Microstructure and mechanical properties of Mg-Nd-Zn-Zr billet prepared by direc

时间：2024-07-28

Xingwei Zheng,Jie Dong,Shiming Wng

aCollege of Engineering Science and Technology,Shanghai Ocean University,201306 Shanghai,PR China

bNational Engineering Research Center of Light Alloy Net Forming,Shanghai Jiao Tong University,200240 Shanghai,PR China

1.Introduction

Direct chill(DC)casting was invented in the 1930s and found its practical application as a common way to produce billet(ingot)for further deformation processing[1].The main benefit of DC casting is that the solidification occurs in a relatively narrow layer of the billet and can be well controlled.

The DC casting of aluminum alloy has been extensively investigated,and excellent reviews are available on the development together with the insights into parameters[1].Magnesium alloys have attracted increasing attentions for their high specific strength and low density in recent years.The application of the magnesium alloys,especially wrought products,is increasing year by year.With the increasing demand of wrought magnesium alloys,more and more attentions have been paid to their DC casting.Progresses have been made on this topic in both experimental and simulating aspects during last decades[2-8].

NZ30K(Mg-3.0Nd-0.4Zn-0.4Zr)alloy is a newly developed magnesium alloy with middle strength and good toughness in as-cast condition.The investigations have shown that the NZ30K alloy possesses good mechanical properties at ambient and elevated temperatures.And the corrosion resistances of as-cast,T4 and T6 NZ30K alloys are better than AZ91D[9,10].Furthermore,our previous investigations also confirm its industrial superiority in terms of formability and forgeability behaviors[11,12].In addition,the NZ30K magnesium alloy has manifested itself as a promising biodegradable material,and attracted a lot of attentions due to a much better bio-corrosion resistance it possesses than AZ31 and WE43 alloys,as exhibited under a human-body mimic environment[13,14].

With increasing demand of NZ30K alloy for high performance wrought products with large size,high-quality NZ30K billets prepared by DC casting are urgent needed.Therefore,NZ30K alloy billets with diameter of 200 mm were prepared by DC casting in this work.The microstructure,macrosegregation and mechanical properties of the NZ30K alloy billet with diameter of 200 mm were investigated.

Fig.1.Schematic(a)and finite element model(b)of DC casting.

2.Experimental material and procedure

2.1.Alloy preparation and characterization

An alloy of nominal composition Mg-3.0Nd-0.4Zn-0.4Zr(wt.%)was prepared by pure Zn,Mg,Mg-30Nd and Mg-30Zr master alloys(wt.%)by melting in an electrical resistance furnace under the protection of mixture gas of SF6,CO2and air in this work.The melt was degassed,slag removed and refined at 740°C.The casting temperature,casting speed and water flow for the NZ30K billet with diameter of 200 mm are maintained constant at 710°C,80 mm/min and 75L/min,respectively.Samples for the microstructure observation were etched with a phosphoric-picral solution(0.7mL phosphoric acid,4.2g picric acid and 100mL ethanol).The grain size was measured by the linear intercept method.

The variations of the chemical compositions along the radius were measured by an Inductively Coupled Plasma Analyzer(ICP).The alloy powder for the ICP measurement was drilled from the cross-section of the billet at each 10 mm.Tensile samples were cut into rectangular tensile specimens with dimension of 10 mm width,2 mm thickness and 30 mm gauge length by an electric sparking wire cutting machine and which were taken from the center,1/2 radius and the edge of the billet.Tensile tests were conducted on a Zwick/Roell-20KN material machine at a crosshead speed of 1 mm/min at room temperature.

2.2.Temperature simulation by finite element

The billet during DC casting comprises three well defined zones as shown in Fig.1(a).The sump consists of the liquid pool and the transition region.The transition region is bound by liquidus and solidus isotherms and can be further divided into the slurry and the mushy zones with the border between them represented by a coherency isotherm(e.g.at a solid fraction of 0.3 in Fig.1(a)).

The cooling zone of the billet during DC casting can be divided into two parts,which are cooled by the primary cooling and the secondary cooling.And the secondary cooling zone can be subdivided into impact zone and downstream zone(as shown in Fig.1b).It is well known that the cooling rate in the impact zone of the secondary cooling is much higher than the downstream zone,therefore,the mesh density in impact zone is much higher than the downstream zone in order to improve simulation accuracy of the temperature field in the junction between impact zone and downstream zone.

Initial conditions:

The temperature of the magnesium liquid is initialized as the pouring temperature(710°C).And the initial temperatures of mold,dummy bar head and cooling water are all initialized as room temperature(20°C)in this work.

Boundary conditions:

The heat transfer is divided into three zones during DC casting,which are primary cooling zone,impact zone of secondary cooling and downstream zone of secondary cooling(as shown in Fig.1a).The thermal boundary condition is formulated according to Eq.(1):

where Tenis the environment temperature and its value is 20°C;hcontactis the effective heat-transfer coefficient between crystallizer and melt and its value is 145 W/m2/K;hairis 100 W/m2/K.

Temperature field was simulated using ANSYS software.The casting material is NZ30K alloy in this work,which is a new developed heat-resistant magnesium alloy,many thermophysical properties are incomplete.The thermophysical properties for the model of the NZ30K alloy are listed in Table 1,which are all calculated by Pandat software.

Table 1 Thermophysical properties of NZ30K alloy applied in the model.

3.Results and discussions

3.1.Effect of diameter on the temperature field

Our previous work shows that the optimal casting speed and casting temperature for NZ30K alloy with diameter of 100 mm is 90 mm/min and 700°C,respectively[15].Casting parameters have significant effect on the temperature field during DC casting,and which has a great influence on the microstructure,mechanical properties and macrosegregation of billet prepared by DC casting.Therefore,temperature field of NZ30K billets of 100 mm and 200 mm prepared with the same casting parameter is simulated in this work.Fig.2 shows the temperature fields of billets prepared by DC casting at the casting temperature of 700°C and casting speed of 90 mm/min.It can be seen that the temperature fields of the billets with different diameters are almost the same.However,the sump depth increases with diameter,more detail information on the measurement of sump depth refers to previous work[15].The sump depths of billets with diameters of 100 and 200 mm are 53 and 90 mm,respectively,according to the simulation result.The sump depth of the NZ30K alloy billet with diameter of 100 mm prepared at casting speed of 90 mm/min and casting temperature of 700°C is 55 mm[15],which is very close to the simulation result in this work.The sump depth(h)mainly depends on casting speed,alloy type and size of the billet,and it can be calculated by the Eq.(2)[1].

whereAis a coefficient which depends on the alloy(latent heat of fusion,density of the solid,specific heat of the solid);

Vcast:the casting speed;

λs:the thermal conductivity of the solid;

Tm:the melting temperature of the alloy;

Tsurf:the surface temperature of the billet;

L:the radius of the billet;

Decreasing of casting speed and increasing of casting temperature are beneficial for reducing the sump depth of DC casting according to the Eq.(2).Therefore,the casting temperature and casting speed for DC casting of the NZ30K alloy billet of 200 mm in diameter are determined as 710°C and 80 mm/min,respectively.

3.2.Microstructure of NZ30K billet

The typical macrostructure and microstructures of the billet with diameter of 200 mm prepared at the casting temperature 710°C and casting speed of 80 mm/min are shown in Fig.3(a).No cold shut,drag mark,crack and segregation knot can be observed on the surface of the billet.The as-cast microstructure of the billet is mainly composed of equiaxed a-Mg and eutectic compounds distributing along grain boundaries.The eutectic is confirmed to be Mg12Nd in our previous work[11,12].The grain sizes in the center and at the edge of billet are 30 and 40 mm,respectively(as shown in Fig.2b and c).

Fig.3.Microstructure of the billet prepared by DC casting along the radius,macrostructure(a)edge(b)center(c).

3.3.Macrosegregation of NZ30K billet

Macrosegregation is an irreparable defect in large sized DC casting billet(ingot).Furthermore,the presence of macrosegregation sets limitation on the size and composition of the billet(ingot)to be cast in a productive and economical way.Thus,emphasis on the importance of reducing macrosegregation in the DC casting billet cannot be overemphasized.Quantitatively,the macrosegregation can be evaluated by a segregation ratio calculated by Eq.(3)[1]:

wherec0is the average composition of billet;

cmaxis the maximum composition of billet;

cminis the minimum composition of billet;

The distributions of Zn and Nd along the radius of billet are shown in Fig.4.According to the Eq.(3)and Fig.4,the segregation ratios of Zn and Nd are 35%and 14%,respectively.And the segregation ratios of Zn and Nd in billet with diameter of 100 mm are 30%and 11%[15],which means that macrosegregation increases with size of the billet.

Fig.4.Macrosegregation of alloying elements along the billet.

3.4.Mechanical properties of NZ30K billet

Based on the results of simulation,microstructure and macrosegregation of NZ30K alloy,the optimal casting speed and casting temperature for NZ30K alloy billet with diameter of 200 mm produced by DC casting are 80 mm/min and 710°C,respectively.The mechanical properties including ultimate tensile strength(UTS),yield strength(YS)andelongation of the NZ30K alloy billet prepared with the optimal parameters are shown in Table 2.It can be seen that the tensile mechanical properties decrease slightly along the radius of the billet,the reason can be attributed to that the grain size of the billet increases from the edge to the center of the billet.The maximum values of UTS and YS and elongation are 198MPa,116MPa and 14.0%,respectively.And the UTS and YS of the NZ30K alloy prepared by mold casting are 175 MPa and 90MPa,respectively[10].While the UTS and YS of the NZ30K billet with the diameter of 100 mm prepared by DC casting are 196MPa,125MPa and 16.5%respectively[15].The improvement of the mechanical properties the NZ30K alloy billet prepared by DC casting,compared with mold casting,can be attributed to that the cooling rate of DC casting is much higher than that of mold casting,which leads to the grain size being refined.On the other hand,macrosegregation of DC casting billet has a detrimental influence on the mechanical properties of the DC casting billet[1].Macrosegregation increases with diameter of billet according to the calculation result of macrosegregation ratio in 3.3 in this work,which is the reason why the mechanical properties of NZ30K billet with the diameter of 200 mm prepared by DC casting in this work is a little lower than that of billets with diameter of 100 mm prepared by DC casting[11].

Table 2 Mechanical properties of the NZ30K billet with diameters of 200 mm.

4.Conclusions

1 Sound NZ30K magnesium alloy billets with diameter of 200 mm can be prepared by DC casting.the optimum casting temperature and casting speed are determined as 710°C and 80 mm/min,respectively.

2 The as-cast microstructures of the billet are mainly composed of equiaxed a-Mg and Mg12Nd eutectic compound distributing on grain boundaries.The grain sizes in the center and at the edge of billet are 30 and 40 mm,respectively.The maximum values of UTS and YS of NZ30K billet are 198MPa and 116MPa,respectively.

Acknowledgments

This project is supported by National Natural Science Foundation of China(Grant No.51775329 and 51605280),the Foundation of Shanghai youth teacher training scheme(Grant No.ZZSHOU16016)and the Doctoral Scientific Research Foundation of Shanghai Ocean University(Grant No.A2-0203-17-100325).

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