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Si-enriched surface layer fabricated through thermochemical treatment
Surface & Coatings Technology 374 (2019) 201–209
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Microstructure and properties of AZ31 with an Al/Si-enriched surface layer fabricated through thermochemical treatment
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Renata Mola , Michał Cieślik ⁎
Department of Metal Science and Manufacturing Processes, Faculty of Mechatronics and Mechanical Engineering, Kielce University of Technology, Al. Tysiąclecia Państwa Polskiego 7, 25-314 Kielce, Poland
ARTICLE INFO
ABSTRACT
Keywords: Magnesium alloy Thermochemical treatment Alloyed surface layer Microstructure Microhardness Wear resistance
The surface enrichment of AZ31 magnesium alloy with Al and Si was achieved using the thermochemical treatment method. The experiments involved heating AZ31 in contact with an Al + Si powder mixture. Two mixtures acting as the source of diffusion elements were tested: 80% Al + 20% Si and 50% Al + 50% Si. The heat treatment of the AZ31 in the Al + 20% Si mixture was performed at three different temperatures: 435, 445 and 455 °C. As the best results were obtained at 445 °C, this temperature was used to heat AZ31 in contact with the Al + 50% Si mixture. The surface modification of AZ31 was attributable to the diffusion processes taking place at the substrate/powder reactive interface. The microstructure of the alloyed layers was characterized using an optical microscope (OM) and a scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectrometer (EDS). An X-ray diffractometer (XRD) was employed to determine the phase composition. The examinations showed that the modified surface layer produced from the Al + 20% Si powder mixture was continuous and had uniform thickness (400–450 μm). The Al/Si-enriched layer was mainly composed of a eutectic, i.e., an Mg17Al12 intermetallic phase and a solid solution of Al in Mg, and unevenly distributed Mg2Si phase particles. The use of the Al + 50% Si powder mixture resulted in the formation of a thinner, continuous layer (about 200 μm) with a much higher volume fraction of the Mg2Si phase. In this case, the layer had a more uniform structure; the particles of the Mg2Si phase were evenly distributed over the eutectic matrix. Due to the presence of hard phases, i.e., Mg2Si and Mg17Al12, both types of Al/Si-enriched surface layers had much higher hardness than the AZ31 substrate. The tribological tests revealed that the alloyed layers had very good wear resistance under dry sliding conditions. Because of the higher volume fraction of the Mg2Si phase particles, the layer fabricated from the Al + 50% Si powder mixture had higher hardness and wear resistance than the layer fabricated from the mixture containing 20% Si.
1. Introduction Magnesium alloys are the lightest metallic structural materials. They are characterized by high specific strength, good electrical and thermal conductivity, good machinability, and high recycling potential. These properties make the materials useful for different automotive and aerospace applications. However, due to low hardness and poor resistance to wear, their use is limited to applications where components are not exposed to severe abrasive wear conditions. A wide range of surface treatment methods have been developed to overcome this problem [1]. Research in this area has involved the formation of surface layers containing hard particles and/or hard intermetallic phases. Yet, articles dealing with the fabrication of alloyed surface layers containing intermetallic phases are less numerous. The literature data show that
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such surface layers can be created by introducing an appropriate element or elements that form intermetallic phases with magnesium. Surface modification techniques used for this purpose include laser surface alloying/cladding [2–13], surface alloying by welding [14,15], thermal spraying [16], cold spraying [17,18], PVD [19,20], casting [21,22], thermochemical treatment in molten salts [23–25], and thermochemical treatment in metal powders [26–40]. The above studies involved using pure Al or Al combined with other elements, i.e., Al + Si, Al + Cu, Al + Ni, Al + Zn, Zn + Sb, Zn + Y, and Al + Mn as alloying elements. This article deals with the fabrication of Al/Si-enriched layers through thermochemical treatment; it is important, however, to mention results concerning such layers on Mg-based materials produced using other methods. The literature data show that a surface layer enriched with Al and Si containing MgeAl intermetallic
Corresponding author. E-mail address: [email protected] (R. Mola).
https://doi.org/10.1016/j.surfcoat.2019.06.003 Received 11 February 2019; Received in revised form 30 April 2019; Accepted 1 June 2019 Available online 03 June 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
Surface & Coatings Technology 374 (2019) 201–209
R. Mola and M. Cieślik
phases and an Mg2Si phase can be formed on pure Mg or Mg alloy by laser alloying/cladding [7–13] or welding [15]. The properties of alloyed layers depend on the microstructure. The main factor affecting the microhardness of the layer is the content of hard Mg2Si phase particles and their distribution in the layer. Studies concerned with the laser alloying/cladding of Mg-based substrates by direct injection of Al12Si [7,8] or Al20Si [10] powders reveal that the microhardness of surface layers may vary, reaching, for example, 220–340 HV [7], 140–285 HV [8] or 135 HV [10]. When surface layers are formed through laser alloying/cladding by direct injection of AlSi powders, a uniform microstructure with evenly distributed Mg2Si particles may be difficult to obtain. In Ref. [7], Qian et al. describe laser alloying of AZ91D. They obtained layers 1.5 mm in thickness. Their study shows that an increase in the microhardness of the surface layer is mainly due to the formation of Mg2Si, Mg17Al12 and Al3Mg2 phases. The results also indicate that the layer microhardness decreases gradually from the uppermost region to the fusion line, which is a result of a non-uniform microstructure. As shown in Ref. [8], Al/Si-enriched layers, 500–650 μm in thickness, produced on various Mg alloys also have a non-uniform microstructure; the region adjacent to the substrate is characterized by higher microhardness (230–285 HV) because this is where the microstructure is mainly composed of MgeAl intermetallic phases and an Mg2Si phase. Lower values (140–170 HV) are reported for the uppermost zone, where the Mg2Si particles are unevenly distributed in the Al-rich matrix. Bobzin et al. [10] investigated the laser cladding of an AZ31B substrate with Al20Si powder. The resulting layer was about 1 mm thick. Although they used powder with a higher content of Si (20%), the alloyed layer had lower microhardness (135 ± 11 HV) than the layer fabricated from Al12Si powder [7,8]. Results presented in Refs. [7 and 9] indicate that unalloyed Al and Si particles can occur in the layer because of insufficient laser power or scan time, with this likely to have some negative effect on the layer properties. Al/Si-enriched layers on Mg-based substrates can also be fabricated using two-stage laser alloying/cladding. First, the alloying elements are preplaced in the form of paste or slurry [11,12], or foil [13] or they are deposited by plasma spraying [10]. The second stage consists in laser melting. When paste or slurry is used and laser melting is involved, high porosity may be observed in the layer. The occurrence of pores is associated with the burning of the organic material acting as the binder [41]. In Ref. [11], Yang et al. analyze the properties of layers fabricated on AZ91D from paste prepared by mixing Al and 12.5% Si powders with resin. The layers are about 1 mm in thickness and contain hard Mg2Si and Mg17Al12 phases. As a result, they are characterized by high microhardness. In this case, the microhardness of the layer is not uniform. The highest values (170 HV) were reported in the uppermost region. The method of surface modification discussed in Ref. [13] involved using a thin AlSi20 plate, which was first diffusion bonded to an Mg substrate and then laser melted. Depending on the process parameters, the laser beam melted the AlSi20 plate only or the AlSi20 plate and a layer of the Mg surface adjacent to it. Two types of microstructure of the remelted layer were fabricated. If the melting zone was limited to the AlSi20 plate, the microstructure of the surface layer was typical of a rapidly solidified hypereutectic AleSi alloy with microhardness in the range 96–120 HV. Since, however, the liquid AlSi20 reacted with the Mg substrate, the following phases were detected in the surface layer: Al3Mg2, Mg17Al12 and Mg2Si. The 400–500 μm thick layer had a microhardness of 215–225 HV. Reference [10] deals with the laser remelting of an AlSi20 plasma sprayed coating deposited on AZ31B. The process did not lead to the interaction of the coating with the substrate or the formation of new phases through reactive diffusion. As a result, the microhardness of the surface layer was low (97 HV). In laser alloying/cladding, the layer microstructure, and, consequently, its microhardness, may change along the length of the alloyed specimens. The reason for that is an increase in the specimen temperature during the alloying process. This leads to higher absorption of the laser-emitted
radiation, more effective use of the laser power, and, consequently, a deeper and wider melt zone increasing along the specimen length [3]. Wear test data [8–11,13] showed that laser alloying/cladding of Mgbased substrate with AlSi resulted in significant reduction in weight loss. Reference [15] analyzes the AleSi cladding of an Mg alloy by direct current-pulse metal insert gas (DC-PMIG) welding with AlSi5 welding wire used as the alloying material. Layers produced in this way are thick (up to 1.8 mm) but non-uniform. MgeAl intermetallic phases and an Mg2Si phase were detected in the thin transition zone close to the substrate. The thick upper zone was composed of α-Al dendrites, an AleSi eutectic and Mg2Si particles. The microhardness of the transition zone was 230–309 HV while that of the upper zone was much lower (115–129 HV) due to a low content of the Mg2Si phase and lack of MgeAl intermetallic phases in the layer microstructure. Thermochemical treatment is one of the most common surface engineering methods. This process employs thermal diffusion to incorporate metal atoms into a material surface to modify its chemical composition and microstructure [42]. A review of the literature [26–40] shows that the thermochemical treatment of an Mg-based substrate in a solid medium, i.e., metal powder or a metal powder mixture, is a simple and cost effective method to fabricate an alloyed layer containing intermetallic phases. In this process, an Mg or Mgbased component surrounded by compacted powder material is heated under inert atmosphere. The heating causes atoms in the outside source to move by diffusion to the surface layer of the specimen. The reactions taking place at the interface result in the formation of new phases. Researchers dealing with surface alloying of Mg experimented with the following powders or powder mixtures used as a solid medium: Al [26–29,35,36], Al + Zn [30–34,36], Zn + Al [37], Sb + Zn [38], Zn + Y [39], and Al + Si [40]. Layers fabricated on pure Mg or Mg alloys using pure Al powder contained MgeAl intermetallic phases. In most cases [26,27,35,36], the layers had a eutectic structure (an Mg17Al12 intermetallic phase and a solid solution of Al in Mg). Liu et al. [28,29] indicate, however, that the layer had a slightly different microstructure with the following structural constituents: a eutectic close to the substrate, an Al3Mg2 intermetallic phase in the center, and a solid solution of Mg in Al at the surface. The literature data show that the heat treatment of an Mg-based specimen in contact with Al + Zn led to the enrichment of the surface layer with both elements; in such a case, Mg-Al-Zn intermetallic phases were detected in the modified layer. Zhang and Kelly [30] produced a modified layer with a thickness of 1–2 mm on AZ91D alloy by heating the material at 430 °C for 12 h in contact with an Al + 30 wt% Zn powder mixture. The surface layer was composed of Mg17Al12 and Mg11Al5Zn4 intermetallic phases and a solid solution of Al and Zn in Mg. Ma et al. [31] used an Al + 50 wt% Zn powder mixture to produce a modified layer at the surface of ZM5, which contained Mg17Al12, Mg11Al5Zn4 and a solid solution of Al and Zn in Mg. Heat treatment was carried out at 470 °C for 12 h. Although the temperature was higher than that applied in the study described in [30], the fabricated layer was very thin (35–75 μm). Hirmke et al. [32,33] also used an Al + Zn powder mixture but with a different composition; they used different Mg alloys as the substrate materials and different process temperatures. They found that the addition of Zn into the powder mixture significantly promoted the formation of an alloyed layer at temperatures below 420 °C. The microstructure and thickness of the layer were dependent on the process parameters. When the content of Zn was over 10 wt%, the alloyed layers contained Mg32(Al,Zn)49, Mg11Al5Zn4 and Mg17Al12 intermetallic phases and a solid solution of Al and Zn in Mg. At low zinc contents (2–5 wt%), no ternary phases were identified in the layer microstructure. The experimental results presented in [34–37] show that an important factor affecting the formation of an Al, Al/Zn or Zn/Al-enriched layer on Mg through thermochemical treatment is good contact between the source of diffusion elements and the substrate material. During the heat treatment process, the powder or powder mixture filling the container 202
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was held under pressure. Adequate contact facilitated the diffusion of alloying elements from the outside source (powder or powder mixture) to the Mg-based substrate and formation of a thick alloyed layer in short heating time. The experimental results provided in [37] refer to a Zn/ Al-enriched layer fabricated on Mg from a Zn + 20% Al powder mixture. The layer was characterized by a microstructure consisting of Mg5Al2Zn2, MgZn and a solid solution of Al and Zn in Mg. Chen et al. [38] used an Sb + Zn powder mixture in the thermochemical treatment of AZ31 alloy. They detected several new compounds in the modified surface layer. The heat treatment of AZ91D in contact with a Zn + Y powder mixture resulted in the formation of a surface layer containing a large amount of an Mg5Al2Zn2 intermetallic phase [39]. The experimental results presented in the above works concerning thermochemical treatment show that the alloyed layer containing intermetallic phases improved the surface properties of Mg-based materials. Since the layers contained intermetallic phases, their hardness was several times higher than that of the substrate; they were also characterized by good wear resistance. An alloyed layer fabricated through thermochemical treatment may also provide effective protection of an Mgbased substrate from corrosion. This paper discusses further investigations on the fabrication of surface layers on an Mg-based substrate through thermochemical treatment. Al + Si powder mixtures were used as the source of diffusion elements. From the literature, it is apparent that the Mg2Si phase can be easily formed in situ by the reaction of Mg with Si; hence the selection of Si powder as a component of the powder mixture [13,43]. As the Mg2Si phase has high hardness, ranging from 350 to 450 HV0.01 [44], the fabrication of a surface layer containing it may significantly improve the hardness and wear resistance of the Mg-based material. This phase is also characterized by low density (1.99 Mg/m3), which implies that the formation of a layer containing a high volume fraction of this phase on a light Mg-based substrate will not affect the total weight of an element. The first results of the research in this area published in [40] suggest that the heat treatment of Mg in contact with an Al + 20% Si powder mixture results in the formation of a surface layer containing an Mg17Al12 intermetallic phase, an Mg2Si phase and a solid solution of Al in Mg. Since pure Mg is rarely applied as a structural material, this study involved using AZ31 magnesium alloy as the substrate. This article considers the influence of the process parameters (the heat treatment temperature and the powder mixture composition) on the microstructure and properties of Al/Si-enriched surface layers fabricated on AZ31 through thermochemical treatment. A detailed microstructural analysis was performed using optical microscopy and scanning electron microscopy. The chemical composition of the layers was identified on the basis of the X-ray EDS analysis data. The XRD method was employed to determine the phase composition. The microhardness and wear resistance tests were used to determine the properties of the fabricated layers.
the container to keep the powder under pressure during heat treatment. The specimens were heated from room temperature to the desired temperature for 30 min, kept at that temperature for another 30 min and cooled back to room temperature. The heat treatment of the AZ31 magnesium alloy in the 80 wt% Al + 20 wt% Si powder mixture was carried out at 435, 445 and 455 °C. When the powder mixture containing 50% Si was used, the heating was performed only at 445 °C. During the entire heat treatment process, a pressure of 1 MPa was applied to ensure good contact between the AZ31 specimen and the powder mixture, acting as the source of diffusion elements. A schematic diagram of the thermochemical treatment process is shown in Fig. 1. 2.2. Layer characterization After the thermochemical treatment process, the specimens were cut into rectangular pieces and prepared for microscopic observations, which involved mechanically polishing them with a 0.05 μm colloidal silica suspension. The specimen preparation for the microscopic analysis did not require etching. The structure of the layers was examined using a Nikon ECLIPSE MA 200 optical microscope and a JEOL JSM5400 scanning electron microscope. The chemical composition of the layers was studied by means of an X-ray energy dispersive spectrometer (EDS) attached to the SEM. The XRD analysis was conducted with a Seifert 3003T/T X-ray diffractometer using a Cu anode. The microhardness was measured with a MATSUZAWA MMT Vickers microhardness tester at a load of 100 g. The wear tests were carried out under dry sliding conditions using a block-on-ring configuration. Blocks with dimensions of 15.75 mm × 6.35 mm × 10 mm were cut from untreated and heattreated AZ31 specimens. The ring, i.e., the lower specimen, was made of 100Cr6 bearing steel (65HRC). The block side to be subjected to wear (15.75 mm × 6.35 mm) was ground with 800-grit SiC paper to obtain a flat surface with similar roughness. The wear tests were conducted for surfaces in linear contact at a load of 5 N, a sliding speed of 0.12 m/s and a sliding distance of 625 m. The specimens were weighed before and after each wear test. The weight loss was determined after removing the debris. A total of nine specimens were tested tribologically: three untreated and six treated with three representing each type of alloyed surface layer.
2. Materials and methods 2.1. Materials and the thermochemical treatment process An AZ31 magnesium alloy ingot composed by weight (wt%) of 3.07 Al, 0.31 Mn, 1.05 Zn, 0.02 Si, 0.001 Cu, 0.008 Fe, and Mg bal. was used in the experiments. The ingot was cut into 40 mm × 20 mm × 10 mm specimens, which were prepared by grinding with up to 800 grit SiC abrasive paper, cleaning with ethanol and drying in air. The fabrication of an Al/Si-enriched layer involved embedding an AZ31 specimen in an Al + Si powder mixture. Two powder mixtures were tested: 80 wt% Al + 20 wt% Si and 50 wt% Al + 50 wt% Si. The powders used were high purity Al powder (99.9%) with a particle size of about 20 μm and Si powder (98.5%) with a particle size of less than 63 μm, where the main impurity was Fe. After the powder mixture was compacted, the steel container was closed with a lid and placed in a vacuum furnace. The furnace was equipped with a pressure pad, which enabled the lid of
Fig. 1. Schematic diagram of the thermochemical treatment process: 1 – steel container filled with a powder mixture, 2 – container lid pressed down with a pressure pad, 3 – layers of Al foil to seal the container. 203
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(point 3) indicates the Mg2Si phase. The analysis of the Al-Mg-Si phase diagram [46] shows that in magnesium-rich alloys, an Mg2Si phase is formed. In this ternary system, there are no three-component phases; Mg2Si is the only two-component phase containing Si. Another element detected in this gray area was Al, which occurred in small amounts. Its presence was due to the fact that fine Mg2Si particles formed agglomerates and, during the EDS analysis, the electron beam may have interacted with Al-rich phases surrounding these particles. In this case, the heat treatment temperature (435 °C) was slightly lower than the eutectic temperature of the AleMg system (437 °C); as a result, the alloyed layer containing new phases formed via reactive diffusion at the substrate/powder interface. Since solid-state diffusion reactions are slow, the layer containing new phases was thin with nonuniform thickness. As can be seen from Fig. 2(b), there are small pores close to the Mg2Si phase agglomerates. The porosity may be associated with the Kirkendall effect as well as the volume reduction during the formation of the Mg2Si phase being a result of reactive diffusion. No pores are visible at the interface between Mg17Al12 and Al3Mg2. Fig. 3 shows optical micrographs of the AZ31 specimens after thermochemical treatment at 445 °C. As can be seen, the heating of the AZ31 substrate in contact with the Al + 20 wt% Si powder mixture led to the formation of a thick (400–450 μm) and continuous surface layer. Its structure differs from that fabricated at 435 °C. The microstructure is not uniform; three characteristic zones marked as A, B and C can be distinguished in the alloyed layer. Figs. 4–6 show details of the layer microstructure in higher-magnification OM and SEM images and Table 2 provides results of the quantitative EDS analysis for points 1–12 marked in these figures. From Fig. 4, it is clear that, in the uppermost region of the surface layer (zone A in Fig. 3), there are large light single-phase areas. Chemically, they are an Mg17Al12 intermetallic phase (points 1 and 2 in Fig. 4(b)). The Mg17Al12 areas are surrounded by a two-phase structure composed of light and dark phases. The former is an Mg17Al12 intermetallic phase, and the latter is a solid solution of Al in Mg (points 3 and 4, respectively). The AleMg phase diagram [45] indicates that the two-phase structure is a eutectic composed of an Mg17Al12 intermetallic phase and a solid solution of Al in Mg. The uppermost region of the layer also contains gray particles distributed irregularly over the single-phase Mg17Al12 area and the eutectic. The EDS analysis at point 5 revealed that the Mg-to-Si ratio for the particles was approximately 2:1 (atomic percentage), which suggested that it was an Mg2Si phase. Also, a small amount of Al was detected in the agglomerates of the Mg2Si phase particles, like in the case of the layers fabricated at the lowest temperature (435 °C). Fig. 5 shows the microstructure of the thickest, central area of the layer (zone B in Fig. 3). There are agglomerates of
Fig. 2. OM (a) and SEM (b) images of the alloyed layer formed on AZ31 thermochemically treated in an 80% Al + 20% Si powder mixture at 435 °C.
3. Results and discussion 3.1. Microstructure of the Al/Si-enriched surface layers The thermochemical treatment of AZ31 in the 80% Al + 20% Si powder mixture was carried out at three different temperatures: 435, 445 and 455 °C. When the lowest temperature (435 °C) was applied, a thin layer, not uniform in thickness (10–50 μm), was formed on the AZ31 substrate, as shown in Fig. 2. The OM image in Fig. 2(a) reveals that in the light matrix of the layer there are irregularly distributed gray areas. The SEM image in Fig. 2(b) shows the layer microstructure at higher magnification with marked points of the EDS analysis. In the light matrix, two main zones can be distinguished: the lower darker and the upper lighter. The results of the quantitative EDS analysis are given in Table 1. According to the AleMg phase diagram [45], the chemical composition of the darker zone (point 1) of the matrix corresponds to an Mg17Al12 intermetallic phase, whereas that of the lighter zone (point 2) is similar to an Al3Mg2 intermetallic phase. The EDS result for the gray area Table 1 Results of the EDS quantitative analysis at points 1–3 in Fig. 2(b). Point
Mg at.%
Al at.%
Si at.%
1 2 3
57.33 39.28 67.55
42.67 60.72 1.99
– – 30.45
Fig. 3. OM image of the alloyed layer formed on AZ31 thermochemically treated in an 80% Al + 20% Si powder mixture at 445 °C. 204
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Fig. 4. Higher magnification OM (a) and SEM (b) images showing the microstructure of the layer at the surface.
Fig. 5. Higher magnification OM (a) and SEM (b) images showing the microstructure of the central area of the alloyed layer.
gray particles visible over the eutectic matrix, which is composed of an Mg17Al12 intermetallic phase (point 6) and a solid solution Al in Mg (point 7). The line scan results in Fig. 7 indicate that these gray particles are rich in Si. In the EDS analysis, they were identified as an Mg2Si phase (point 8 in Fig. 5). Fig. 6 shows the microstructure of the area of the surface layer adjacent to the AZ31 substrate (zone C in Fig. 3). As can be seen, the main structural constituent of this area is a eutectic containing an Mg17Al12 intermetallic phase (point 9) and a solid solution Al in Mg (point 10). There are also light needle-like phases distributed irregularly over the eutectic. The EDS line scan results in Fig. 7 suggest that these phases are rich in Mn, Al, and Fe. Particles of the light phases are also observed in the AZ31 substrate. The occurrence of the multicomponent phases in the modified surface layer was due to the fact that the AZ31 alloy contained Mn and a small amount of Fe as an impurity; Fe as an impurity was also found in the Si powder used for the Al + Si mixture. Very fine particles of the light multicomponent phases rich in Fe, Al and Si were also visible in the other areas of the layer, i.e., at the very surface (Fig. 4(b)) and in the central area (Fig. 5(b)). Similar light multicomponent phase particles were observed in the layers fabricated at the lowest temperature, i.e., 435 °C (Fig. 2(b)). The layers obtained at 445 °C also featured a small number of tiny, evenly distributed pores. The porosity is clearly visible in high magnification SEM images (Figs. 4(b) and 6(b)). In small magnification OM images, it is difficult to differentiate between a small pore and a multicomponent phase because they are both dark in color. In SEM images, however, the difference is more noticeable because the pores are dark and the multicomponent phase is white. The high magnification SEM image in Fig. 6(b) shows that the microstructure of the AZ31 magnesium alloy is
locally modified in the area adjacent to the surface layer. There are fine, plate-like discontinuous Mg17Al12 precipitates, which indicates that the surface enrichment of AZ31 with Al occurred close to the layer fabricated through thermochemical treatment. The results of the EDS quantitative analysis confirm that the area with a lamellar structure (point 11) is richer in Al than the AZ31 alloy matrix (point 12). Fig. 8 shows the X-ray diffraction spectra of the surface layer of AZ31 enriched with Al and Si using thermochemical treatment. The results indicate the presence of an Mg17Al12 intermetallic phase, an Mg2Si phase, and a solid solution of Al in Mg, identified in the EDS analysis. The results of the microstructural investigations suggest that the mechanism of formation of an alloyed layer on AZ31 at 445 °C was as follows. The heating of the AZ31 alloy in contact with the Al + 20% Si powder mixture resulted in the diffusion of Al and Si atoms into the substrate material. The temperature of the heating process (445 °C) was slightly higher than the eutectic temperature of the AleMg system (437 °C). During heating, the concentration of Al and Si in the surface layer of the AZ31 substrate increased gradually and a transient liquid phase, being a result of the formation of a low melting eutectic, occurred at the interface. The liquid phase formation largely accelerated the interdiffusion of elements between the powder mixture and the substrate material and, consequently, the formation of new phases. After cooling, an Al/Si-enriched surface layer containing intermetallic phases formed on AZ31. The presence of the eutectic (Mg17Al12 and a solid solution of Al in Mg) in the surface layer microstructure confirmed that the reactions at the reactive interface proceeded to completion via the mechanism of liquid phase formation. During the thermochemical 205
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Fig. 6. Higher magnification OM (a) and SEM (b) images showing the microstructure of the interface between the surface layer and the substrate.
Fig. 7. SEM image of the microstructure of the area of the surface layer adjacent to the AZ31 with EDS line scan results showing the distribution of elements along the index line.
Table 2 Results of the EDS quantitative analysis at points 1-12 in Figs. 4-6. Point
Mg at%
Al at%
Si at%
Zn at%
1 2 3 4 5 6 7 8 9 10 11 12
60.03 61.35 65.71 90.38 65.37 61.88 89.04 66.47 64.00 89.77 83.23 91.75
39.97 38.65 34.29 9.62 4.64 38.12 10.96 35.68 10.23 16.52 8.25
29.99 33.53 -
0.32 0.25 -
treatment process, Al diffused deeply into AZ31 making the area adjacent the alloyed surface layer locally enriched with this element. The experiments also involved fabricating Al/Si-enriched layers from the 80% Al + 20% Si mixture at 455 °C. The layer microstructure is exemplified in Fig. 9. As can be seen, a layer produced under such conditions is very thick (about 1 mm). There is a large number of unevenly distributed pores. The highest level of porosity is observed close to the eutectic zone. This suggests that, at a temperature of 455 °C, which is much higher than the eutectic temperature of the AleMg system, the reactions at the substrate/powder interface proceeded rapidly. A high amount of liquid formed at the reactive interface and when the solidification shrinkage occurred, pores were generated. The
Fig. 8. X-ray diffraction spectra for the alloyed layer on AZ31.
high porosity indicates that the temperature of 455 °C was too high to fabricate suitable Al/Si-enriched layers, i.e., ones that are uniform, continuous and with low porosity. Thermochemical treatment of AZ31 was also carried out in the presence of the Al + 50% Si powder mixture at 445 °C. This temperature was selected on the basis of the experiments described above. As 206
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Fig. 9. OM image of the alloyed layer formed on AZ31 thermochemically treated in an 80% Al + 20% Si powder mixture at 455 °C.
shown in the OM image in Fig.10, the alloyed layer produced under such conditions was continuous but thinner (about 200 μm) than that obtained at the same temperature from the Al + 20% Si powder mixture. The layer is uniform in structure with some small, evenly distributed pores. Details of the layer microstructure are shown in the SEM image in Fig. 10 (b). The EDS results for the points marked in this figure are provided in Table 3. The chemical composition of the light area (point 1) indicates an Mg17Al12 intermetallic phase. The dark area (point 2) is a solid solution of Al in Mg. There are large amounts of evenly distributed gray phase particles (point 3) over the eutectic matrix (an Mg17Al12 intermetallic phase and a solid solution of Al in Mg). In the EDS analysis, they were identified as the Mg2Si phase. The above results show that the higher the content of Si in the Al + Si powder mixture acting as the source of diffusion elements, the higher the volume fraction of the Mg2Si phase in the layer microstructure.
Fig. 10. OM (a) and SEM (b) images of the alloyed layer formed on AZ31 thermochemically treated in an 50% Al + 50% Si powder mixture at 445 °C. Table 3 Results of the EDS quantitative analysis at points 1–3 in Fig. 10(b). Point
Mg at.%
Al at.%
Si at.%
1 2 3
63.24 89.17 67.60
36.76 10.83 1.36
– – 31.04
ranging between 290 and 367 HV0.1. The higher microhardness was a result of the high volume fraction of the hard Mg2Si phase particles, evenly distributed in the layer microstructure. The microhardness data indicate that the Al/Si-enriched layers containing Mg2Si and Mg17Al12 phases were much harder than the AZ31 substrate. The microhardness results obtained in this study for the AZ31 alloy surface-enriched with Al and Si (183–348 HV and 290–367 HV for the 80% Al + 20% Si and 50% Al + 50% Si powder mixtures, respectively) were compared with the data available in the literature. The microhardness of the material enriched with Al, Al/Zn, Zn/Al and Zn/Y is reported to be 181–203 HV [35], 183–223 HV [34], 156–180 HV [37], and 175–265 HV [39], respectively. It is evident that the highest microhardness was obtained for the Al/Si-enriched layer. This was due to the presence of fine particles of the Mg2Si phase, whose microhardness is much higher than that of the binary MgeAl and ternary Mg-Al-Zn intermetallic phases. As pointed out in the introduction to this article, Al/Si-enriched layers on an Mg-based substrate are mainly produced by laser alloying/cladding [7–13]. The thickness of an alloyed layer and the extent of alloying are mainly dependent on the process parameters
3.2. Hardness of the Al/Si-enriched surface layers Fig. 11 shows indentations left after a Vickers hardness test in the AZ31 substrate and the Al/Si-enriched layer fabricated through thermochemical treatment using the 80% Al + 20% Si powder mixture. The microhardness of the AZ31 varied from 44 to 48 HV0.1. The microhardness of the alloyed surface layer was in the range between 183 and 348 HV0.1. Lower values, i.e., 183–189 HV0.1, were obtained for the eutectic (Mg17Al12 and a solid solution of Al in Mg) in the area adjacent to the AZ31. Higher values, i.e., 205–310 HV0.1, were reported for areas where the eutectic co-occurred with the Mg2Si-phase particles. As indicated above, the Mg2Si phase present in the layer microstructure is mainly in the form of agglomerates. The material microhardness measured in the agglomerate areas was the highest, reaching 348 HV0.1. For the layer fabricated from the 50% Al + 50% Si powder mixture, the microhardness was found to be higher, with relatively stable values 207
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Fig. 12. Weight loss against sliding distance: (a) bare AZ31, (b) Al/Si-enriched layer fabricated on AZ31 from the 80% Al + 20% Si mixture, (c) Al/Si-enriched layer fabricated on AZ31 from the 50% Al + 50% Si mixture.
a higher volume fraction of hard, evenly distributed Mg2Si particles. The comparative studies concerning the wear behavior of thermochemically treated Mg-based substrates reveal that when they are enriched with Al and Si, their wear resistance improves considerably and it is higher than that obtained for Mg-based materials enriched with Al or Al/Zn [36]. A substantial increase in the wear resistance of Al/Sienriched Mg-based materials is due to the occurrence of very hard Mg2Si-phase particles in the layer microstructure. The results presented in this article are in agreement with the literature data concerning Al/ Si-enriched layers fabricated using laser-based methods [8–11,13]; in all the cases studied, the alloyed layer had good resistance to abrasive wear. To sum up, the thermochemical treatment method makes it possible to fabricate Al/Si enriched layers on Mg-based substrates that are characterized by very good wear resistance. The experiments described here were conducted for AZ31 as the substrate material but this technique can be successfully used for other Mg alloys exposed to severe abrasive wear conditions.
Fig. 11. Indentations left in the Al/Si-enriched surface layer and the AZ31 substrate after the Vickers hardness test.
such as laser power, scan speed, laser spot size and injection rate of alloying elements or thickness of preplaced layer [9]. The process parameters need to be carefully optimized to obtain a desired alloying effect. Porosity formation is also an important problem in laser alloying/cladding. Al/Si-enriched layers fabricated in this way generally have a non-uniform structure with the microstructure and properties likely to change along the length and depth. To summarize, the thermochemical treatment method used in this study is well-suited to fabricate Al/Si alloyed layer on Mg-based substrates with uniform structure along the depth and length, which is difficult when laser-based methods are applied. The composition of the powder mixture affected the layer microstructure. When the content of Si was higher (50%), the alloyed layer was uniform with a high volume fraction of the Mg2Siphase. The microhardness of the layer was high and relatively stable. The thermochemical treatment method is also much simpler and more cost effective than laser alloying/cladding.
4. Conclusions The alloyed layers were fabricated on AZ31 magnesium alloy through thermochemical treatment. The surface enrichment involved heating an AZ31 specimen in contact with an Al + Si powder mixture. Two powder mixtures were analyzed: 80% Al + 20% Si and 50% Al + 50% Si. For the former, three heat treatment temperatures were used (435, 445 and 455 °C). With 445 °C found to be optimal, the experiments for the other mixture (50% Si) were carried out only at that temperature. The main conclusions of the study are as follows:
3.3. Wear behavior of the Al/Si-enriched surface layers Fig. 12 illustrates the weight loss for bare AZ31 and AZ31 with alloyed layers tested under dry sliding conditions. It can be seen that the loss of weight reported for the unalloyed AZ31 (3.83 mg) is much higher than the average values obtained for the Al/Si-enriched specimens, and these are: 0.43 mg for the layer fabricated from the powder mixture containing 20% Si and 0.12 mg for the layer produced from the mixture where Si constituted 50%. The significant decrease in the weight loss was due to the presence of hard constituents in the surface layer microstructure, i.e., the Mg2Si phase and the Mg17Al12 intermetallic phase. The wear resistance of the layer fabricated from the mixture containing a higher amount of Si (50%) was better than that reported for the layer obtained from the mixture containing only 20% of this element. The microstructure of the more wear resistant layer had
1. The Al/Si-enriched layer fabricated from the 80% Al + 20% Si powder mixture at 445 °C is thick and continuous with low porosity and thickness ranging from 400 to 450 μm. 2. The heat treatment of AZ31 in contact with a 50% Al + 50% Si powder mixture at 445 °C results in the formation of a uniform layer 208
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with a thickness of about 200 μm. 3. The structural constituents of the Al/Si-enriched layers are a eutectic (an Mg17Al12 intermetallic phase and a solid solution of Al in Mg) and Mg2Si-phase particles. In the layers fabricated from the powder mixture containing 20% Si, the Mg2Si particles are unevenly distributed over eutectic. However, when the powder mixture contains 50% Si, there is a much higher volume fraction of Mg2Si phase particles and their distribution over the eutectic is even. The presence of the eutectic in the layer microstructure suggests that during the thermochemical treatment at 445 °C, the reactions at the powder/substrate interface proceed with partial melting. 4. The microhardness of the alloyed surface layers containing hard structural constituents, i.e., an Mg17Al12 intermetallic phase and an Mg2Si phase, is much higher than that of the AZ31 substrate. The layer microhardness depends on the content of Si in the powder mixture. The higher the percentage of Si in the Al + Si powder mixture, the higher volume fraction of the hard Mg2Si phase particles and the higher the microhardness of the layer produced. Thus, the microhardness of the layer fabricated from the powder mixture containing 50% Si (290–367 HV) is higher than that obtained from the Al + 20% Si mixture (183–348 HV). 5. The Al/Si-enriched surface layers significantly improve the wear resistance of AZ31. A greater amount of the Si powder in the mixture leads to the formation of a layer with a high volume fraction of the Mg2Si phase evenly distributed in the layer microstructure. Hence the excellent wear resistance of the alloyed surface layer produced from the Al + 50% Si mixture.
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Bucki, A. Dziadoń, Formation of Al-alloyed layer on magnesium with use of casting techniques, Arch. Foundry Eng. 16 (1) (2016) 112–116. [23] C. Zhong, M.F. He, L. Liu, Y.J. Chen, B. Shen, Y.T. Wu, Y.D. Deng, W.B. Hu, Formation of an aluminum-alloyed coating on AZ91D magnesium alloy in molten salts at lower temperature, Surf. Coat. Technol. 205 (2010) 2412–2418. [24] C. Zhong, M. He, L. Liu, Y. Wu, Y. Chen, Y. Deng, B. Shen, W. Hu, Lower temperature fabrication of continuous intermetallic coatings on AZ91D magnesium alloy in molten salts, J. Alloys Compd. 504 (2010) 377–381. [25] M. He, L. Liu, Y. Wu, C. Zhong, W. Hu, D. Pan, Kinetics and mechanism of multilayer Mg-Al intermetallic compound coating formation of magnesium alloy by AlCl3-NaCl molten salt bath treatment, J. Alloys Compd. 551 (2013) 389–398. [26] I. Shigematsu, M. Nakamura, N. Saitou, K. Shimojima, Surface treatment of AZ91D magnesium alloy by aluminum diffusion coating, J. Mater. Sci. Lett. 19 (2000) 473–475. [27] L. Zhu, G. Song, Improved corrosion resistance of AZ91D magnesium alloy by an aluminum-alloyed coating, Surf. Coat. Technol. 200 (2006) 2834–2840. [28] F. Liu, X. Li, W. Liang, X. Zhao, Y. Zhang, Effect of temperature on microstructures and properties of aluminized coating on pure magnesium, J. Alloys Compd. 478 (2009) 579–585. [29] F. Liu, W. Liang, X. Li, X. Zhao, Y. Zhang, H. Wang, Improvement of corrosion resistance of pure magnesium via vacuum pack treatment, J. Alloys Compd. 461 (2008) 399–403. [30] M.X. Zhang, P.M. Kelly, Surface alloying of AZ91D alloy by diffusion coating, J. Mater. Res. 17 (2002) 2477–2479. [31] Y. Ma, X.K. Xu, W. Wen, X. He, P. Liu, The effect of solid diffusion surface alloying on properties of ZM5 magnesium alloy, Surf. Coat. Technol. 190 (2005) 165–170. [32] J. Hirmke, M.X. Zhang, D.H. StJohn, Surface alloying of AZ91E alloy by Al-Zn packed powder diffusion coating, Surf. Coat. Technol. 206 (2011) 425–433. [33] J. Hirmke, M.X. Zhang, D.H. StJohn, Influence of chemical composition of Mg alloys on surface alloying by diffusion coating, Metall. Mater. Trans. A 43 (2012) 1621–1628. [34] R. Mola, Fabrication and microstructural characterization of Al/Zn-enriched layers on pure magnesium, Mater. Charact. 78 (2013) 121–128. [35] R. Mola, K. Jagielska-Wiaderek, Formation of Al-enriched surface layers through reaction at the Mg-substrate/Al-powder interface, Surf. Interface Anal. 46 (2014) 577–580. [36] R. Mola, The properties of Mg protected by Al- and Al/Zn-enriched layers containing intermetallic phases, J. Mater. Res. 30 (23) (2015) 3682–3691. [37] R. Mola, The microstructure of alloyed layers formed on Mg by the powder-pack method, Proceedings of 25th Anniversary International Conference on Metallurgy and Materials, METAL, 2016, pp. 1492–1497. [38] Y. Chen, T.M. Liu, L.W. Lu, Z.C. Wang, Thermally diffused antimony and zinc coatings on magnesium alloys AZ31, Surf. Eng. 28 (5) (2012) 382–386. [39] H. Wang, B. Yu, W. Wang, G. Ren, W. Liang, J. Zhang, Improved corrosion resistance of AZ91D magnesium alloy by zinc-yttrium coating, J. Alloys Compd. 582 (2014) 457–460. [40] R. Mola, E. Stępień, M. Cieślik, Characterization of the surface layer of Mg enriched with Al and Si by thermochemical treatment, Arch. Foundry Eng. 17 (4) (2017) 195–199. [41] G. Buza, V. Jano, M. Sveda, O. Verezub, Z. Kalazi, G. Kaptay, A. Roosz, On the possible mechanisms of the porosity formation during laser melt injection technology, Mater. Sci. Forum 589 (2008) 79–84. [42] F. Czerwiński, Thermochemical treatment of metals, Heat Treatment - Conventional and Novel Applications. InTech Chapter 5, Open Access, 2012, pp. 73–112 (www.intechopen.com). [43] M.A. Malik, K. Majchrzak, K.N. Braszczyńska-Malik, Microstructural analysis of AM50/Mg2Si cast magnesium composite, Arch. Foundry Eng. 12 (4) (2012) 109–112. [44] T. Yamaguchi, T. Serikawa, M. Henmi, H. Oginuma, K. 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Acknowledgments The research reported herein was supported by Poland's National Science Centre (Grant No. DEC-2017/01/X/ST08/00791). References [1] J.E. Gray, B. Luan, Protective coatings on magnesium and its alloys – a critical review, J. Alloys Compd. 336 (2002) 88–113. [2] R. Galun, A. Weisheit, B.L. Mordike, Improving the surface properties of magnesium by laser alloying, Corros. Rev. 16 (1998) 53–74. [3] S. Ignat, P. Sallamand, D. Grevey, M. Lambertin, Magnesium alloys laser (Nd:YAG) cladding and alloying with side injection of aluminium powder, Appl. Surf. Sci. 225 (2004) 124–134. [4] S.R. Paital, A. Bhattacharya, M. Moncayo, Y.H. Ho, K. Mahdak, S. Nag, R. Banerjee, N.B. Dahotre, Improved corrosion and wear resistance of Mg alloys via laser surface modification of Al on AZ31B, Surf. Coat. Technol. 206 (2012) 2308–2315. [5] J.D. Majumdar, T. Maiwald, R. Galun, B.L. Mordike, I. Manna, Laser surface alloying on Mg alloy with Al + Mn to improve corrosion resistance, Lasers in Engineering 12 (3) (2002) 147–169. [6] Y. Gao, C. Wang, H. Pang, H. Liu, M. Yao, Broad-beam laser cladding of Al-Cu alloy coating on AZ91HP magnesium alloy, Appl. Surf. Sci. 253 (2007) 4917–4922. [7] M. Qian, D. Li, C. Jin, Microstructure and corrosion characteristics of laser-alloyed magnesium alloy AZ91D with Al-Si powder, Sci. Technol. Adv. Mater. 9 (2008) 1–7. [8] B. Carcel, J. Sampedro, A. Ruescas, X. Toneu, Corrosion and wear resistance improvement of magnesium alloys by laser cladding with Al-Si, Phys. Procedia 12 (2011) 353–363. [9] A. Singh, S.P. Harimkar, Laser surface engineering of magnesium alloys: a review, JOM 64 (6) (2012) 716–733. [10] K. Bobzin, N. Kopp, T. Warda, C. Schulz, G. Rolink, A. Weisheit, Investigation of wear and corrosion protection of AlSi20 coatings produced by plasma spraying and laser cladding on AZ31, J. Therm. Spray Technol. 22 (2–3) (2013) 207–2012. [11] Y. Yang, H. Wu, Improving the wear resistance of AZ91D magnesium alloys by laser cladding with Al-Si powders, Mater. Lett. 63 (2009) 19–21. [12] Y. Lei, R. Sun, Y. Tang, W. Niu, Experimental and thermodynamic investigations into the microstructure of laser clad Al-Si coatings on AZ91D alloys, Surf. Coat. Technol. 207 (2012) 400–405. [13] A. Dziadoń, R. Mola, L. Błaż, The microstructure of the surface layer of magnesium laser alloyed with aluminium and silicon, Mater. Charact. 118 (2016) 505–513. [14] M.R. Elahi, M.H. Sohi, A. Safaei, Liquid phase surface alloying of AZ91D magnesium alloy with Al and Ni powders, Appl. Surf. Sci. 258 (2012) 5876–5880. [15] C. Liu, Z. Li, X. Pei, L. Yang, Y. Zhang, S. Wei, Microstructure and properties of Al-Si alloy cladding on AZ91D magnesium alloy by low heat input DC-PMIG welding, Surf. Coat. Technol. 329 (2017) 42–48.
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Microstructure and properties of AZ31 with an Al/Si-enriched surface layer fabricated through thermochemical treatment
T
Renata Mola , Michał Cieślik ⁎
Department of Metal Science and Manufacturing Processes, Faculty of Mechatronics and Mechanical Engineering, Kielce University of Technology, Al. Tysiąclecia Państwa Polskiego 7, 25-314 Kielce, Poland
ARTICLE INFO
ABSTRACT
Keywords: Magnesium alloy Thermochemical treatment Alloyed surface layer Microstructure Microhardness Wear resistance
The surface enrichment of AZ31 magnesium alloy with Al and Si was achieved using the thermochemical treatment method. The experiments involved heating AZ31 in contact with an Al + Si powder mixture. Two mixtures acting as the source of diffusion elements were tested: 80% Al + 20% Si and 50% Al + 50% Si. The heat treatment of the AZ31 in the Al + 20% Si mixture was performed at three different temperatures: 435, 445 and 455 °C. As the best results were obtained at 445 °C, this temperature was used to heat AZ31 in contact with the Al + 50% Si mixture. The surface modification of AZ31 was attributable to the diffusion processes taking place at the substrate/powder reactive interface. The microstructure of the alloyed layers was characterized using an optical microscope (OM) and a scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectrometer (EDS). An X-ray diffractometer (XRD) was employed to determine the phase composition. The examinations showed that the modified surface layer produced from the Al + 20% Si powder mixture was continuous and had uniform thickness (400–450 μm). The Al/Si-enriched layer was mainly composed of a eutectic, i.e., an Mg17Al12 intermetallic phase and a solid solution of Al in Mg, and unevenly distributed Mg2Si phase particles. The use of the Al + 50% Si powder mixture resulted in the formation of a thinner, continuous layer (about 200 μm) with a much higher volume fraction of the Mg2Si phase. In this case, the layer had a more uniform structure; the particles of the Mg2Si phase were evenly distributed over the eutectic matrix. Due to the presence of hard phases, i.e., Mg2Si and Mg17Al12, both types of Al/Si-enriched surface layers had much higher hardness than the AZ31 substrate. The tribological tests revealed that the alloyed layers had very good wear resistance under dry sliding conditions. Because of the higher volume fraction of the Mg2Si phase particles, the layer fabricated from the Al + 50% Si powder mixture had higher hardness and wear resistance than the layer fabricated from the mixture containing 20% Si.
1. Introduction Magnesium alloys are the lightest metallic structural materials. They are characterized by high specific strength, good electrical and thermal conductivity, good machinability, and high recycling potential. These properties make the materials useful for different automotive and aerospace applications. However, due to low hardness and poor resistance to wear, their use is limited to applications where components are not exposed to severe abrasive wear conditions. A wide range of surface treatment methods have been developed to overcome this problem [1]. Research in this area has involved the formation of surface layers containing hard particles and/or hard intermetallic phases. Yet, articles dealing with the fabrication of alloyed surface layers containing intermetallic phases are less numerous. The literature data show that
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such surface layers can be created by introducing an appropriate element or elements that form intermetallic phases with magnesium. Surface modification techniques used for this purpose include laser surface alloying/cladding [2–13], surface alloying by welding [14,15], thermal spraying [16], cold spraying [17,18], PVD [19,20], casting [21,22], thermochemical treatment in molten salts [23–25], and thermochemical treatment in metal powders [26–40]. The above studies involved using pure Al or Al combined with other elements, i.e., Al + Si, Al + Cu, Al + Ni, Al + Zn, Zn + Sb, Zn + Y, and Al + Mn as alloying elements. This article deals with the fabrication of Al/Si-enriched layers through thermochemical treatment; it is important, however, to mention results concerning such layers on Mg-based materials produced using other methods. The literature data show that a surface layer enriched with Al and Si containing MgeAl intermetallic
Corresponding author. E-mail address: [email protected] (R. Mola).
https://doi.org/10.1016/j.surfcoat.2019.06.003 Received 11 February 2019; Received in revised form 30 April 2019; Accepted 1 June 2019 Available online 03 June 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
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phases and an Mg2Si phase can be formed on pure Mg or Mg alloy by laser alloying/cladding [7–13] or welding [15]. The properties of alloyed layers depend on the microstructure. The main factor affecting the microhardness of the layer is the content of hard Mg2Si phase particles and their distribution in the layer. Studies concerned with the laser alloying/cladding of Mg-based substrates by direct injection of Al12Si [7,8] or Al20Si [10] powders reveal that the microhardness of surface layers may vary, reaching, for example, 220–340 HV [7], 140–285 HV [8] or 135 HV [10]. When surface layers are formed through laser alloying/cladding by direct injection of AlSi powders, a uniform microstructure with evenly distributed Mg2Si particles may be difficult to obtain. In Ref. [7], Qian et al. describe laser alloying of AZ91D. They obtained layers 1.5 mm in thickness. Their study shows that an increase in the microhardness of the surface layer is mainly due to the formation of Mg2Si, Mg17Al12 and Al3Mg2 phases. The results also indicate that the layer microhardness decreases gradually from the uppermost region to the fusion line, which is a result of a non-uniform microstructure. As shown in Ref. [8], Al/Si-enriched layers, 500–650 μm in thickness, produced on various Mg alloys also have a non-uniform microstructure; the region adjacent to the substrate is characterized by higher microhardness (230–285 HV) because this is where the microstructure is mainly composed of MgeAl intermetallic phases and an Mg2Si phase. Lower values (140–170 HV) are reported for the uppermost zone, where the Mg2Si particles are unevenly distributed in the Al-rich matrix. Bobzin et al. [10] investigated the laser cladding of an AZ31B substrate with Al20Si powder. The resulting layer was about 1 mm thick. Although they used powder with a higher content of Si (20%), the alloyed layer had lower microhardness (135 ± 11 HV) than the layer fabricated from Al12Si powder [7,8]. Results presented in Refs. [7 and 9] indicate that unalloyed Al and Si particles can occur in the layer because of insufficient laser power or scan time, with this likely to have some negative effect on the layer properties. Al/Si-enriched layers on Mg-based substrates can also be fabricated using two-stage laser alloying/cladding. First, the alloying elements are preplaced in the form of paste or slurry [11,12], or foil [13] or they are deposited by plasma spraying [10]. The second stage consists in laser melting. When paste or slurry is used and laser melting is involved, high porosity may be observed in the layer. The occurrence of pores is associated with the burning of the organic material acting as the binder [41]. In Ref. [11], Yang et al. analyze the properties of layers fabricated on AZ91D from paste prepared by mixing Al and 12.5% Si powders with resin. The layers are about 1 mm in thickness and contain hard Mg2Si and Mg17Al12 phases. As a result, they are characterized by high microhardness. In this case, the microhardness of the layer is not uniform. The highest values (170 HV) were reported in the uppermost region. The method of surface modification discussed in Ref. [13] involved using a thin AlSi20 plate, which was first diffusion bonded to an Mg substrate and then laser melted. Depending on the process parameters, the laser beam melted the AlSi20 plate only or the AlSi20 plate and a layer of the Mg surface adjacent to it. Two types of microstructure of the remelted layer were fabricated. If the melting zone was limited to the AlSi20 plate, the microstructure of the surface layer was typical of a rapidly solidified hypereutectic AleSi alloy with microhardness in the range 96–120 HV. Since, however, the liquid AlSi20 reacted with the Mg substrate, the following phases were detected in the surface layer: Al3Mg2, Mg17Al12 and Mg2Si. The 400–500 μm thick layer had a microhardness of 215–225 HV. Reference [10] deals with the laser remelting of an AlSi20 plasma sprayed coating deposited on AZ31B. The process did not lead to the interaction of the coating with the substrate or the formation of new phases through reactive diffusion. As a result, the microhardness of the surface layer was low (97 HV). In laser alloying/cladding, the layer microstructure, and, consequently, its microhardness, may change along the length of the alloyed specimens. The reason for that is an increase in the specimen temperature during the alloying process. This leads to higher absorption of the laser-emitted
radiation, more effective use of the laser power, and, consequently, a deeper and wider melt zone increasing along the specimen length [3]. Wear test data [8–11,13] showed that laser alloying/cladding of Mgbased substrate with AlSi resulted in significant reduction in weight loss. Reference [15] analyzes the AleSi cladding of an Mg alloy by direct current-pulse metal insert gas (DC-PMIG) welding with AlSi5 welding wire used as the alloying material. Layers produced in this way are thick (up to 1.8 mm) but non-uniform. MgeAl intermetallic phases and an Mg2Si phase were detected in the thin transition zone close to the substrate. The thick upper zone was composed of α-Al dendrites, an AleSi eutectic and Mg2Si particles. The microhardness of the transition zone was 230–309 HV while that of the upper zone was much lower (115–129 HV) due to a low content of the Mg2Si phase and lack of MgeAl intermetallic phases in the layer microstructure. Thermochemical treatment is one of the most common surface engineering methods. This process employs thermal diffusion to incorporate metal atoms into a material surface to modify its chemical composition and microstructure [42]. A review of the literature [26–40] shows that the thermochemical treatment of an Mg-based substrate in a solid medium, i.e., metal powder or a metal powder mixture, is a simple and cost effective method to fabricate an alloyed layer containing intermetallic phases. In this process, an Mg or Mgbased component surrounded by compacted powder material is heated under inert atmosphere. The heating causes atoms in the outside source to move by diffusion to the surface layer of the specimen. The reactions taking place at the interface result in the formation of new phases. Researchers dealing with surface alloying of Mg experimented with the following powders or powder mixtures used as a solid medium: Al [26–29,35,36], Al + Zn [30–34,36], Zn + Al [37], Sb + Zn [38], Zn + Y [39], and Al + Si [40]. Layers fabricated on pure Mg or Mg alloys using pure Al powder contained MgeAl intermetallic phases. In most cases [26,27,35,36], the layers had a eutectic structure (an Mg17Al12 intermetallic phase and a solid solution of Al in Mg). Liu et al. [28,29] indicate, however, that the layer had a slightly different microstructure with the following structural constituents: a eutectic close to the substrate, an Al3Mg2 intermetallic phase in the center, and a solid solution of Mg in Al at the surface. The literature data show that the heat treatment of an Mg-based specimen in contact with Al + Zn led to the enrichment of the surface layer with both elements; in such a case, Mg-Al-Zn intermetallic phases were detected in the modified layer. Zhang and Kelly [30] produced a modified layer with a thickness of 1–2 mm on AZ91D alloy by heating the material at 430 °C for 12 h in contact with an Al + 30 wt% Zn powder mixture. The surface layer was composed of Mg17Al12 and Mg11Al5Zn4 intermetallic phases and a solid solution of Al and Zn in Mg. Ma et al. [31] used an Al + 50 wt% Zn powder mixture to produce a modified layer at the surface of ZM5, which contained Mg17Al12, Mg11Al5Zn4 and a solid solution of Al and Zn in Mg. Heat treatment was carried out at 470 °C for 12 h. Although the temperature was higher than that applied in the study described in [30], the fabricated layer was very thin (35–75 μm). Hirmke et al. [32,33] also used an Al + Zn powder mixture but with a different composition; they used different Mg alloys as the substrate materials and different process temperatures. They found that the addition of Zn into the powder mixture significantly promoted the formation of an alloyed layer at temperatures below 420 °C. The microstructure and thickness of the layer were dependent on the process parameters. When the content of Zn was over 10 wt%, the alloyed layers contained Mg32(Al,Zn)49, Mg11Al5Zn4 and Mg17Al12 intermetallic phases and a solid solution of Al and Zn in Mg. At low zinc contents (2–5 wt%), no ternary phases were identified in the layer microstructure. The experimental results presented in [34–37] show that an important factor affecting the formation of an Al, Al/Zn or Zn/Al-enriched layer on Mg through thermochemical treatment is good contact between the source of diffusion elements and the substrate material. During the heat treatment process, the powder or powder mixture filling the container 202
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was held under pressure. Adequate contact facilitated the diffusion of alloying elements from the outside source (powder or powder mixture) to the Mg-based substrate and formation of a thick alloyed layer in short heating time. The experimental results provided in [37] refer to a Zn/ Al-enriched layer fabricated on Mg from a Zn + 20% Al powder mixture. The layer was characterized by a microstructure consisting of Mg5Al2Zn2, MgZn and a solid solution of Al and Zn in Mg. Chen et al. [38] used an Sb + Zn powder mixture in the thermochemical treatment of AZ31 alloy. They detected several new compounds in the modified surface layer. The heat treatment of AZ91D in contact with a Zn + Y powder mixture resulted in the formation of a surface layer containing a large amount of an Mg5Al2Zn2 intermetallic phase [39]. The experimental results presented in the above works concerning thermochemical treatment show that the alloyed layer containing intermetallic phases improved the surface properties of Mg-based materials. Since the layers contained intermetallic phases, their hardness was several times higher than that of the substrate; they were also characterized by good wear resistance. An alloyed layer fabricated through thermochemical treatment may also provide effective protection of an Mgbased substrate from corrosion. This paper discusses further investigations on the fabrication of surface layers on an Mg-based substrate through thermochemical treatment. Al + Si powder mixtures were used as the source of diffusion elements. From the literature, it is apparent that the Mg2Si phase can be easily formed in situ by the reaction of Mg with Si; hence the selection of Si powder as a component of the powder mixture [13,43]. As the Mg2Si phase has high hardness, ranging from 350 to 450 HV0.01 [44], the fabrication of a surface layer containing it may significantly improve the hardness and wear resistance of the Mg-based material. This phase is also characterized by low density (1.99 Mg/m3), which implies that the formation of a layer containing a high volume fraction of this phase on a light Mg-based substrate will not affect the total weight of an element. The first results of the research in this area published in [40] suggest that the heat treatment of Mg in contact with an Al + 20% Si powder mixture results in the formation of a surface layer containing an Mg17Al12 intermetallic phase, an Mg2Si phase and a solid solution of Al in Mg. Since pure Mg is rarely applied as a structural material, this study involved using AZ31 magnesium alloy as the substrate. This article considers the influence of the process parameters (the heat treatment temperature and the powder mixture composition) on the microstructure and properties of Al/Si-enriched surface layers fabricated on AZ31 through thermochemical treatment. A detailed microstructural analysis was performed using optical microscopy and scanning electron microscopy. The chemical composition of the layers was identified on the basis of the X-ray EDS analysis data. The XRD method was employed to determine the phase composition. The microhardness and wear resistance tests were used to determine the properties of the fabricated layers.
the container to keep the powder under pressure during heat treatment. The specimens were heated from room temperature to the desired temperature for 30 min, kept at that temperature for another 30 min and cooled back to room temperature. The heat treatment of the AZ31 magnesium alloy in the 80 wt% Al + 20 wt% Si powder mixture was carried out at 435, 445 and 455 °C. When the powder mixture containing 50% Si was used, the heating was performed only at 445 °C. During the entire heat treatment process, a pressure of 1 MPa was applied to ensure good contact between the AZ31 specimen and the powder mixture, acting as the source of diffusion elements. A schematic diagram of the thermochemical treatment process is shown in Fig. 1. 2.2. Layer characterization After the thermochemical treatment process, the specimens were cut into rectangular pieces and prepared for microscopic observations, which involved mechanically polishing them with a 0.05 μm colloidal silica suspension. The specimen preparation for the microscopic analysis did not require etching. The structure of the layers was examined using a Nikon ECLIPSE MA 200 optical microscope and a JEOL JSM5400 scanning electron microscope. The chemical composition of the layers was studied by means of an X-ray energy dispersive spectrometer (EDS) attached to the SEM. The XRD analysis was conducted with a Seifert 3003T/T X-ray diffractometer using a Cu anode. The microhardness was measured with a MATSUZAWA MMT Vickers microhardness tester at a load of 100 g. The wear tests were carried out under dry sliding conditions using a block-on-ring configuration. Blocks with dimensions of 15.75 mm × 6.35 mm × 10 mm were cut from untreated and heattreated AZ31 specimens. The ring, i.e., the lower specimen, was made of 100Cr6 bearing steel (65HRC). The block side to be subjected to wear (15.75 mm × 6.35 mm) was ground with 800-grit SiC paper to obtain a flat surface with similar roughness. The wear tests were conducted for surfaces in linear contact at a load of 5 N, a sliding speed of 0.12 m/s and a sliding distance of 625 m. The specimens were weighed before and after each wear test. The weight loss was determined after removing the debris. A total of nine specimens were tested tribologically: three untreated and six treated with three representing each type of alloyed surface layer.
2. Materials and methods 2.1. Materials and the thermochemical treatment process An AZ31 magnesium alloy ingot composed by weight (wt%) of 3.07 Al, 0.31 Mn, 1.05 Zn, 0.02 Si, 0.001 Cu, 0.008 Fe, and Mg bal. was used in the experiments. The ingot was cut into 40 mm × 20 mm × 10 mm specimens, which were prepared by grinding with up to 800 grit SiC abrasive paper, cleaning with ethanol and drying in air. The fabrication of an Al/Si-enriched layer involved embedding an AZ31 specimen in an Al + Si powder mixture. Two powder mixtures were tested: 80 wt% Al + 20 wt% Si and 50 wt% Al + 50 wt% Si. The powders used were high purity Al powder (99.9%) with a particle size of about 20 μm and Si powder (98.5%) with a particle size of less than 63 μm, where the main impurity was Fe. After the powder mixture was compacted, the steel container was closed with a lid and placed in a vacuum furnace. The furnace was equipped with a pressure pad, which enabled the lid of
Fig. 1. Schematic diagram of the thermochemical treatment process: 1 – steel container filled with a powder mixture, 2 – container lid pressed down with a pressure pad, 3 – layers of Al foil to seal the container. 203
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(point 3) indicates the Mg2Si phase. The analysis of the Al-Mg-Si phase diagram [46] shows that in magnesium-rich alloys, an Mg2Si phase is formed. In this ternary system, there are no three-component phases; Mg2Si is the only two-component phase containing Si. Another element detected in this gray area was Al, which occurred in small amounts. Its presence was due to the fact that fine Mg2Si particles formed agglomerates and, during the EDS analysis, the electron beam may have interacted with Al-rich phases surrounding these particles. In this case, the heat treatment temperature (435 °C) was slightly lower than the eutectic temperature of the AleMg system (437 °C); as a result, the alloyed layer containing new phases formed via reactive diffusion at the substrate/powder interface. Since solid-state diffusion reactions are slow, the layer containing new phases was thin with nonuniform thickness. As can be seen from Fig. 2(b), there are small pores close to the Mg2Si phase agglomerates. The porosity may be associated with the Kirkendall effect as well as the volume reduction during the formation of the Mg2Si phase being a result of reactive diffusion. No pores are visible at the interface between Mg17Al12 and Al3Mg2. Fig. 3 shows optical micrographs of the AZ31 specimens after thermochemical treatment at 445 °C. As can be seen, the heating of the AZ31 substrate in contact with the Al + 20 wt% Si powder mixture led to the formation of a thick (400–450 μm) and continuous surface layer. Its structure differs from that fabricated at 435 °C. The microstructure is not uniform; three characteristic zones marked as A, B and C can be distinguished in the alloyed layer. Figs. 4–6 show details of the layer microstructure in higher-magnification OM and SEM images and Table 2 provides results of the quantitative EDS analysis for points 1–12 marked in these figures. From Fig. 4, it is clear that, in the uppermost region of the surface layer (zone A in Fig. 3), there are large light single-phase areas. Chemically, they are an Mg17Al12 intermetallic phase (points 1 and 2 in Fig. 4(b)). The Mg17Al12 areas are surrounded by a two-phase structure composed of light and dark phases. The former is an Mg17Al12 intermetallic phase, and the latter is a solid solution of Al in Mg (points 3 and 4, respectively). The AleMg phase diagram [45] indicates that the two-phase structure is a eutectic composed of an Mg17Al12 intermetallic phase and a solid solution of Al in Mg. The uppermost region of the layer also contains gray particles distributed irregularly over the single-phase Mg17Al12 area and the eutectic. The EDS analysis at point 5 revealed that the Mg-to-Si ratio for the particles was approximately 2:1 (atomic percentage), which suggested that it was an Mg2Si phase. Also, a small amount of Al was detected in the agglomerates of the Mg2Si phase particles, like in the case of the layers fabricated at the lowest temperature (435 °C). Fig. 5 shows the microstructure of the thickest, central area of the layer (zone B in Fig. 3). There are agglomerates of
Fig. 2. OM (a) and SEM (b) images of the alloyed layer formed on AZ31 thermochemically treated in an 80% Al + 20% Si powder mixture at 435 °C.
3. Results and discussion 3.1. Microstructure of the Al/Si-enriched surface layers The thermochemical treatment of AZ31 in the 80% Al + 20% Si powder mixture was carried out at three different temperatures: 435, 445 and 455 °C. When the lowest temperature (435 °C) was applied, a thin layer, not uniform in thickness (10–50 μm), was formed on the AZ31 substrate, as shown in Fig. 2. The OM image in Fig. 2(a) reveals that in the light matrix of the layer there are irregularly distributed gray areas. The SEM image in Fig. 2(b) shows the layer microstructure at higher magnification with marked points of the EDS analysis. In the light matrix, two main zones can be distinguished: the lower darker and the upper lighter. The results of the quantitative EDS analysis are given in Table 1. According to the AleMg phase diagram [45], the chemical composition of the darker zone (point 1) of the matrix corresponds to an Mg17Al12 intermetallic phase, whereas that of the lighter zone (point 2) is similar to an Al3Mg2 intermetallic phase. The EDS result for the gray area Table 1 Results of the EDS quantitative analysis at points 1–3 in Fig. 2(b). Point
Mg at.%
Al at.%
Si at.%
1 2 3
57.33 39.28 67.55
42.67 60.72 1.99
– – 30.45
Fig. 3. OM image of the alloyed layer formed on AZ31 thermochemically treated in an 80% Al + 20% Si powder mixture at 445 °C. 204
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Fig. 4. Higher magnification OM (a) and SEM (b) images showing the microstructure of the layer at the surface.
Fig. 5. Higher magnification OM (a) and SEM (b) images showing the microstructure of the central area of the alloyed layer.
gray particles visible over the eutectic matrix, which is composed of an Mg17Al12 intermetallic phase (point 6) and a solid solution Al in Mg (point 7). The line scan results in Fig. 7 indicate that these gray particles are rich in Si. In the EDS analysis, they were identified as an Mg2Si phase (point 8 in Fig. 5). Fig. 6 shows the microstructure of the area of the surface layer adjacent to the AZ31 substrate (zone C in Fig. 3). As can be seen, the main structural constituent of this area is a eutectic containing an Mg17Al12 intermetallic phase (point 9) and a solid solution Al in Mg (point 10). There are also light needle-like phases distributed irregularly over the eutectic. The EDS line scan results in Fig. 7 suggest that these phases are rich in Mn, Al, and Fe. Particles of the light phases are also observed in the AZ31 substrate. The occurrence of the multicomponent phases in the modified surface layer was due to the fact that the AZ31 alloy contained Mn and a small amount of Fe as an impurity; Fe as an impurity was also found in the Si powder used for the Al + Si mixture. Very fine particles of the light multicomponent phases rich in Fe, Al and Si were also visible in the other areas of the layer, i.e., at the very surface (Fig. 4(b)) and in the central area (Fig. 5(b)). Similar light multicomponent phase particles were observed in the layers fabricated at the lowest temperature, i.e., 435 °C (Fig. 2(b)). The layers obtained at 445 °C also featured a small number of tiny, evenly distributed pores. The porosity is clearly visible in high magnification SEM images (Figs. 4(b) and 6(b)). In small magnification OM images, it is difficult to differentiate between a small pore and a multicomponent phase because they are both dark in color. In SEM images, however, the difference is more noticeable because the pores are dark and the multicomponent phase is white. The high magnification SEM image in Fig. 6(b) shows that the microstructure of the AZ31 magnesium alloy is
locally modified in the area adjacent to the surface layer. There are fine, plate-like discontinuous Mg17Al12 precipitates, which indicates that the surface enrichment of AZ31 with Al occurred close to the layer fabricated through thermochemical treatment. The results of the EDS quantitative analysis confirm that the area with a lamellar structure (point 11) is richer in Al than the AZ31 alloy matrix (point 12). Fig. 8 shows the X-ray diffraction spectra of the surface layer of AZ31 enriched with Al and Si using thermochemical treatment. The results indicate the presence of an Mg17Al12 intermetallic phase, an Mg2Si phase, and a solid solution of Al in Mg, identified in the EDS analysis. The results of the microstructural investigations suggest that the mechanism of formation of an alloyed layer on AZ31 at 445 °C was as follows. The heating of the AZ31 alloy in contact with the Al + 20% Si powder mixture resulted in the diffusion of Al and Si atoms into the substrate material. The temperature of the heating process (445 °C) was slightly higher than the eutectic temperature of the AleMg system (437 °C). During heating, the concentration of Al and Si in the surface layer of the AZ31 substrate increased gradually and a transient liquid phase, being a result of the formation of a low melting eutectic, occurred at the interface. The liquid phase formation largely accelerated the interdiffusion of elements between the powder mixture and the substrate material and, consequently, the formation of new phases. After cooling, an Al/Si-enriched surface layer containing intermetallic phases formed on AZ31. The presence of the eutectic (Mg17Al12 and a solid solution of Al in Mg) in the surface layer microstructure confirmed that the reactions at the reactive interface proceeded to completion via the mechanism of liquid phase formation. During the thermochemical 205
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Fig. 6. Higher magnification OM (a) and SEM (b) images showing the microstructure of the interface between the surface layer and the substrate.
Fig. 7. SEM image of the microstructure of the area of the surface layer adjacent to the AZ31 with EDS line scan results showing the distribution of elements along the index line.
Table 2 Results of the EDS quantitative analysis at points 1-12 in Figs. 4-6. Point
Mg at%
Al at%
Si at%
Zn at%
1 2 3 4 5 6 7 8 9 10 11 12
60.03 61.35 65.71 90.38 65.37 61.88 89.04 66.47 64.00 89.77 83.23 91.75
39.97 38.65 34.29 9.62 4.64 38.12 10.96 35.68 10.23 16.52 8.25
29.99 33.53 -
0.32 0.25 -
treatment process, Al diffused deeply into AZ31 making the area adjacent the alloyed surface layer locally enriched with this element. The experiments also involved fabricating Al/Si-enriched layers from the 80% Al + 20% Si mixture at 455 °C. The layer microstructure is exemplified in Fig. 9. As can be seen, a layer produced under such conditions is very thick (about 1 mm). There is a large number of unevenly distributed pores. The highest level of porosity is observed close to the eutectic zone. This suggests that, at a temperature of 455 °C, which is much higher than the eutectic temperature of the AleMg system, the reactions at the substrate/powder interface proceeded rapidly. A high amount of liquid formed at the reactive interface and when the solidification shrinkage occurred, pores were generated. The
Fig. 8. X-ray diffraction spectra for the alloyed layer on AZ31.
high porosity indicates that the temperature of 455 °C was too high to fabricate suitable Al/Si-enriched layers, i.e., ones that are uniform, continuous and with low porosity. Thermochemical treatment of AZ31 was also carried out in the presence of the Al + 50% Si powder mixture at 445 °C. This temperature was selected on the basis of the experiments described above. As 206
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Fig. 9. OM image of the alloyed layer formed on AZ31 thermochemically treated in an 80% Al + 20% Si powder mixture at 455 °C.
shown in the OM image in Fig.10, the alloyed layer produced under such conditions was continuous but thinner (about 200 μm) than that obtained at the same temperature from the Al + 20% Si powder mixture. The layer is uniform in structure with some small, evenly distributed pores. Details of the layer microstructure are shown in the SEM image in Fig. 10 (b). The EDS results for the points marked in this figure are provided in Table 3. The chemical composition of the light area (point 1) indicates an Mg17Al12 intermetallic phase. The dark area (point 2) is a solid solution of Al in Mg. There are large amounts of evenly distributed gray phase particles (point 3) over the eutectic matrix (an Mg17Al12 intermetallic phase and a solid solution of Al in Mg). In the EDS analysis, they were identified as the Mg2Si phase. The above results show that the higher the content of Si in the Al + Si powder mixture acting as the source of diffusion elements, the higher the volume fraction of the Mg2Si phase in the layer microstructure.
Fig. 10. OM (a) and SEM (b) images of the alloyed layer formed on AZ31 thermochemically treated in an 50% Al + 50% Si powder mixture at 445 °C. Table 3 Results of the EDS quantitative analysis at points 1–3 in Fig. 10(b). Point
Mg at.%
Al at.%
Si at.%
1 2 3
63.24 89.17 67.60
36.76 10.83 1.36
– – 31.04
ranging between 290 and 367 HV0.1. The higher microhardness was a result of the high volume fraction of the hard Mg2Si phase particles, evenly distributed in the layer microstructure. The microhardness data indicate that the Al/Si-enriched layers containing Mg2Si and Mg17Al12 phases were much harder than the AZ31 substrate. The microhardness results obtained in this study for the AZ31 alloy surface-enriched with Al and Si (183–348 HV and 290–367 HV for the 80% Al + 20% Si and 50% Al + 50% Si powder mixtures, respectively) were compared with the data available in the literature. The microhardness of the material enriched with Al, Al/Zn, Zn/Al and Zn/Y is reported to be 181–203 HV [35], 183–223 HV [34], 156–180 HV [37], and 175–265 HV [39], respectively. It is evident that the highest microhardness was obtained for the Al/Si-enriched layer. This was due to the presence of fine particles of the Mg2Si phase, whose microhardness is much higher than that of the binary MgeAl and ternary Mg-Al-Zn intermetallic phases. As pointed out in the introduction to this article, Al/Si-enriched layers on an Mg-based substrate are mainly produced by laser alloying/cladding [7–13]. The thickness of an alloyed layer and the extent of alloying are mainly dependent on the process parameters
3.2. Hardness of the Al/Si-enriched surface layers Fig. 11 shows indentations left after a Vickers hardness test in the AZ31 substrate and the Al/Si-enriched layer fabricated through thermochemical treatment using the 80% Al + 20% Si powder mixture. The microhardness of the AZ31 varied from 44 to 48 HV0.1. The microhardness of the alloyed surface layer was in the range between 183 and 348 HV0.1. Lower values, i.e., 183–189 HV0.1, were obtained for the eutectic (Mg17Al12 and a solid solution of Al in Mg) in the area adjacent to the AZ31. Higher values, i.e., 205–310 HV0.1, were reported for areas where the eutectic co-occurred with the Mg2Si-phase particles. As indicated above, the Mg2Si phase present in the layer microstructure is mainly in the form of agglomerates. The material microhardness measured in the agglomerate areas was the highest, reaching 348 HV0.1. For the layer fabricated from the 50% Al + 50% Si powder mixture, the microhardness was found to be higher, with relatively stable values 207
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Fig. 12. Weight loss against sliding distance: (a) bare AZ31, (b) Al/Si-enriched layer fabricated on AZ31 from the 80% Al + 20% Si mixture, (c) Al/Si-enriched layer fabricated on AZ31 from the 50% Al + 50% Si mixture.
a higher volume fraction of hard, evenly distributed Mg2Si particles. The comparative studies concerning the wear behavior of thermochemically treated Mg-based substrates reveal that when they are enriched with Al and Si, their wear resistance improves considerably and it is higher than that obtained for Mg-based materials enriched with Al or Al/Zn [36]. A substantial increase in the wear resistance of Al/Sienriched Mg-based materials is due to the occurrence of very hard Mg2Si-phase particles in the layer microstructure. The results presented in this article are in agreement with the literature data concerning Al/ Si-enriched layers fabricated using laser-based methods [8–11,13]; in all the cases studied, the alloyed layer had good resistance to abrasive wear. To sum up, the thermochemical treatment method makes it possible to fabricate Al/Si enriched layers on Mg-based substrates that are characterized by very good wear resistance. The experiments described here were conducted for AZ31 as the substrate material but this technique can be successfully used for other Mg alloys exposed to severe abrasive wear conditions.
Fig. 11. Indentations left in the Al/Si-enriched surface layer and the AZ31 substrate after the Vickers hardness test.
such as laser power, scan speed, laser spot size and injection rate of alloying elements or thickness of preplaced layer [9]. The process parameters need to be carefully optimized to obtain a desired alloying effect. Porosity formation is also an important problem in laser alloying/cladding. Al/Si-enriched layers fabricated in this way generally have a non-uniform structure with the microstructure and properties likely to change along the length and depth. To summarize, the thermochemical treatment method used in this study is well-suited to fabricate Al/Si alloyed layer on Mg-based substrates with uniform structure along the depth and length, which is difficult when laser-based methods are applied. The composition of the powder mixture affected the layer microstructure. When the content of Si was higher (50%), the alloyed layer was uniform with a high volume fraction of the Mg2Siphase. The microhardness of the layer was high and relatively stable. The thermochemical treatment method is also much simpler and more cost effective than laser alloying/cladding.
4. Conclusions The alloyed layers were fabricated on AZ31 magnesium alloy through thermochemical treatment. The surface enrichment involved heating an AZ31 specimen in contact with an Al + Si powder mixture. Two powder mixtures were analyzed: 80% Al + 20% Si and 50% Al + 50% Si. For the former, three heat treatment temperatures were used (435, 445 and 455 °C). With 445 °C found to be optimal, the experiments for the other mixture (50% Si) were carried out only at that temperature. The main conclusions of the study are as follows:
3.3. Wear behavior of the Al/Si-enriched surface layers Fig. 12 illustrates the weight loss for bare AZ31 and AZ31 with alloyed layers tested under dry sliding conditions. It can be seen that the loss of weight reported for the unalloyed AZ31 (3.83 mg) is much higher than the average values obtained for the Al/Si-enriched specimens, and these are: 0.43 mg for the layer fabricated from the powder mixture containing 20% Si and 0.12 mg for the layer produced from the mixture where Si constituted 50%. The significant decrease in the weight loss was due to the presence of hard constituents in the surface layer microstructure, i.e., the Mg2Si phase and the Mg17Al12 intermetallic phase. The wear resistance of the layer fabricated from the mixture containing a higher amount of Si (50%) was better than that reported for the layer obtained from the mixture containing only 20% of this element. The microstructure of the more wear resistant layer had
1. The Al/Si-enriched layer fabricated from the 80% Al + 20% Si powder mixture at 445 °C is thick and continuous with low porosity and thickness ranging from 400 to 450 μm. 2. The heat treatment of AZ31 in contact with a 50% Al + 50% Si powder mixture at 445 °C results in the formation of a uniform layer 208
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with a thickness of about 200 μm. 3. The structural constituents of the Al/Si-enriched layers are a eutectic (an Mg17Al12 intermetallic phase and a solid solution of Al in Mg) and Mg2Si-phase particles. In the layers fabricated from the powder mixture containing 20% Si, the Mg2Si particles are unevenly distributed over eutectic. However, when the powder mixture contains 50% Si, there is a much higher volume fraction of Mg2Si phase particles and their distribution over the eutectic is even. The presence of the eutectic in the layer microstructure suggests that during the thermochemical treatment at 445 °C, the reactions at the powder/substrate interface proceed with partial melting. 4. The microhardness of the alloyed surface layers containing hard structural constituents, i.e., an Mg17Al12 intermetallic phase and an Mg2Si phase, is much higher than that of the AZ31 substrate. The layer microhardness depends on the content of Si in the powder mixture. The higher the percentage of Si in the Al + Si powder mixture, the higher volume fraction of the hard Mg2Si phase particles and the higher the microhardness of the layer produced. Thus, the microhardness of the layer fabricated from the powder mixture containing 50% Si (290–367 HV) is higher than that obtained from the Al + 20% Si mixture (183–348 HV). 5. The Al/Si-enriched surface layers significantly improve the wear resistance of AZ31. A greater amount of the Si powder in the mixture leads to the formation of a layer with a high volume fraction of the Mg2Si phase evenly distributed in the layer microstructure. Hence the excellent wear resistance of the alloyed surface layer produced from the Al + 50% Si mixture.
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