Fulin Wen ,Jinhu Zho,b,* ,Miowng Yun ,Jingfeng Wng,b,* ,Dengzhi Zheng ,Jinyong Zhng ,Ke He,Jingjing Shnggun,Yu Guo

Abstract The solid-liquid compound casting of Mg-AZ91D and Ti-TC4 alloys was developed by using pure Ni electro-deposited coating.The pouring temperatures of 660?C,690?C,720?C and 750?C were chosen to investigated the effects of casting temperatures on microstructural evolution,properties,and fracture behaviors of Ni-coated TC4/AZ91D bimetals by the solid-liquid compound casting (SLCC).The scanning electron microscopy (SEM) and the energy dispersive spectroscopy (EDS) results showed that the interfacial zone mainly composed of nickel,Mg2Ni and Mg-Al-Ni in the bimetals cast at 660?C.As the pouring temperature was increased to 750?C,the width of the interface zone,which mainly composed of δ(Mg),Mg2Ni,Mg-Al-Ni,Mg3TiNi2 and Al3Ni,gradually increased.The microhardness tests showed that the micro-hardness of the interface zone was smaller than that of TC4 substrate but larger than that of the cast AZ91D matrix.At the pouring temperature of 720?C,the Ni-coated TC4/AZ91D bimetals had the most typical homogeneous interface,which had granular Mg-Al-Ni ternary phase but no ribbon-like Al3Ni binary phase,and achieved the highest shear strength of 97.35MPa.Meanwhile,further fracture behavior analysis showed that most fracture failure of Ni-coated TC4/AZ91D bimetals occurred at the Mg2Ni+δ(Mg) eutectic structure and Al3Ni hard intermetallic.

Keywords: Dissimilar materials;Solid-liquid compound casting;Interface;Titanium alloy;AZ91D Mg;Intermetallic compound;Mechanical property.

1.Introduction

Magnesium alloys,which provide many excellent properties such as low density,high strength-to-weight ratio and good casting properties were widely used in automotive and aerospace industries[1-3].However,the high-temperature mechanical properties and the corrosion resistance of Mg alloys are poor.By contrast,the titanium alloy possesses some advantages including excellent high temperature performance,corrosion resistance,good fracture toughness and high specifi strength which also attracted considerable attention in the newly developed industries [4-6].Recently,the products made of one of these two alloys are always difficul to meet the actual requirements for the industrial applications in some cases.The Mg-Ti composite materials are widely used in the biological fields aviation industries and automobile industries due to the combination properties such as high specifi strength,excellent electromagnetic shielding and low density[7].Thus,different joining techniques of Mg-Ti composite materials have been developed,such as transient liquid phase bonding (TLP) [8-11],laser welding-brazing (LWB) [12,13]and friction stir welding (FSW) [14-17].By contrast,few works have been focused on the solid-liquid compound casting technology.

The solid-liquid compound casting (SLCC) was an alternative technology which has the advantages of low production cost,simple processing procedure,low energy consumption and high interface bonding strength of the products.The most obvious feature of this method was that it can fabricate bimetallic composite castings with complex shapes and internal structures.In general,the interface reaction products would determine the joint properties of the solid-liquid compound castings.Therefore,the interfacial reinforcement has always been a research hotspot in the fiel of solid-liquid compound casting.Rübner et al.[18] enhanced the Al/Al interfacial reaction by the means of mechanical vibration,ultrasonic,internal induction heating,centrifugal casting,pressure,vacuum and other external physical field Zhao et al.[19,20]adopted solid-liquid compound casting by pouring the molten metal around the Al-coated aluminum,aluminum alloy (A356) and magnesium alloy (AZ91D) to fabricate Al/Al,Al/A356 and Al-Zn/AZ91D bimetallic materials successfully.Nevertheless,the application of the Mg/Ti solid-liquid compound castings was very limited due to the deficien y of the interfacial reinforcement methods and mechanism investigations.The melting point of TC4 is 1668?C,while the melting and boiling points of AZ91D are 596?C and 1091?C,respectively,indicating the low mutual solid solubility of these two metals [21].Besides,there were not any reaction products or atomic diffusion between the Mg and Ti during the melting and solidificatio process because of the immiscibility characteristics of these two metallic elements [22].To overcome the above obstacles,using an intermediate layer such as Al,Ni and Cu which can react with Mg and TC4 was adopted to obtain the metallurgical bonding joints.Meanwhile,Zhao et al.[23] adopted selective laser melting (SLM)additive manufacturing method and lattice materials to fabricate the AZ91D/TC4 solid-liquid compound casts in our previous study and found that high specifi surface area and three-dimensional embedded structure between AZ91D/TC4 could reinforce the interfacial metallurgical bonding and mechanical bonding.

In this study,it was a new exploration that the solid-liquid compound casting was used to connect AZ91D/TC4 dissimilar metals.Enhancing the liquid-solid interfacial reaction of AZ91D/TC4 via the intermediate layer of Ni would be proved effective.Meanwhile,the effects of casting pouring temperature on the evolution of microstructures and mechanical properties were investigated in detail.

2.Experimental procedures

2.1.Materials

As the most widely-used alloys,the magnesium alloy AZ91D (Mg-9%Al-1%Zn) and the titanium alloy Ti-6Al-4V (Ti-6%Al-4%V) were used as a molten magnesium bath and as the substrate material,respectively,to fabricate theTC4/AZ91D bimetallic castings.Before electroplating,the TC4 rod with a diameter of 5mm was cut into 30mm length and the acid solution (containing 25% hydrofluori acid,75%nitric acid) was used to remove oxide fil on TC4 rods.The electroplating process of TC4 rods was performed in a beaker fille with 500mL plating solution with a pH value of 4 at 30°C and the composition of it was listed in Table 1.As shown in Fig.1,the nickel plate was used as the anode while the TC4 rod was used as the cathode.Meanwhile,the directcurrent power was utilized as the power source and the current density was set as 1A/dm2.Fig.2 showed the results of scanning electron microscopy (SEM) and the corresponding EDS analysis of the Ni coating.

Table 1 Composition of Ni electro-plating solution.

Fig.1.The schematic of Ni electroplating onto TC4 surface.

2.2.Casting process

In order to fabricate the Ni-coated TC4/AZ91D bimetal compound casts,the AZ91D ingots covered with the RJ-2 flu were melted in an alumina crucible,which was placed in an electrical resistance furnace.Prior to the pouring process,the electrodeposited TC4 rods were put into the prepared mould which was preheated in a box-type furnace at 250?C for 2h.Then,the AZ91D melts were poured into the mould with a steady casting speed as the schematic illustration of solid-liquid compound casting process shown in Fig.3.As shown,The K-type thermocouple was installed 5mm away from the surface of the TC4 rods to record the temperature curve of the AZ91D melt during solidificatio process.The time-temperature curves were monitored by paperless recording instrument and the results were presented in Fig.4.To ensure that the AZ91D melt have a sufficien contact with the Ni coatings of TC4 rods,the mould should not be removed until the magnesium alloy completely cooled down and solidified

Fig.2.The SEM and EDS analysis for Ni interlayer electroplated on TC4 substrate.

Fig.3.The schematic illustration of solid-liquid compound casting.

Fig.4.The temperature curves of Ni-coated TC4/AZ91D bimetals with different pouring temperatures.

Fig.5.Schematic of shear tester and the dimension of the shear test sample.

2.3.Characterizations

Before observation,the samples were cut from the bimetallic materials by an electrical discharge machine,and then the silicon carbide (SiC) paper and diamond pastes were used to ground and polish the samples.A solution of 4% concerted nitric acid and 96%ethyl alcohol was used to etch all the samples.The cross-sectional observation of solid-liquid reaction region was taken by the TESCAN VEGA 3 LMH scanning electron microscope (SEM) with energy dispersive spectrometer (EDS).The A D/max 2500PC X-ray diffraction (XRD)was conducted to confir the phases compositions within the interface of the samples.

2.4.Mechanical properties

The MH-5L microhardness tester was adapted to examine the microhardness distribution of the Ni-coated TC4/AZ91D bimetallic composites using a load of 25g for a dwell time of 15s at the interface region.During the testing process,a sufficien distance of indentations was guaranteed to avoid the interference between adjacent ones.

In this study,the shear tests were employed to measure the bonding strength of the AZ91D/TC4 bimetallic composites.Fig.5 presented the schematic sketch of the shear tester and the dimensions of the shear test sample.At least three specimens of each group were tested at a loading rate of 0.5mm/min in order to minimize the errors.Finally,the shear strength of the interface could be calculated based on the equation as follows:

whereδTis the shear strength of the bimetallic samples,F maxis the maximum load,R is the diameter of TC4 rod and h is the height of specimen.

3.Results

3.1.Interfacial compositions and microstructures

Fig.6 showed the interfacial macro-photographs of the Nicoated TC4/AZ91D bimetal samples.It was evident that a large number of unreacted interface was appeared when the pouring temperature was set as 660?C.However,as the pouring temperatures was increased to 690?C,720?C and 750?C,all the interfaces of Ni-coated TC4/AZ91D bimetallic composites showed a better bonding between the TC4 rod and AZ91D melt,which illustrated that the pouring temperature had significan effects on the interface reaction between the TC4 and AZ91D.

Fig.7 illustrated the X-ray diffraction results of the constitutive phases of the reaction region.As can be seen,the interface zone of the bimetallic materials fabricated at 660?C,690?C,720?C and 750?C were mainly composed of six phases,i.e.nickel,magnesium,Mg2Ni,Mg-Al-Ni,Mg3TiNi2and Al3Ni,while the nickel was only found in the interface of Ni-coated TC4/AZ91D bimetallic casting fabricated at 660?C.The SEM microstructure observation and EDS spot scan analysis,as shown in Figs.8 and 10,could be considered as the proofs to confir these phases.

Fig.8a exhibited the SEM micrograph of the interface microstructures of the TC4/AZ91D bimetallic composites cast at 720?C without Ni coating.As can be seen,the gaps were almost present in the whole interface between the AZ91D and TC4 and a large gap with a maximum size of about 10 um was observed in the interface,indicating the poor wettability between the AZ91D melt and the TC4 substrate.

Fig.8(b)-(e) illustrated the interface microstructures of the Ni-coated TC4/AZ91D bimetals made at different pouring temperatures.It could be observed that the AZ91D and TC4 rod were distinctly separated with no evidence of defects along the interface and the reaction region was composed by the microstructure with different contrast and morphologies.During the period of gradual change of pouring temperature,ostensibly three different layers (I,II and III) in the interface region could be distinguished by colors and shapes.Based on the XRD analysis (Fig.7) and EDS results (Fig.10 and Table 2),it could be speculated that Layer I was mainly made up ofδ-Mg+Mg2Ni eutectic structure,layer II was mainly composed of massive Mg-Ni-Al and Al3Ni intermetallic compounds,and layer III was primarily composed of Mg-Ni-Al ternary phase andδ-Mg.And,more remarkable,the phase composition of each layer was varied with the increase of the pouring temperature.

Fig.9 displayed the EDS line scan spectra (marked as dashed lines in Fig.8) across the interface of Ni-coated TC4/AZ91D bimetals which were cast at different casting conditions.Yet,on the whole,the magnesium content in layer I and layer III increased with the increase of pouring temperature.In contrast,the nickel content in layer I decreased gradually as the pouring temperature increased from 660?C to 750?C.During the period of gradual change,the aluminum content increased gradually in the interface zone as shown in Fig.9(a)-(c).When the pouring temperature increased to 750?C,the distribution of elements across the interface was extremely uneven,at which the Mg element was gathered at the layer I and layer III,but Ni and Al elements segregated at the layer II.To further demonstrate the microstructure evolution of Ni-coated TC4/AZ91D bimetals produced at different pouring temperatures,areas marked in Fig.8 were observed and analyzed at high magnification the results were corresponding to Fig.10(a-g) in sequence,and the EDS analysis results of the spots marked in Fig.10 were summarized in Table 2.

Fig.6.The macro-photographs of the Ni-coated TC4/AZ91D bimetal samples;(a) 660 ?C,(b) 690 ?C,(c) 720 ?C and (d) 750 ?C.

As the pouring temperature was set as 660?C,almost half of the Ni coating still untransformed in layer I which was besieged by the Mg2Ni intermetallic compound as shown in Fig.8(b).Normally,there should not be nickel areas remaining in the layer of Mg2Ni eutectic structure.According to the cooling curve as shown in Fig.4 and Mg-Ni phase diagram [24],the temperatures of the thermocouples were slightly higher than the temperature of eutectic reaction of Mg and Ni,thus indicating that the stage when the temperature maintained above the effective level available for the eutectic transformation and element diffusion was insufficien during the cast process of 660?C.However,the joint of Nicoated TC4/AZ91D was obtained without any gaps or cracks,and the width of interface was approximately 45μm in some parts of the interface.In light of the results of the XRD patterns (Fig.7),EDS results (Fig.10 and Table 2) and Mg-Ni-Al system,theτ1-Ni2Mg3Al,δ-Mg and Mg2Ni may form simultaneously in the Al-Mg-Ni system to form the network structure as shown in layer III [25,26].In the middle of reaction zone (layer II) as shown in Fig.8(b),some massive phases were observed,which suggested that they were discontinuous blocky Mg-Al-Ni ternary compounds.In addition,the (Mg2Ni andδ-Mg) eutectic structure was the major constituent of the layer adjacent to the TC4 substrate (layer I)formed through the eutectic transformation [24]:When the pouring temperature was increased to 690?C,all the width of the layer I,II and III increased.The untransformed Ni coating which was showed in Fig.8(c)disappeared in layer I.Meanwhile,the size of the blocky Mg-Al-Ni ternary compounds in layer II became larger than that of which fabricated at 660?C.Basically,owning to the strengthening of diffusion capacity,the relative content of Mg and Al elements in layer II had an obvious increase as shown in Fig.10(b) point 6.Therefore,a littleδ-Mg separated out in layer III due to the segregation of Mg element.

Fig.7.The XRD patterns of the constituent phases at the interfaces of the Ni-coated TC4/AZ91D bimetals cast at 660?C-750?C.

Table 2 EDS analysis results corresponding to points marked in Fig.10.

Fig.8.SEM micrographs of interface microstructures of the TC4/AZ91D bimetals (a) without nickel coating cast at 720 ?C;with nickel coating and cast at(b) 660 ?C,(c) 690 ?C,(d) 720 ?C and (e) 750 ?C.

As the pouring temperature increased to 720?C,the width of the interface region reached to 80μm.Generally,the increased pouring temperature promoted the diffusion of elements,therefore,the nickel element distributed homogeneous in the Mg-Al-Ni ternary compound at the interface zone.As the EDS analysis of the point 7 and point 10 marked in Fig.10(b) and (d),the phase composition of these two points were 65.53%Mg,12.28%Ni and 22.20%Al and 65.96%Mg,21.71% Ni and 12.29% Al,respectively.It could be found that the content of Ni element degraded obviously and the content of Al element increased gradually.As a consequence,the massive block Mg-Al-Ni ternary compound disappeared all the way through the reaction zone and the layer II and layer III were entirely occupied by the network Mg-Al-Ni ternary compounds andδ-Mg.

When the pouring temperature of 750?C was adopted,the largest width of 120μm of the interface reaction region was obtained and the recorded temperature of the thermocouple was also the highest as shown in Fig.4.As a consequence,during the solidification more Mg element diffused from the AZ91D melt into the nickel coating,leading to plenty ofδ-Mg participated in layer I.Meanwhile,the segregation of Al and Ni elements was also remarkable as shown in Fig.9(d).And the phase composition of point 16 in Fig.10(f) were 40.38% Ni,41.62% Al and 18% Mg.Based on the Al-Ni system,the grey and continuous banded structures in layer II could be verifie as the Al3Ni intermetallic which formed through the eutectic transformation [27]:

3.2.Microhardness distribution

Fig.9.EDS line scan spectra across the interfaces of Ni-coated TC4/AZ91D bimetals,as marked by dashed lines in Fig.8,respectively.

Fig.11 showed the microhardness profile across the bonding interface of Ni-coated TC4/AZ91D bimetallic composite of the various pouring temperatures from 660?C to 750?C.It could be observed that the microhardness distribution at the interface zones exhibited a same trend when the pouring temperature was set as 660?C,690?C and 720?C,but which were different with the micro-hardness profile for 750?C.As the pouring temperature increased from 660?C to 750?C,the morphology of layer I consisted eutectic structure (Mg2Ni+δ-Mg) gradually transformed,and the area ratio ofδ-Mg also increased.Therefore,the microhardness of layer I decreased with the increase of the pouring temperature.Layer II mainly composed of Mg-Al-Ni ternary phases when the pouring temperature was set as 660?C,690?C and 720?C,thus the microhardness value of this layer was relatively stable and lower than layer I.However,when the pouring temperature was increased to 750?C,the interfacial microhardness value in layer II increased sharply to 334 HV,according to the above analysis,this change of the microhardness value could be attributed to the generation of Al3Ni hard intermetallic.For layer III,it mainly composed of Mg-Ni-Al ternary phases andδ-Mg thus had a stable hardness value of about 90 HV.

3.3.Shear strength and fractography

Fig.12 presented the results of shear strength tests of TC4/AZ91D bimetallic composites.As can be seen,due to the inner cladding,which the surface of TC4 rod was covered by AZ91D melt,was adapted in this study,the directly joint of TC4/AZ91D by SLCC without a nickel interlayer had the shear strength of 25MPa.By contrast,with the pouring temperature increased from 660?C to 720?C,the shear strength of the Ni-coated TC4/AZ91D joints raised from 47MPa to 97MPa,indicating that the interfacial bonding was strengthened by the increase of pouring temperature.However,the shear strength began to decline as the pouring temperature was increased to 750?C,the decrease of the shear strength corresponded to the pushing effect of the generation of excess eutectic phase and the Al3Ni hard intermetallic at the joint.

In order to further analyze the fracture behaviors of Nicoated TC4/AZ91D bimetallic composites,the fractured surfaces were examined using SEM micrographs.It could be observed that,all the fracture occurred at the bonding interface.Fig.13(a) showed the fracture morphology of the joint fabricated at 660?C.As can be seen,the failure surface was divided into two distinct regions by a yellow dotted line.On the left side of the yellow dotted line,some river-like patterns appeared on the fracture surface revealed the lamellar tearing occurred between the compound layers in this region.On the right side of the yellow dotted line,the fracture morphology was relatively flat and the EDS spot analysis in this region showed the composition was close to TC4,indicating that the fracture failure here could be imputed to the absence of bonding.

Fig.10.High-magnificatio of SEM micrographs of the joints corresponding to Fig.8A-G in sequence.

Fig.11.Vickers microhardness profile across the interfaces of Ni-coated TC4/AZ91D bimetals casted at different pouring temperatures.

Fig.12.Shear strength of the TC4/AZ91D bimetals without and with Ni coating in different pouring temperatures.

Fig.13(b)showed the fracture surface of the joint obtained at 690?C which displayed features with great height fluctu ation.Comparing with the fracture morphology of the joint fabricated at 660?C,the amount of compound layers which suffered lamellar tearing had a remarkable increase in this interfacial region.The fractured layers are distributed all the fracture surface which indicating a cleavage fracture in this case.The EDS point analysis of the fractured layers marked as spot scan shown in Fig.13(c) demonstrates that the phase composition of this point was 49.78% Mg,28.36% Al and 21.86% Ni,which corresponds to the interfacial blocky Mg-Al-Ni ternary phase.Meanwhile,EDS analysis also revealed that the Ti content in lower region was much higher than the upper region at the fracture surface.Therefore,it could be concluded that the fracture is propagated both at Ti-Ni interface to the Mg-Ni interface.And,the uniform interfacial structure in this case proved to be harmful to the shear strength of the samples.

Further,Fig.13(d) shows an even fracture surface of the Ni-coated TC4/AZ91D compound casting which was cast at 720?C.Fig.13(e) showed the high magnificatio of the areas marked in Fig.13(d).As shown,many fin particles were found at the fracture surface.According to the EDS results,these particles were Mg-Al-Ni ternary phase.Due to the dispersion strengthening of these particles,the shear strength of the samples in this case was improved significantl .

Fig.13(f) shows the fracture surface of the Ni-coated TC4/AZ91D compound castings cast at 750?C,the moreδ-Mg was observed at the fracture surface,indicating the interfacial reaction was more sufficient However,the joints shear strength decreased and the fracture surface was characterized with scraggly area in this case.A distinct fault could be observed in the intermediate region of the fracture surface.Fig.13(g) shows the high magnificatio of the areas marked in Fig.13(f).According to the EDS spot scan analysis,the fault could be confirme as Al3Ni hard intermetallic.

4.Discussion

Fig.14 illustrated the bonding process of the Ni-coated TC4/AZ91D bimetallic composites.The schematic figur mainly presented the diffusion behaviors of the elements and the precipitated phases at different pouring temperatures.Fig.14(a) showed the calculated formation enthalpy of binary system based on the Miedema model and Toop model[28,29].As it shown,the Mg and Ti could not react spontaneously because of the positive formation enthalpy value.By contrast,the Al-Ni and Mg-Ni could form binary compounds easily due to the negative standard molar enthalpy of formation,with a disparate driving force.During the fillin and solidificatio process of magnesium melt,the nickel coating not merely wetted the surface of TC4 rod but also protected the TC4 rod from oxidation as shown in Fig.14(b).

As can be seen from Fig.14(c),during the solidificatio process,the Ni element dissolved and diffused from Ni coating to magnesium melt which was promoted by the concentration gradient.Due to the high solubility of Ni element in Mg melt,the Ni coating dissolved and vanished quickly.As a result,the solubility of Ni element reached the limit firstl at the Mg/Ni interface.However,the magnitude of diffusion coefficient on both sides of the liquid/solid interface varied,the diffusivity of nickel in magnesium(D0NiinMg=1.2×10-9m2s-1) was lower than that of magnesium in nickel (D0MginNi=5.6×10-4m2s-1) [11].Therefore,it was easy to conclude that the diffusion rate of Mg into the Ni coating was much faster than that of Ni into the Mg melt liquid zone.Hence,the Mg2Ni rod phases was firstl precipitated at the interface of layer I as shown in Fig.14(d).When the pouring temperature was set as 690?C,the diffusion of Ni element from the layer I to layer II was inadequate,leading to the participation of the nickel-rich blocky Mg-Al-Ni ternary compounds along the boundary of layer II adjacent to layer I as shown in Fig.14(d).

Fig.13.SEM fractographies and EDS analysis of Ni-coated TC4/AZ91D bimetals cast at:(a) 660 ?C,(b) 690 ?C,(c) area H in b,(d) 720 ?C,(e) area I in d,(f) 750 ?C and (g) area J in f.

Afterwards with the pouring temperature increased from 690?C to 720?C,Ni elements diffused further into the region away from the interface,resulting in the relatively uniform distribution of elements as the line scan results showed in Fig.9(b).Thus,the uniform structure of layer I,II and III was obtained in the interface zone of Nicoated TC4/AZ91D bimetal samples in this case as shown in Fig.14(e).Meanwhile,based on the Mg-Ni-Ti ternary phase diagram reported by Denys et al.,the formation temperature of Mg3TiNi2was 500?C [30].According to the results of the XRD patterns displayed in Fig.7(b) and the analysis of EDS spot scan showed in Fig.10(f),the phase composition of point 14 was 53% Mg,21.71% Ni,15.5%Ti and 9.79% Al.Thus,the production of Mg-Ni-Ti ternary phase could be determined at the Ni-Ti interface as shown in Fig.14(e).

Fig.14.Schematic of bonding mechanism:(a) calculated formation enthalpy of binary system;(b) fillin process;(c) diffusion process of Ni,Mg,Al,Ti atoms at the interface region;(d) solidificatio behavior of the joint at 690 ?C;(e) 720 ?C;(f) 750 ?C.

As the pouring temperature further increased to 750?C,the diffusivity of elements was enhanced.More Mg atoms tended to segregate in layer I and layer III causing the participation ofδ-Mg in these interface reaction region as shown in Fig.14(f).However,Ni and Al atoms preferred to gather in the region of layer II.Due to the lower formation enthalpy of the Al and Ni binary system than the Mg and Ni binary system,Al3Ni binary phase was generated more easily in this region.In general,the mechanical properties of intermetallic compounds can be evaluated by the value of bulk modulus(B)/shear modulus (G),when B/G is less than 1.75,the material is brittle.Otherwise,it is ductile [31].Based on the research of Zheng et al.[32],the value of B/G of Mg2Ni and Al3Ni are 2.33 and 1.75,respectively.It indicated that the Al3Ni is brittle phases while Mg2Ni is ductile phases.Therefore,it could be concluded that the Al3Ni weaken the shear strength of the samples when the temperature was set as 750?C.

Fig.4 showed the time-temperature curves of the thermocouples at the different pouring temperatures.As shown,when the pouring temperature was increased to 750?C,the temperature of the thermocouple was the highest and reached to 687?C,which was 85?C higher than that was poured at 660?C.For the fabrication of Ni-coated TC4/AZ91D bimetals,microstructure evolution of the interfacial area was affected by the features of magnesium melt such as fluidit,wettability and diffusivity.However,these properties of the liquid metal mainly depended on the casting temperatures.

In general,the wettability acted as a significan factor in determining the reciprocal diffusion behavior between the Nicoated TC4 rod and molten AZ91D melt.To analyze the wettability of AZ91D melt to Ni coating,a model based on the thermodynamics,molecular interaction and Gibbs adsorption isotherm was adopted to infer the relation between the temperature (T) and the contact angle (θ) which was performed by Adamson [33]:

WhereT∞is the temperature when the contact angle goes to zero which was called a pseudo-critical temperature,a and b represented the balance of intermolecular forces and was define as constants,independent of temperature.C is the integration constant.In this study,when the pouring temperature was set as 660?C,the contact angle in Eq.(4) was bigger than that of 690?C,720?C and 750?C,thus leading to an inadequate interface reaction with liquid of magnesium as shown in Fig.8(b).With the increase of pouring temperature,the contact angle decreased gradually and it promoted the interfacial reaction of Ni-coated TC4/AZ91D bimetals.

According to the above descriptions,the different elements concentration determined the microstructural evolution within the interfacial zone during the SLCC process.To study the diffusion behaviors of elements,the Arrhenius equation was adopted based on the vacancy and interstitial diffusion mechanisms to explained the temperature (T) dependences of the diffusion coefficien (D) [34]:

Where D0is preexponential factor which has no relation with temperature,EAis the activation energy for diffusion,and R is the universal gas constant.According to the description of formula 5,the diffusivities of magnesium,nickel and aluminum atoms at the interface could be accelerated by the increased pouring temperature.When the AZ91D melt was cast at a pouring temperature of 660?C and 690?C,the temperature of system dropped rapidly and seriously decreased the mobility of the magnesium,nickel and aluminum atoms.Thus the unreacted Ni coating and the massive Mg-Al-Ni ternary phase existed in the interface region of layer I and layer II in these two cases,respectively.As the pouring temperature increased to 720?C and 750?C,the increased width of the interface zone could prove the motility of atoms under different temperatures was improved obviously.

5.Conclusions

(1) With the application of the Ni coating on TC4 substrate,the AZ91D/TC4 bimetallic material could be prepared successfully at the pouring temperatures of 690?C,720?C and 750?C,expect 660?C.As the pouring temperature was increased from 690?C to 720?C,the width of interface reaction zone comprising theδ-Mg,Mg2Ni andτ1-Ni2Mg3Al phases augmented incessantly.When the pouring temperature was increased to 750?C,Al3Ni phase began to appear in the interface region mixed with the other phases mentioned above.

(2) The microhardness of the interfacial reaction zone of Ni-coated TC4/AZ91D bimetals was significantl higher than that of as-cast AZ91D matrix but lower than that of titanium substrate.The increased pouring temperature resulted in an increment of the microhardness and brittleness in the interface zone.

(3) The shear strength of the Ni-coated TC4/AZ91D was increased as the pouring temperature increased from 690?C to 720?C due to the formation of the Mg-Al-Ni particles.However,as the pouring temperature was increased from 720?C to 750?C,the shear strength of the Ni-coated TC4/AZ91D decreased from～97.05MPa to～86.05MPa due to the generation of Al3Ni hard intermetallic and the expansion of the interface zone.

Compliance with ethical standards

This research completely confir to the ethical standards.

Declaration of Competing Interest

The authors declare that they have no conflic of interest.

Acknowledgment

The authors would like to acknowledge the financia supports from the National Natural Science Foundation of China(No.51875062).