Improving mechanical properties of AZ91D magnesium/A356 aluminum bimetal prepared by compound casting via a high velocity oxygen fuel sprayed Ni coating

Guangyu Li, Wenming Jiang, Feng Guan, Junwen Zhu, Yang Yu, Zitian Fan

State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China

Abstract In this paper, a Ni coating was deposited on the surface of the A356 aluminum alloy by high velocity oxygen fuel spraying to improve the performance of the AZ91D magnesium/A356 aluminum bimetal prepared by a compound casting.The effects of the Ni coating as well as its thickness on microstructure and mechanical properties of the AZ91D/A356 bimetal were systematically researched for the firs time.Results demonstrated that the Ni coating and its thickness had a significan effect on the interfacial phase compositions and mechanical properties of the AZ91D/A356 bimetal.The 10μm's Ni coating cannot prevent the generation of the Al-Mg intermetallic compounds (IMCs) at the interface zone of the AZ91D/A356 bimetal, while the Ni coating with the thickness of 45μm and 190μm can avoid the formation of the Al-Mg IMCs.When the Ni coating was 45μm, the Ni coating disappeared and transformed into Mg-Mg2Ni eutectic structures+Ni2Mg3Al particles at the interface zone.With a thickness of 190μm's Ni coating, part of the Ni coating remained and the interface layer was composed of the Mg-Mg2Ni eutectic structures+ Ni2Mg3Al particles, Mg2Ni layer, Ni solid solution (SS) layer, Al3Ni2 layer, Al3Ni layer and sporadic Al3Ni+Al-Al3Ni eutectic structures from AZ91D side to A356 side in sequence.The interface layer consisting of the Mg-Ni and Al-Ni IMCs obtained with the Ni coating had an obvious lower hardness than the Al-Mg IMCs.The shear strength of the AZ91D/A356 bimetal with a Ni coating of 45μm thickness enhanced 41.4% in comparison with that of the bimetal without Ni coating, and the fracture of the bimetal with 45μm's Ni coating occurred between the Mg matrix and the interface layer with a mixture of brittle fracture and ductile fracture.

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Keywords: Magnesium/aluminum bimetal; Microstructure; Mechanical properties; Ni coating; Compound casting; High velocity oxygen fuel spraying.

1.Introduction

The magnesium/aluminum bimetal synthesizes both advantages of the magnesium (Mg) and aluminum (Al) and is widely used throughout automotive, aerospace and weaponry to meet the demand for higher and higher comprehensive performance [1-5].The lost foam compound casting (LFCC) is a promising approach to achieve the joint of the Al and the Mg alloys with complicated shapes and large connection area thanks to its some unique advantages [6-9].For example,no additional immobilization is required for the inlay [10],and the foam-decomposed gases prevent the oxidation of the Al inlays and the Mg melts [11], and a metallurgical combination is prone to implement [12].Nevertheless, the thick interface layer constituted of brittle and hard Al-Mg IMCs sharply weakens the connection strength, therefore limiting the application of the Al/Mg bimetal [13-15].Hence, how to reduce or eliminate these detrimental Al-Mg IMCs is a research focus in the fiel of the Al/Mg bimetal.

At present, introducing an intermediate medium, such as Zn [16-18], Sn [15], Al [19,20], Mg-Al eutectic alloy [21],SiC [22], Al2O3[23], Ni [19,24-26], Cu [27,28], CuNi [29],Fe [30], Ti [31], Ag [32-34], Ce [35]and Zr [36]and thelike, between the Al and Mg is a frequently-used and resultful mean.Thereinto, the Ni interlayer is proved to be an economical and helpful choose in the welding field For instance, Zhang et al.added a Ni foil between the pure Al and pure Mg to take away the Al-Mg IMCs in the Mg-Al dissimilar joints fabricated by diffusion bonding, and the shear strength enhanced to 19.5MPa compared to that without Ni foil (9.4MPa) [19,25].Yin et al.researched the influenc of the Ni coating on the Al/Mg joint prepared by diffusion bonding, and an optimum bonding strength of 5.8MPa was get at 420°C [24].Khodabakhshi et al.used the laser welding to bond the AA6022 and AZ31 with a Ni as interlayer, and effect of the laser power on the microstructures and phases produced is evaluated [26].Sun et al.explored the effect of the nickel interlayer on the microstructure and mechanical properties of the AZ31/AA5754 joint obtained by resistance spot welded, and research results indicated that the Ni interlayer can improve the joint strength [30].But there are few investigations about the effect of the nickel interlayer on the Al/Mg bimetallic castings in the compound casting.Zhang et al.developed a ‘zincate+nickelling' surface treatment method to cover a Ni coating on the surface of the Al alloy, and the shear strength of Al/Mg bimetallic castings with Ni coating arrived 47.5MPa at a preheating temperature of 560°C [37].Liu.et al.studied the influenc of the plasma sprayed Ni coating on the microstructure and mechanical performance of the Al/Mg bimetal firstl , and the shear strength arrived a maximum value of 25.4MPa at a pouring temperature of 700°C[38].Li et al.compared the effect of different ways of nickel plating (electronickelling, chemical nickel-plating and plasma sprayed nickel)on the microstructure and performance of the Al/Mg bimetallic castings, and the results indicate that the Al/Mg bimetallic castings with the plasma sprayed nickel have an optimum shear performance [39].

High velocity oxygen fuel (HVOF) spraying, shooting the supersonic molten particles to the plated parts, is good at preparing a strong and dense coating with low porosity and oxide inclusion, and has a rapid development in recent years[40-42].Thus, it is a suitable technology to cover a Ni layer on the sophisticated surface efficientl.Moreover, the thickness of the Ni coating also affects the interfacial microstructure and performance potentially.However, the research about the effect of the HVOF sprayed Ni coating as well as its thickness on the Al/Mg bimetal is an unexplored field

In our current paper, the HVOF spraying was firs introduced to deposit a Ni coating on the surface of the A356 alloy,and the influenc of the thickness of the Ni coating on the microstructure and mechanical properties of the AZ91D/A356 bimetallic castings fabricated by LFCC was researched.

2.Experimental procedure

2.1.Experimental materials

Fig.1.The preparation process of the AZ91D/A356 bimetal with HVOF-Ni coating: (a) HVOF spraying process; (b) A356 with Ni coating was inserted the foam pattern; (c) Schematic of the LFCC.

The A356 with a composition of Al-0.44Mg-6.81Si-0.21Fe-0.02Ti was chosen as the solid part and the AZ91D whose composition was Mg-9.08Al-0.62Zn-0.23Mn was used as the pouring liquid alloy.The polystyrene was used to prepare the foam pattern, whose density was 12kg/m3.The size of the nickel powder used in the process of the HVOF spraying was 15-45μm purchased from Beijing Institute of Nonferrous Metals.

2.2.Preparation process of Ni coating

Firstly, the columniform A356 bar with a diameter of 10mm and a height of 110mm was treated by sand blast to wipe off the impurities and oxide layer as well as increase surface roughness.Afterwards, Ni coatings with different thicknesses were sprayed on the surface of the A356 by a HVOF spraying, whose principle was displayed in Fig.1(a).The nickel powders along with oxygen and fuel gas were put into a high-pressure chamber and were ignited in a combustor.The Ni powers were sprayed out with a form of diamond wave in a supersonic velocity (1500-2000m/s) under the high pressure to deposit on the surface of the A356, and a Ni coating was formed finall.During the process of the HVOF spraying,the oxygen fl w was 920L/min,the fl w of kerosene was 25L/h, the powder feeder fl w was 80g/min and the spraying distance was 400mm.An A356 bar without Ni coating was also chosen as comparison.

2.3.Compound casting process

The A356 bars with and without Ni coating were embedded into the foam pattern by a form of interference fit as demonstrated in Fig.1(b).Finally, the LFCC process was performed, whose detailed process has been introduced in our previous paper [43], as shown in Fig.1(c).

Fig.2.SEM microstructures of the Ni coatings with different thicknesses:(a) 10μm; (b) 45μm; (c) 190μm; (d) EDS result of region 1 in Fig.2(c).

2.4.Characterization

The bonding zones including A356 base and AZ91D base of the AZ91D/A356 bimetallic castings were cut out to conduct analysis and testing.The AZ91D/A356 bimetal specimens were grinded and polished.Thereafter, these specimens were etched with 4% nital solution on the AZ91D side and a 3% hydrofluori acid solution on the A356 side.The microstructure of the AZ91D/A356 bimetal specimens was observed with a Quanta 400 scanning electron microscope(SEM), and the chemical compositions of the interface layer were also analyzed by an energy-dispersive X-ray spectroscope (EDS).A Tecnai G2 F30 Field Emission Transmission Electron Microscopy (FTEM) was applied to further observe and analyze the microstructure and compositions.The TEM samples were obtained by a Helios NanoLab G3 CX Focused Ion Beam (FIB) system.The hardness was tested by a HV-1000 Vickers with a 300g load and a 15s dwell time.The shear strengths of the AZ91D/A356 bimetals were measured by a Zwick Z100 universal testing machine with a compression rate of 0.2mm/min, whose principle has been described detailly in our previous paper [44].

3.Results

3.1.HVOF-Ni coating

Fig.2 exhibits the microstructures of the Ni coatings with different thicknesses.The average thickness of the Ni coating is 10μm, 45μm, 190μm, respectively.It is apparent that the Ni coatings are dense, while the thickness of the Ni coating is not uniform, especially for the 10μm's Ni coating, as displayed in Fig.2(a).The Ni coating prepared by HVOF spraying is almost not oxidized according to the EDS result in Fig.2(d).

Fig.3.SEM interfacial microstructures and line scanning images of the AZ91D/A356 bimetals with different thickness's Ni coating: (a, b) Without Ni coating; (c, d) With 10μm Ni coating; (e, f) With 45μm Ni coating;(g, h) With 190μm Ni coating.

3.2.Microstructure

Fig.3 presents the SEM interfacial microstructure and line scanning image of the AZ91D/A356 bimetal with different thickness's Ni coating, and it is clear that the Ni coating as well as its thickness has a significan impact on the interfacial microstructure of the AZ91D/A356 bimetal.According to the line scanning image, the interface layers of the AZ91D/A356 bimetal with 0 and 10μm Ni coating are composed of the Al-Mg IMCs mainly, while the Ni, Mg-Ni and Al-Ni IMCs are the primary compositions for the AZ91D/A356 bimetal with 45μm and 190μm Ni coating.The interface layer of the AZ91D/A356 bimetal without Ni coating is composed of three reaction layers: Al3Mg2+Mg2Si, Al12Mg17+Mg2Si and Al12Mg17+δ-Mg eutectic structure, respectively, as shown in Fig.3(a), according to our previous investigation [43].The average thickness of the interface layer is about 1450μm.

Fig.4.SEM interfacial microstructures of the AZ91D/A356 bimetal with 10 μm's Ni coating: (a-d) Microstructures corresponding to A, B, C and D region in Fig.3(c); (e-h) EDS results of point 1, 2, 3 and 4 in Fig.4.

When the Ni coating is 10μm, the pure Ni layer is not found at the interface region of the AZ91D/A356 bimetal, as presented in Fig.3(c).Some bright white particles are found at the interfacial region and the A356 base except the Al-Mg IMCs, as exhibited in Fig.4(a)-(d).The EDS results(Fig.4(e)-(h)) show that these bright white particles are Al-Al3Ni eutectic structure based on the Al-Ni binary phase diagram [45].Thus, the interface layer of the AZ91D/A356 bimetal can be divided into two regions, one consists of the Al-Mg IMCs and Al-Al3Ni eutectic structure, the other is made up of the A356 base and Al-Al3Ni eutectic structure.The whole thickness of the interface layer is about 1650μm,which is thicker than that of the interface without Ni coating.

With the thickness of the Ni coating increasing to 45μm,there are few Al-Mg IMCs found at the interfacial region of the AZ91D/A356 bimetal, and the interface layer can be divided into two regions based on the morphology roughly,namely, region I and II, as exhibited in Fig.3(e).One is the mixed zone of the Mg-Ni and Al-Ni IMCs near to AZ91D base (region I), the other is the dispersed Al-Al3Ni eutectic structure next to A356 base (region II), and the phase compositions are detailly analyzed as below, as indicated in Fig.5(a)-(f).

The microstructure and EDS results of the region I is presented in Fig.5(a) and Table 1.As can be seen, most of the Ni coating cannot keep its original shape and reacts with Al and Mg, and an Al-Mg-Ni mixed zone is formed.In region G (Fig.5(b)), a thin Mg solid solution layer (Mg SS, point 1) is close to the AZ91D base.Point 2 contains 14.64 at% Al, 52.68 at% Mg and 32.69 at% Ni, demonstrating that the phase may be Ni2Al3Mg according to the atomic ratio (Ni:Mg:Al≈2:3:1) and Al-Mg-Ni ternary phase diagram [46].There are some Mg-Al12Mg17eutectic structure(point 3)around the Ni2Al3Mg particles.Region H(Fig.5(c))consists of light grey flocculen structure (point 4) and dark grey lumpish structure (point 5).The flocculen structure may be a mixed phases made up of tiny Ni2Mg3Al particles and Mg-Al12Mg17eutectic structure on the basis of the composition and contrast.Region I is composed of Ni2Mg3Al particles+Mg-Al12Mg17eutectic structure (point 6), Ni solid solution (Ni SS, point 8), Al3Ni2(point 10) and Al3Ni (point 8)according to Fig.5(d), (e) and Table 1.The average thickness of region I is about 223μm.In region II, some Al+Al3Ni eutectic particles are dispersed in the A356 base, as presented in Fig.5(f).An interesting phenomenon is that these Al+Al3Ni eutectic particles are located on the eutectic silicon mainly.The average thickness of the region II is about 1065μm.Thus, the whole thickness of the interface layer is about 1288μm.

When the Ni coating sequentially increases to 190μm, the original Ni coating still retains, but the thickness reduces to about 139μm, as be seen in Fig.3(g).The interface layer can be divided three regions on basis of the microgram and line scanning result, as displayed in Fig.3(g) and (h), and the phase compositions of every region are analyzed in Fig.6 and Table 2.Region III contains dispersive polygonal phases and lamellar structures mainly, as presented in Fig.6(d) and(e).These dispersive polygonal phases may be Ni2Mg3Al in line with EDS results (point 2 and 5) and Al-Mg-Ni ternary phase diagram, as exhibited in Table 2.Furthermore, the size of the Ni2Mg3Al near the Ni coating is bigger than that close to the AZ91D base seemingly.The lamellar structures are Mg-Mg2Ni eutectic structure according to the composition of point 4 and Mg-Ni binary phase diagram.A TEM test was applied to further verify the phase composition in Region III, and the FIB sampling location and the TEM results are displayed in Fig.7.As can be seen from the TEM bright fiel image(Fig.7(b)),the sampling location includes three phases,namely A, B and C region, respectively.Region B and region C are Mg and Mg2Ni phase according to the SAED patterns in Fig.7(d)and(e),respectively,which is in accordance with the above analysis.The selected area electron diffraction (SAED)pattern in region A only shows a set of spots and has a goodmatch with the Ni2Mg3Al phase, indicating that this zone is composed of a single-phase particle, as shown in Fig.7(c).A little Mg (Al, Ni) solid solution (point 1) andα-Mg also(point 3) exist in the region III according to Fig.6(b) and Table 2.The thickness of the region III is about 173μm.

Table 1Results of EDS analysis at different positions of the interface corresponding to Fig.5.

Table 2Results of EDS analysis at different positions of the interface corresponding to Fig.6.

Fig.5.High magnificatio SEM images of the AZ91D/A356 bimetal with 45 μm's Ni coating: (a, f) SEM images corresponding to E and F region in Fig.3(e), respectively; (b-d) SEM images corresponding to G, H and I region in Fig.5(a), respectively; (e) SEM image corresponding to J region in Fig.5(d).

Fig.6.High magnificatio SEM images of the AZ91D/A356 bimetal with 190 μm's Ni coating: (a, f) SEM images corresponding to G and H region in Fig.3(g), respectively; (b-d) SEM images corresponding correspond to K, L and M region in Fig.6(a), respectively; (e) SEM images corresponding to N region in Fig.6(d).

Fig.7.FIB sampling place and TEM test results: (a) FIB sampling place;(b) TEM bright fiel image; (c-e) SAED patterns corresponding to A, B and C region in Fig.7(b).

Fig.8.Curves of the stress's change against the displacement for the AZ91D/A356 bimetals with different thickness's Ni coatings.

Four dense diffusion layers are present in region IV, and they are Mg2Ni layer, Ni SS layer, Al3Ni2layer and Al3Ni layer from the AZ91D side to the A356 side successively, as demonstrated in Fig.6(c), (d), (e) and Table 2.Some Al3Ni particles (Fig.6(d)) and Al-Al3Ni eutectic particles (Fig.6(f))disperse on the grain boundary of the A356 base in region V,and the thickness of the region V is about 1070μm.

3.3.Mechanical properties

The curves of the stress's change against the displacement of the AZ91D/A356 bimetals with different thickness's Ni coatings are presented in Fig.8.It can be seen that the shear strength of the AZ91D/A356 bimetal increases with the augment of the Ni coating's thickness in the range from 0 to 45μm.However, the shear strength of the AZ91D/A356 bimetal with 190μm's Ni coating decreases, even lower than that without Ni coating.The shear strength of the AZ91D/A356 bimetal with 45μm's Ni coating has the maximal shear strength, which enhances 41.4% than that of the bimetal without Ni coating.

The Vickers hardness in the interface zone was measured for the AZ91D/A356 bimetal with different thickness's Ni coatings, and the corresponding results are made a line chart,as illustrated in Fig.9.When the thickness of the Ni coating is 10μm, the microhardness barely changes compared to that without Ni coating in the interface zone.It is apparent that the microhardness consisted of the Al-Ni and Mg-Ni IMCs is lower than the Al-Mg IMCs and pure Ni layer prepared by HVOF (209HV).Moreover, the hardness of the Al-Ni and Mg-Ni IMCs (162-173HV) with 45μm's Ni coating is a little higher than that with 190μm's Ni coating (120.5HV).And, it seems that the disperse Al+Al3Ni eutectic structure can boost the hardness of the A356 base slightly.The indentation sizes also present consistent conclusion with the above description,as indicated in Fig.10.

Fig.9.Vickers hardness of the AZ91D/A356 bimetals with different thickness's Ni coatings.

3.4.Fracture characteristics

The SEM fractographs of the AZ91D/A356 bimetal with different thickness's Ni coatings are exhibited in Figs.11 and 12.The brittle rupture is the main failure mode for the AZ91D/A356 bimetal without Ni coating and with 10μm's Ni coating, as shown in Fig.11.Some Al-Al3Ni eutectic structures are also detected in the fracture surface of the AZ91D/A356 bimetal with 10μm's Ni coating, as demonstrated in Fig.11(d) and (f).The AZ91D/A356 bimetal with 45μm's Ni coating breaks in the joint zone of the AZ91D matrix and Mg-Mg2Ni eutectic structure+Ni2Mg3Al chiefl ,as presented in Fig.12(a) and (c).The fracture surface displays a mixture of brittle and ductile fracture on account ofthe existence of dimples, which is not detected in other samples, as indicated in Fig.12(b) and (d).With regard to the AZ91D/A356 bimetal with 190μm's Ni coating, the fracture morphology shows a typical brittle fracture mode,as exhibited in Fig.12(e) and (g).Different from the 45μm's Ni coating,a number of Mg2Ni particles are found in the fracture surface closed to the A356 side, as displayed in Fig.12(e), demonstrating the fracture position is located between the AZ91D base and Mg2Ni layer.

Fig.10.Optical microscope images of the A356/AZ91D bimetals with different thickness's Ni coatings: (a) Without Ni coating; (b) 10 μm's Ni coating; (c)45 μm's Ni coating; (d) 190 μm's Ni coating.

Fig.11.SEM fractographs of the AZ91D/A356 bimetals with and without Ni coating: (a, b) SEM frapctographs on A356 side and AZ91D side without Ni coating; (c, e) SEM fractographs on A356 side and AZ91D side with 10 μm's Ni coating; (d, f) SEM fractographs corresponding to A and B region.

Fig.12.SEM fractographs of the AZ91D/A356 bimetals with 45 μm's and 190 μm's Ni coatings: (a, c) SEM fractographs on A356 side and AZ91D side with 45 μm's Ni coating; (b, d) SEM fractographs corresponding to C and D region; (e, g) SEM fractographs on A356 side and AZ91D side with 190μm's Ni coating; (f, h) SEM fractographs corresponding to E and F region.

Fig.13.Optical microscope images of cross sections of the fracture of the AZ91D/A356 bimetals with different thickness's Ni coatings: (a, b) A356 and AZ91D sides without Ni coating, respectively; (c, d) A356 and AZ91D sides with 10 μm's Ni coating, respectively; (e, f) A356 and AZ91D sides with 45 μm's Ni coating, respectively; (g, h) A356 and AZ91D sides with 190 μm's Ni coating, respectively.

Fig.13 presents the optical microscope images of cross sections of the fracture of the AZ91D/A356 bimetals with different thickness's Ni coatings.As can be seen, it is in good agreement with the results of the SEM fractographs of the AZ91D/A356 bimetals.The breakage occurs at the junction of the Al3Mg2+Mg2Si layer and Al12Mg17+Mg2Si layer for the AZ91D/A356 bimetals with 0 and 10μm's Ni coatings, as demonstrated in Fig.13(a)-(d).With a 45 μm's Ni coating, the AZ91D/A356 bimetals break in the region I(Ni2Mg3Al+Mg-Mg2Ni), as displayed in Fig.13(e) and (f).When the thickness of the Ni coating is 190μm, the rupture is mainly located between the AZ91D base and Mg2Ni layer,as indicated in Fig.13(g) and (h).There is no Al-Ni+Mg-Ni IMCs layer found in the fracture section, which may be attributed to the crush and falling off in the shear condition.

4.Discussion

4.1.Interfacial formation mechanism

According to investigations, the interfacial microstructures of the AZ91D/A356 bimetals with different thickness's Ni coatings have an obvious difference, which demonstrates that the thickness of the Ni coating has an evident influenc on the interface layer, and the influenc mechanism can be explained as Fig.14.

Fig.14.Schematic diagram of the interfacial formation of the AZ91D/A356 bimetals with different thickness's Ni coatings: (a1-a3) 10′s Ni coating; (b1-b4)45′s Ni coating; (c1-c7) 190′s Ni coating.

When the thickness of the Ni coating is 10μm, the thin Ni coating cannot impede the liquid Mg alloy contacting with the solid Al alloy by reason of the complete decomposition of the Ni layer, as indicated in Fig.14(a1) and (a2).Thus, the interfacial phase compositions of the AZ91D/A356 bimetal with 10μm's Ni coating are Al-Mg IMCs mainly, which is similar with that without Ni coating.The Al-Al3Ni eutectic structure dispersedly distributed in the interfacial zone may be attributed to the reaction of the Ni element with the Al element by the following equation: Al+Ni→Al+Al3Ni, as presented in Fig.14(a3).There are no Mg-Ni IMCs existing in the interfacial zone, which indicates that the Ni element is easier to react with Al element.

In the case of the 45 μm's Ni coating,the formation mechanism can be described with Fig.14(b1)-(b4).Since the melting point of Ni (1453°C) is much higher than the pouring temperature (730°C), the diffusion plays a major role in the formation process of the interface layer.When the AZ91D liquid was poured on the surface of the A356 inlay, the Ni element would diffuse to Al side and Mg side, respectively under the drive of the high temperature, and Mg element and Al element also would diffuse toward the Ni side.With the decreasing of the temperature, elements diffusion would reach its limitation and then stop.When the Ni coating's thickness was 45μm, the pure Ni coating almost disappeared completely (Fig.14(b2)), indicating that the 45μm's Ni coating was exactly consumed up under this process conditions.Therefore, the Ni coating with 45μm's thickness can block the reaction between Al and Mg, and the interface layer was mainly composed of the Al-Ni and Mg-Ni IMCs vicariously.The formation process of these Al-Ni and Mg-Ni IMCs is indicated in Fig.14(b3) and (b4).

The Ni element diffused to the Al side and reacted withα-Al to form Al-Al3Ni eutectic structure at the Al side.At the same time, with the Ni element diffused to the Mg side, the mixed structure of Mg-Mg2Ni eutectic structure and Ni2Mg3Al phase formed.It is well known that the lower of the formation Gibbs free energy (ΔG) of the compounds,the easier it is to form.The AZ91D contains 9.08% Al element, therefore the Mg side can be considered as Al-Ni-Mg ternary systems.In this work, the generated compounds include Al3Ni, Al3Ni2, Ni2Mg3Al and Mg2Ni, and theΔG of these compounds can be calculated by Eq.(1):

wherex1, x2andx3are the mole fractions of the constituents,0Giis the Gibbs energy of the pure element i,the second term is the contribution of the ideal mixing to the Gibbs energy,andexGis the excess Gibbs energy [43,47].

The change of theΔG for these compounds with temperature is listed in Fig.15.TheΔG of the Ni2Mg3Al is the lowest in these compounds, so the Ni2Mg3Al phase is formed firstl.When the Al element is depleted, the Ni element reacts with the Mg element.At this time, the content of elements exactly arrives to the eutectic composition, and the Mg-Mg2Ni eutectic structure forms through the eutectic reaction.

Fig.15. ΔG of Al3Ni, Al3Ni2, Ni2Mg3Al and Mg2Ni as a function of temperature.

The Ni coating remains on account of the thick Ni coating (190μm) is not consumed by interdiffusion completely,therefore the interfacial microstructure is different from that with 45μm's Ni coating, and the formation mechanism of the interface layer can be explained with Fig.14(c1)-(c7).In Mg-Ni interface, a thin and dense Mg2Ni layer generated firstl at the synergy of the chilling action and the interdiffusion between Ni element and Mg element, as displayed in Fig.14(c2).The formation mechanism of the reaction layer constituted by Mg-Mg2Ni eutectic structure and Ni2Mg3Al particles is similar to that with 45μm's thickness.However,the amount of the Ni2Mg3Al phases with 45μm's Ni coating is higher than that with 190μm's Ni coating, which may be ascribed to the Al element diffusing to the Mg side because of the complete decomposition of the Ni coating.At the Al-Ni interface, the interfacial formation process can be divided three stage:(i)Al3Ni2and Al3Ni layer formed in succession as a result of the interdiffusion between Al element and Ni element, and the Al3Ni2is antecedent to the Al3Ni to form because of the lowerΔG than the Al3Ni, as shown in Fig.14(c2); (ii) the growth stresses of the Al3Ni layer, reported by Sistaninia [48], caused the consecutive Al3Ni layer rupturing to nubs, as indicated in Fig.14(c3) and (c4); (iii) in the position far away from the Ni coating, the concentration of the Ni element is poor and the Al-Al3Ni eutectic structure was formed.

4.2.Relationship between microstructure and mechanical properties

When the Ni coating thickness is 45μm, the presence of the Ni coating successfully prevents the formation of the Al-Mg IMCs replaced by the Al-Ni and Mg-Ni IMCs.Moreover,the hardnesses of the Al-Ni and Mg-Ni IMCs are lower than that of the Al-Mg IMCs significantl.It is well known that the intermetallic compounds with low hardness at the interface have greater capacity to resist the deformation when subjected to a shear force [49,50].In consequence, the fracture mode changes from a brittle fracture to a mixture of brittle fracture and ductile fracture, as shown in the Fig.12(a)-(d), thereby improving shear strength.

Although the 10μm's Ni coating cannot hold back the generation of the Al-Mg IMCs, the presence of the Al-Al3Ni eutectic particles may be as the obstacles of the crack propagation, thus slightly increasing the shear strength of the Al/Mg bimetal compared to without Ni coating.

With the Ni coating thickness increasing to 190μm, although there is no Al-Mg IMCs generated in the interface zone, as well as the hardness of the intermetallic layer is lower than the Al-Mg IMCs, the interface structure was not uniform due to contain different reaction layers with different stress differences.The stress concentration would be more severe when subjected to shear force.The fracture location was mainly in the region between the Mg2Ni layer and the Mg matrix, indicating that there is a poor bonding at the Mg-Ni interface.

Thus, an ideal interfacial microstructure should be composed of homogeneous and low hardness's IMCs, which depends on the thickness of the Ni coating and the technological parameter.This principle may be also suitable for other interlayers and compound castings.

5.Conclusions

Current work focuses on the effect of the HVOF sprayed Ni coating as well as its thickness on the microstructure and mechanical properties of the AZ91D/A356 bimetal fabricated by LFCC, and the following conclusions are drawn:

(1) The Ni coating and its thickness had a significan effect on the interfacial phase compositions and mechanical properties of the AZ91D/A356 bimetal.The 10 μm's Ni coating cannot prevent the generation of the Al-Mg IMCs at the interface zone of the AZ91D/A356 bimetal,while the Ni coating with the thickness of 45μm and 190μm can avoid the formation of the Al-Mg IMCs.

(2) The interfacial phase compositions were diverse for the AZ91D/A356 bimetal with different thickness's Ni coatings.The Al-Mg IMCs and a few Al-Al3Ni eutectic structures were the main compositions for the AZ91D/A356 bimetal with 10μm's Ni coating.The interface layer was mainly composed of the Mg-Mg2Ni eutectic structures+Ni2Mg3Al particles and dispersed Al-Al3Ni eutectic structure for the AZ91D/A356 bimetal with 45 μm's Ni coating.The interfacial microstructure of the AZ91D/A356 bimetal with 190 μm's Ni coating can be divided into three regions, namely,the Mg-Mg2Ni eutectic structures+ Ni2Mg3Al particles closed to the Mg base,the compact diffusion layers constituted by the Mg2Ni layer, Ni SS layer, Al3Ni2layer and Al3Ni layer, and the dispersed Al3Ni particles and Al-Al3Ni eutectic structures, respectively.

(3) The hardness of the interface layer containing Mg-Ni and Al-Ni IMCs obtained with the Ni coating was clearly lower than that of the Al-Mg IMCs.The AZ91D/A356 bimetal with a 45μm's Ni coating ob-tained a maximum value of the shear strength in this study, which was 41.4% higher than that without Ni coating, and the fracture mode was a mixture of the brittle fracture and ductile fracture.

Acknowledgments

The authors gratefully acknowledge the supports provided by the National Natural Science Foundation of China(No.52075198), the National Key Research and Development Program of China (Nos.2020YFB2008300 and 2020YFB2008304), the State Key Laboratory of High Performance Complex Manufacturing in CSU (No.Kfkt2019-01),and the Analytical and Testing Center, HUST.