, Al balance, in wt%. AA5052 substrate was cut in dimensions of 150×150×2 mm3. The consumable rod was 20 mm in diameter and 100 mm in length. The axial feeding rate of 125 mm/min, the rotational speed of 800 rev/min, and the transverse speed of 125 mm/min were used in the friction surfacing experiments. In order to apply zinc powder to the Al-Mg consumable rod, four 2.5 mm diameter holes drilled with a 30 mm depth in the consumable rod cross section. The holes were drilled at a 3 mm radial distance from the center and at an angle of 90º relative to each other. According to Figure 1 the Zn powders with average size 2.5 µm were inserted inside the holes. Friction surfacing of zinc free consumable rod was also performed. According to the preliminary studies in the rotational speed (ɷ) range of 600-1000 rpm, the traverse speed (vx) 75150 mm / min and the axial feeding rate (vz) 100-200mm / min, the maximum coating efficiency achieved at ɷ , vx , and vz 800 rpm, 125 mm / min, and 125 mm / min, respectively. Therefore, all coating in this study performed with the last cited friction surfacing parameters. After coating to determine the effect of homogenizing and aging treatment on the microstructure and mechanical properties of the coated samples, homogenizing at 375 °C and the artificial aging at 180 °C were carried out for 4, 8 and 12 hours. In order to investigate the microstructure, the coated samples were cut transversely and the microstructure was studied after grinding/polishing operations and etching by Poulton's etchant
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using optical and electron microscopes. The grain sizes were determined based on ASTM: E11213 and the general intercept procedure. To identify phases in the microstructure, X-ray diffraction (XRD) analysis was used. The hardness of the coated samples was measured using the Brinel hardness instrument with a 15 kg load and a 10 s dwell time. In order to evaluate the strength of the coating, a shear punch test was used in accordance with the procedure reported in the reference [13]. 3. Results and Discussion Figure 3 shows the microstructure of the Al-Mg consumable rod as well as the friction surfaced samples containing zinc and zinc free. The average grain size in the consumable rod is 250µm. The microstructure of the consumable rod according to the EDAX analysis contains Mg2Si, and iron-rich particles. The average particle size of Mg2Si, and iron-rich in the consumable rod is 4.5µm, and 1.5µm, respectively. The coating microstructure in both Zn containing and Zn free samples containing very fine grains compared to the microstructure of the consumable rod. The equiaxed fine-grained microstructures in the coatings are due to dynamic recrystallization during the friction surfacing process and the severe plastic deformation applied to the consumable rod. Although the exact mechanism of recrystallization during the coating process is not clear, dynamic recovery (DRV), discontinuous dynamic recrystallization (DDRX), continuous dynamic recrystallization (CDRX), and geometric dynamic recrystallization (GDRX) [14, 15] may occur depending on the nature of the process and properties of aluminum alloy. The stacking fault energy is an important parameter that influences the microstructural evolutions in aluminum alloys at high temperature deformation through the effect of cross-slip and dislocation climb. Although the addition of alloying elements reduces the stacking fault energy of aluminum, the addition of zinc with a stacking fault energy of 220 mJ / m2 will not significantly alter the stacking fault energy of 188 mJ / m2 aluminum during alloying [16, 17]. Therefore, it can be expected that the addition of zinc will not have a significant change in the recrystallization mechanism and the formation of equiaxed microstructures during the friction surfacing process. It should be noted that although both Zn containing and Zn free samples have equiaxed fine grains, the average grain size of the coating containing Zn (6.2 µm) is smaller than that without Zn (7.5 µm). The smaller grain size in the Zn containing coating can be attributed to the effect of solid solution pinning, which reduces the driving force of the grain boundary migration [12, 18,
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19] during dynamic recrystallization. Another reason to explain smaller grain size in the Zn containing coating is that the presence of Zn powders in the consumable rod cross section changes the frictional conditions and creates less frictional heat during coating. Therefore, it is expected that lower heat during the process will result in less grain growth in the Zn containing sample. However, in Zn free sample, the integrity of the consumable rod cross-section and the stability of friction conditions result in more frictional heat generation. Figure 3 shows the microstructure of the sample containing Zn powder after homogenizing and solid solution heat treatment in 350ºC for 4, 8 and 12 hours, followed by aging for 4, 8, and 12 hours at a temperature of 180ºC. Also, the microstructure of Zn free sample for comparison after solid solution and aging heat treatment for 12 hours is shown in Figure 3 (d). As can be seen, with increasing solid solution and aging time, the grain size increases in the Zn containing sample. The grain size of the coating in the samples containing Zn powder after 4, 8 and 12 hours of solid solution and aging treatment is 8.1, 11.2 and 12.5 µm, respectively. Considering a linear approximation between the grain size and the heat treatment time, the grain growth rate in the coating containing Zn powder is approximately 0.55 µm/hr. However, in Zn free sample after 12 hours of heat treatment, the grain size of the coating is 25.4 µm. Again, given the linear approximation between the coating grain size and the duration of the heat treatment, it is observed that the growth rate of grain in the coating without powder is approximately 1.46 µm/hr. As it can be seen, the addition of Zn powder greatly increases the thermal stability of the coating and reduces kinetic of grain growth. Undoubtedly, as shown in Figure 3 (a-c) and Figure 4 (a-c), the presence of pinning particles on the grain boundary plays a key role in creating the thermal stability characteristic. In order to more accurately analyze precipitates and particles formed during heat treatment, the SEM image and distribution of the main elements of Zn, Si and Mg in the coating containing Zn powder, is shown in Figure 5. In the pre heat treatment condition of the coating containing Zn powder, it is observed that the Zn distribution is completely homogeneous. After 4 hours of heat treatment, Zn-rich particles accumulate in some places, which are mostly grain boundaries. By increasing the heat treatment time to 8 hours, the non-uniformity in the distribution of Zn-containing particles reached its maximum. As can be seen, these particles accumulate mainly on the grain boundaries, and a small part of them are placed inside the grain. After 12 hours, it is observed that the concentration of Zn particles in the grain boundaries is reduced and the distribution of these particles in the whole structure becomes 5
more uniform. By increasing the heat treatment time, the amount of accumulation of Zn containing particles reduced. In other words, by carrying out heat treatment up to 8 hours, the Zn is mainly accumulated on the grain boundaries due to the higher energy level of grain boundaries and, during the aging treatment, causes the formation of more Zn containing particles in this zone. In order to identify the particles and precipitates formed during the heat treatment, XRD analysis was performed on coatings with and without Zn powder. Figure 6 shows the results of the XRD analysis and Figure 7 shows the particles and precipitates in the SEM image of microstructure. Based on XRD analysis in Zn free sample in both pre and post heat treatment condition, only Mg2Si and iron-rich particles are detected. SEM images in Figure 7 (c and d) confirm XRD results. According to the EDAX analysis, very small amounts of Si particles are detectable in the microstructure of Zn free sample. As can be seen, the size, type and morphology of iron-rich particles do not differ significantly in pre and post heat treatment conditions. However, Mg2Si particles after heat treatment become coarser than before heat treatment. Based on the XRD results in pre heat treatment condition of sample containing Zn powder, in addition to the ironrich particles and Mg2Si, the MgZn and Mg7Zn3 particles formed. As seen in the SEM image (Figure 7 (a)), the Zn rich particles are in the range of 100-250 nm and uniformly distributed in the aluminum matrix. After the heat treatment for 4 and 8 hours, the nature of the particles containing Zn is not changed (according to XRD results (Figure 6)). However, based on the SEM images (Figure 7 (b and e)), the particle size of the zinc is larger and, as can be seen in Figure 7 (e), after 8 hours of heat treatment, the high density of the Zn containing precipitates is formed at the grain boundary. The major precipitates in the grain boundary based on EDAX analysis are Mg7Zn3. In addition to the MgZn and Mg7Zn3 precipitates, a small amount of AlMg4Zn11 precipitates is observed in SEM image. The reason for not identifying these precipitates in the XRD analysis is probably due to the small amount of these precipitates in the microstructure. AlMg4Zn11 precipitates are observed in the process of precipitation of Al-Mg-Zn when prolonged heat treatment is performed [20]. Due to the long duration of homogenizing and aging heat treatment, formation of this precipitate is expected. Based on XRD analysis, by increasing the heat treatment time, in addition to the MgZn and Mg7Zn3 particles, AlMg4Zn11 precipitates also form in the microstructure. According to Figure 7(f), the important point in the coated sample heat treated after 12 hours is decreasing of Mg7Zn3 precipitates concentration in the grain 6
boundaries and the formation of Zn containing particles in a homogeneous and uniform manner throughout the microstructure. In Figure 8, the results of the hardness of the coating in various conditions of heat treatment are shown. The hardness of the Zn free and Zn containing samples in the pre heat treatment condition indicates that the hardness is 60.62 and 61.30 BHN, respectively. As previously mentioned, the presence of smaller grains and the high percentage of MgZn and Mg7Zn3 particles in the zinc containing sample can result the higher hardness in this sample. After heat treatment in the sample containing Zn, it is observed that with increasing heat treatment time up to 4 hours, the hardness of the coating is 53.23 BHN, which is less than the pre-heat treatment condition. The reason for this can be due to the growth of the grain, as well as the coarsening and inhomogeneous distribution of MgZn and Mg7Zn3 particles. After 8 hours heat treatment, the coating hardness reaches 70.14 BHN, which shows an increase of 14% as compared to before heat treatment. Due to the fact that the grain size in this specimen is greater than that before the heat treatment, it is possible to attribute the reason for increasing the hardness to the formation of thick Mg7Zn3 interconnected precipitates along the grain boundary (grain boundary reinforcement with intermetallic phase), as well as the formation of AlMg4Zn11 precipitates. After 12 hours heat treatment, the hardness of the coating is reduced to 69.67 BHN. Although AlMg4Zn11 precipitates have been formed more in this sample, this can be attributed to the grain growth as well as the discontinuity of the Mg7Zn3 precipitates at the grain boundary. Also, it can be seen that due to grain growth in the Zn free sample after 12 hours heat treatment hardness reduced to 56.9 BHN. The results of the shear punch test are shown in Figure 9. Also, the strength- normalized displacement curve of coated sample is shown in the Figure 10. Before the heat treatment, the strength of the Zn containing and Zn free samples is 212.07 MPa and 178.68 MPa, respectively. However, the consumable rod strength is 168.02 MPa. The strengthening by precipitates and grain boundary are the main mechanism for increasing the strength of the Zn containing sample. After heat treatment for 4 hours, the Zn containing sample strength is reduced to 169.47 MPa. Grain growth as well as non-homogeneous precipitates distribution in this sample can lead to a decrease in strength. By increasing the heat treatment time to 8 hours and the formation of zinc containing intermetallic compounds in the grain boundaries, it will increase the strength to
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213.83 MPa. On the other hand, it should be noted that the higher strength in this sample is accompanied by a reduction in the elongation of the coating. Again, with the increase in the heat treatment time, the coating strength reduced to 183.07 MPa, but the elongation of coating increased. The reduction in the interconnected intermetallic compounds containing Zn in the grain boundaries can result in decreasing of strength after 12 hours heat treatment. The strength of the Zn free sample after 12 hours heat treatment is equal to 162.01 MPa. It is observed that the variation in the strength of the coating is similar to the hardness variation. This means that with an increase in the heat treatment time, the strength and hardness of Zn containing coting first reaches its maximum after 8 hours, but after 12 hours decreases. 4. Conclusions This study investigated the effect of zinc powder and different heat treatment conditions (homogenizing and artificial aging) on the microstructure and mechanical properties of Al-Mg alloy friction surfaced on an AA5052 aluminum alloy substrate. The main conclusions can be summarized as follows: -
Considering a linear approximation between the grain size and the heat treatment time, the grain growth rate in the coating containing Zn and Zn free is approximately 0.55 µm/hr and 1.46 µm/hr, respectively. The addition of Zn powder greatly increases the thermal stability of the coating and reduces kinetic of grain growth.
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By increasing the heat treatment time, the amount of accumulation of Zn containing particles reduced. In other words, by carrying out heat treatment up to 8 hours, the Zn is mainly accumulated on the grain boundaries due to the higher energy level of grain boundaries and, during the aging treatment, causes the formation of more Zn containing particles in this zone.
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Based on the XRD results, after longtime solid solution and aging treatment in addition to the Mg2Si, MgZn, Mg7Zn3, and iron rich particles the AlMg4Zn11 precipitates formed.
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After 8 hours of heat treatment, the high density of the Zn containing precipitates is formed at the grain boundary. The major precipitates in the grain boundary based on EDAX analysis are Mg7Zn3.
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It is observed that the variation in the strength of the coating is similar to the hardness variation. This means that with an increase in the heat treatment time, the strength and
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hardness of Zn containing coting first reaches its maximum after 8 hours, but after 12 hours decreases.
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Figure caption Figure 1- SEM image of zinc powders used in this work. Figure 2- The microstructure of a) consumable rod, b) coated sample with Zn powder, c) coated sample without Zn powder, d) EDAX analysis of Mg2Si, and e) EDAX analysis of iron-rich particles. Figure 3- Optical microscopy images of samples after homogenization and solid solution treatment; a) Zn containing sample after 4h, b) Zn containing sample after 8h, c) Zn containing sample after 12 h, and d) Zn free sample after 12h. Figure 4- The SEM image and the distribution of pinning particles in heat treated coated samples; a) after 4 hours, b) after 8 hours, and c) after 12 hours. Figure 5- The SEM image and the distribution of the main elements in the sample containing Zn powder in the pre and post heat treatment conditions; a) coated sample without heat treatment, b) after 4 hours, c) after 8 hours, and d) after 12 hours. Figure 6- XRD plots for coated samples before and after heat treatment. Figure 7- The SEM image of coated sample; a) Zn containing sample before heat treatment, b) Zn containing sample after 4 hours heat treatment, c) Zn free sample before heat treatment, d) Zn free sample after 12 hours heat treatment, e) Zn containing sample after 8 hours heat treatment, and f) Zn containing sample after 12hr heat treatment. Figure 8- The hardness value for coated samples. Figure 9- Shear strength of consumable rod and coated samples. Figure 10- The strength-normalized displacement curve of Zn containing and Zn free samples.
Figure 1- SEM image of zinc powders used in this work.
(a)
(b)
(c)
(d)
(e) Figure 2- The microstructure of a) consumable rod, b) coated sample with Zn powder, c) coated sample without Zn powder, d) EDAX analysis of Mg2Si, and e) EDAX analysis of iron-rich particles.
Pinning particles
Pinning particles
(a)
(b)
(c)
(d)
Pinning particles
Figure 3- Optical microscopy images of samples after homogenization and solid solution treatment; a) Zn containing sample after 4h, b) Zn containing sample after 8h, c) Zn containing sample after 12 h, and d) Zn free sample after 12h.
Pinning particles
Pinning particles
(a)
(b) Pinning particles
(c) Figure 4- The SEM image and the distribution of pinning particles in heat treated coated samples; a) after 4 hours, b) after 8 hours, and c) after 12 hours.
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(b)
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Figure 5- The SEM image and the distribution of the main elements in the sample containing Zn powder in the pre and post heat treatment conditions; a) coated sample without heat treatment, b) after 4 hours, c) after 8 hours, and d) after 12 hours.
Figure 6- XRD plots for coated samples before and after heat treatment.
Mg7Zn3
MgZn Mg2Si
MgZn Mg2Si
Al6(Fe,Mn)
Al6(Fe,Mn)
(a)
Mg7Zn3
(b) Al6(Fe,Mn)
Mg2Si Si Al6(Fe,Mn) Mg2Si Si
(c)
(d)
MgZn
AlMg4Zn11
AlMg4Zn11
Mg7Zn3 MgZn
Mg7Zn3
(e)
(f)
Figure 7- The SEM image of coated sample; a) Zn containing sample before heat treatment, b) Zn containing sample after 4 hours heat treatment, c) Zn free sample before heat treatment, d) Zn free sample after 12 hours heat treatment, e) Zn containing sample after 8 hours heat treatment, and f) Zn containing sample after 12hr heat treatment.
80 70
Brinell hardness
60 50 Without Zn
40
With Zn 30 20 10 0 As recived
After aging 4h
After aging 8h
Figure 8- The hardness value for coated samples.
After aging 12h
250
Shear strength (MPa)
200
150 Without Zn 100
With Zn
50
0 As recived
After aging 4h
After aging 8h
After aging AA5083 12h consumable rod
Figure 9- Shear strength of consumable rod and coated samples.
250
With Zn- after 4hr aging With Zn- after 8hr aging
Strength (MPa)
200
With Zn- after 12hr aging With Zn- Pre Heat treatment Without Zn
150
100
50
0 0
0.2
0.4
0.6
0.8
1
1.2
Normalized displacment
Figure 10- The strength-normalized displacement curve of Zn containing and Zn free samples.
Fine grained (Al-Mg) / Znp coating was fabricated on the AA5052 substrate.
Alloying during friction surfacing process caused precipitation hardening behavior.
The addition of Zn powder greatly increases the thermal stability of the coating and reduces kinetic of grain growth.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: