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Page 1: Microstructure and Mechanical Properties of Mg-6Zn-2Sn-1Al ......method analysis, the Mg-6Zn-2Sn-1Al alloy was manufactured. Also, the extruded Mg-6Zn-2Sn-1Al alloys were evaluated

Microstructure and Mechanical Propertiesof Mg-6Zn-2Sn-1Al Alloy Designed by Numerical

Analysis with Extrusion ProcessK.W. Lee, B.D. Lee, U.H. Baek, E.H. Kwon, and J.W. Han

(Submitted March 28, 2013; in revised form July 16, 2014; published online January 7, 2015)

In the extrusion process, the temperature of the workpiece results in non-uniformity of dimensions,microstructure, and mechanical properties of the product. Although many researchers are expected toparticipate in the extrusion behavior of Mg alloys, no specific information is available yet to clarify theirroles in extrusion process of Mg alloys because of a wide variety of compositions. In this study, a goodunderstanding the role of die in the extrusion process is expected to contribute to the improvement ofprocessing efficiency for Mg-6Zn-2Sn-1Al alloy. To design Mg-6Zn-2Sn-1Al alloy, the parameters, such astemperature and angle of the designed materials, were determined using the commercial softwareDEFORM 3D. With this simulation model, the real-time extrusion temperature and angle of the die wereadjusted according to the simulation results. Using the optimal extrusion process predicted by finite elementmethod analysis, the Mg-6Zn-2Sn-1Al alloy was manufactured. Also, the extruded Mg-6Zn-2Sn-1Al alloyswere evaluated on the microstructure and mechanical properties.

Keywords extrusion process, FEM analysis, mechanical proper-ties, Mg alloys, microstructure

1. Introduction

The weight of transportation vehicles, such as automobilesand airplanes, needs to be reduced to improve fuel consumptionand help address some of the concerns regarding greenhouseemissions (Ref 1, 2). Magnesium alloys have the lowest densityamong existing common alloys. Unfortunately, such magne-sium alloys have limited use owing to their low ductility andcorrosion resistance compared to other light materials. Therelatively low strength and ductility of Mg alloys due to thehexagonal close-packed (HCP) structure with limited slipsystems is a major impediment to their widespread application.Many limitations have been overcome with the advances in thealloy and plastic deformation technology (Ref 3-5).

The established trend in weight reduction will continue andmagnesium alloys will penetrate further into power trainapplications. As far as some applications are concerned, suchas transmission housings and crankcases, the existing perfor-mance criteria are placed on the elevated-temperature( ‡ 150 �C) mechanical properties. Magnesium is unsuitablefor use at temperatures greater than 120 �C owing to its poorcreep resistance and strength at elevated temperatures (Ref 6).Therefore, it is important to develop some high-strengthmagnesium alloys for elevated-temperature automotive

applications. Many studies have examined the age hardeningproperties of Mg-RE (RE = rare earth elements) casting alloysfor the development of excellent elevated-temperature mechan-ical properties (Ref 7-9). On the other hand, only limitedapplications are expected from Mg-RE alloys due to the highcost of RE. Therefore, it is essential to explore promisingmagnesium alloys with excellent elevated-temperature mechan-ical properties that do not use rare earth elements. Many studieshave been carried out to improve the elevated-temperaturemechanical properties of AZ91 by alloying and micro-alloyingover the past few years. On the other hand, no desirable resultshave been achieved.

The aim of the study was to develop an efficient simulationmodel for the extrusion process to enhance the mechanicalproperties of Mg-6Zn-2Sn-1Al alloy using the hot-extruded andheat-treatment process. In this study, the mechanical propertiesand precipitate of Mg-6Zn-2Sn-1Al extruded alloys wereexamined by FEM (Finite Elements Method) analysis to helpdetermine the optimal conditions to improve the mechanicalproperties at room and elevated temperatures. Based on thethermodynamic calculations, heat treatment was conducted at150, 200, and 250 �C. This study is expected to contribute tothe design of new Mg alloys by helping control the propertiesof the alloy. These alloys will also provide useful informationon new Mg alloys to industries that utilize Mg through theappropriate combination of information.

2. Numerical Models

A finite element simulation of extrusion was performedusing commercial DEFORM_3D designed for modeling theforging and extrusion processes. This package is used widely inthe metal forming industry because of its accuracy and

K.W. Lee, B.D. Lee, U.H. Baek, E.H. Kwon, and J.W. Han, Inhauniversity, Incheom KS006, Republic of Korea. Contact e-mails:[email protected] and [email protected].

JMEPEG (2015) 24:1253–1261 �ASM InternationalDOI: 10.1007/s11665-014-1215-1 1059-9495/$19.00

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effectiveness. The system provides a thermo-mechanical inter-active simulation of metal flow where the flow stress isrepresented as a function of the strain, strain rate, andtemperature in analytical or table form. The velocity of theram was kept constant during the simulation for this extrusion.The simulation of extrusion was performed under the followingconditions:

• The as-cast ingots were extruded into the cylinder-shapedspecimen. The extrusion ratio is 10:1.

• The initial billet and material temperatures were 250, 300,and 350 �C. The ram velocity was 10 mm/s.

The bearing angle of the die was 0�, 5�, and 10�. The frictioncoefficient used was 0.3. Table 1 lists the billet dimensions andprocess parameters used in the numerical simulation. Figure 1shows the geometry of the extrusion die billet and container.

3. Experimental Methods

The alloy ingots with nominal compositions of the Mg-6Zn-2Sn-1Al alloy were prepared from high purity Mg(>99.95 wt.%), Al (>99.9 wt.%), Sn (>99.9 wt.%), and Zn(>99.9 wt.%) by melting in an electric resistance furnace atapproximately 740 �C under a mixed atmosphere of CO2 andSF6. The chemical components of the casted Mg alloys were6.42 wt.% Zn, 2.09 wt.% Sn, and 1.02 wt.% Al, respectively.

The ingots of Mg-6Zn-2Sn-1Al alloy were held for 6 h at300 �C, and the diameter of the ingots was approximately70 mm. Before extrusion, extrusion die and the Mg alloy wereheated to 250, 300, and 350 �C for 60 min, respectively. And,

Table 1 Extrusion process and simulation parameters

Materials size, mm (X9 Y9 Z) 209 209 100Materials temperature, �C 250, 300, 350U 0�, 5�, 10�Ram speed, mm/s 10Friction factor at all interfaces 3

Fig. 1 Schematic illustration of extrusion die used for numerical simulation

Fig. 2 The thermodynamic calculation (a) equilibrium phase diagram (b) volume fraction of Mg-6Zn-2Sn-1Al alloy

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Fig. 3 Deformation characteristic in extruded billet (a) temperature distribution (b) point position for point tracking during the extrusion (c) tem-perature of point P1, P2, and P3

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the Mg alloy was extruded into bars at 250, 300, and 350 �Cwith an extrusion ratio of 10:1. The extrusion rate was 10 mm/sfor all trials, and the extruded Mg alloy bar was air cooled. Theextruded samples were cut for microstructure analysis. Crosssections were prepared for optical microscopy, and the meangrain size was measured using an image analyzer in thecomputer program. The tensile test specimens were preparedfrom longitudinal axis of the extrusion direction. The preparedspecimens were annealed for 1 h at 150, 200, and 250 �C.

Depending on the annealing conditions, the mechanicalproperties were analyzed at room and elevated temperatures witha strain rate of 10�3 s�1. The temperature of the tensile test was20, 100, and 200 �C. The microhardness was measured using aVickers hardness (HV) tester. HV was measured at a minimumof ten points to reduce the error. The heat-treatment conditionswere as follows: the as-extrudedMg alloy was solution-treated at300 �C for 6 h, and the Mg alloy was aged for 1 h at 150, 200,and 250 �C under vacuum conditions. The phase and micro-structure of the alloys were analyzed by x-ray diffraction (XRD)and scanning electron microscopy (SEM), respectively. Themeasurements for phase identification were conducted using aRigaku DMAX 2500 x-ray powder diffractometer using Cu-Kaand operated at 40 mA and 40 kV. The XRD step scan wasperformed over the range, 20�-90� 2h with 3� increments and a

1 min dwell time for each step. The microstructure of thedifferent alloys, the different phases present, and the influence ofthermal treatment on the microstructural evolution were exam-ined by SEM (HITACHI S-4300SE). This instrument wasequipped with an Energy Dispersive Spectrophotometer (EDS)for elemental microanalysis of the sample and for measuring thecompositions of the different phases.

4. Results and Discussion

4.1 Thermodynamic Calculations

The phase diagrams of the ternary Mg-xZn-2Sn alloy andquaternary Mg-xZn-2Sn-1Al alloy were calculated using thePandat program. Figure 2(a) shows the phase transformation ofthe Mg-xZn-2Sn alloy and Mg-xZn-2Sn-1Al alloy as a functionof temperature. The phase zone of a-Mg in the Mg-xZn-2Snalloy and Mg-xZn-2Sn-1Al alloy was approximately 400 �C.a-Mg + Mg2Sn were observed when the temperature of thealloy was less than 300 �C. In the case of the Mg-xZn-2Snalloy, a-Mg + Mg2Sn + MgZn phases were detected at tem-peratures <250 �C. On the other hand, the phase zone in theMg-xZn-2Sn-1Al alloy showed an a-Mg + Mg2Sn + MgZn

Fig. 4 Deformation characteristic in extruded billet (a) strain distribution (b) strain rates of point P1, P2, and P3

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phase and a-Mg + Mg2Sn + MgAlZn + MgZn phase at tem-peratures >250 �C. In addition, the phase zone of a-Mg anda-Mg + Mg2Sn in the Mg-xZn-2Sn-1Al alloy was diminishedslightly.

When alloys with a new composition are designed, thisinformation will be helpful for determining the appropriateannealing temperature. Therefore, a thermodynamic calculationwill be used for providing important data for estimating theeffects of grain refinement by precipitate phases in heat-treatment or hot-extruded process. In this study, the precipitatefractions of MgZn, Mg2Sn, and MgAlZn phases were calcu-lated by thermodynamic calculations to examine the annealingtemperatures of Mg-xZn-2Sn-1Al alloys. Figure 2(b) showedthat the precipitate phases of Mg-xZn-2Sn-1Al alloy increasedwith decreasing temperature, and the liquidus line was locatedat approximately 370 �C. Accordingly, the suitable annealingtemperature for the Mg-xZn-2Sn-1Al alloy is less than 370 �C.

4.2 Numerical Analysis on Extrusion Process

The bearing angle of the die was 0�, 5�, and 10�. The initialbillet and material temperature (=Mg alloy) were 250, 300, and350 �C, respectively. A simulation of the extrusion wasperformed at ram velocities of 10 mm/s. Figure 3(a) showsthe temperature distribution depending on the changing bearingangle of the die. The temperature distribution of the alloy with abearing angle of 0� is disproportionate. The temperature

distribution of the alloy with a bearing angle of 5� is relativelyhigh in the outer spots with an output grater than 10�. On theother hand, the temperature of the alloy with a bearing angle of10� was distributed uniformly.

Figure 3(b) presents the point positions at different stepsduring hot extrusion. Figure 3(c) shows variations of temper-ature depending on the times during the extrusion, and points of1, 2, and 3 in the figure were used to label the specimen spot.As shown in Fig. 3(b), the point positions pass the entrance andoutput of the die with different bearing angles. When extrusionbegins, the material around the entrance is crushed into the die.The peak stress and strain rate values appear when significantdeformation occurs. Therefore, the temperature of point P3 isthe highest. The temperature of the alloy with a bearing angleof 0� increased rapidly.

Figure 4(a) shows the strain rate depending on the changingbearing angle of the die. The strain rate of the alloy passed diewith a bearing angle of 10� was less than the alloy with abearing angle of 0� and 5�. Therefore, the strain rate of the alloywith a bearing angle of 10� showed good deformation. In bothsides, the alloy showed a high strain rate. Figure 4(b) presentsthe changes in strain rate as a function of time during extrusion.Points 1, 2, and 3 in the figure were used to label the specimenspot, respectively. Significant deformation occurs when P3passes the entrance of the die. Therefore, the apparent peakvalue in the strain rate curve of point P3 appears for the firsttime, as shown in Fig. 4(b).

Fig. 5 Deformation characteristic in extruded billet (a) temperature, and (b) strain distribution

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In Fig. 4(b), the strain rate curves of P1 and P2 have asimilar shape, whereas those of P3 were dramatically different.The strain rate of the alloy with a bearing angle of 0�, 5�, and10� increases with deformation and has only a single peakvalue at P3. On the other hand, the strain rate of the alloy with abearing angle of 0� during the deformation is higher than that ofthe alloy with a bearing angle of 5� and 10�. In the alloy with abearing angle of 0�, the strain rates of P3 increased dramaticallyat the first time more than that of the alloy with a bearing angleof 5� and 10�. The reason is that P3 suffers severe deformationat the initial stages of extrusion and reaches a strain rate peakvalue at the first time. Afterward, P3 passes the output of thedie, and significant deformation is given to P3, hence, thesecond peak strain rate emerges. Therefore, another peak(P1 and P2) and a peak at P3 should be incorporated in theshortest time. The reason is that deformation and microstructureof the alloy are distributed, and the strain of the alloy with abearing angle of 10� was distributed uniformly.

Figure 5 shows the strain rate and temperature distributionof an alloy with a bearing angle of 10� depending on thechanging temperature of the billet (250, 300, and 350 �C).Figure 5(a) shows the temperature distribution depending onthe changing temperature of the billet (250, 300, and 350 �C).Figure 5(b) shows that the temperature of the alloy with a billetof 250 and 300 �C was distributed uniformly. On the otherhand, the temperature distribution of the alloy with a billet of350 �C was higher. Therefore, the microstructure of the product

produced by the extrusion process with a billet temperature of350 �C will be coarse and irregular. The strain rate of the alloywith a billet temperature of 250 �C during the entire deforma-tion was higher than that of the alloy with a billet temperatureof 300 and 350 �C. The strain rates of the alloy with a billettemperature of 300 and 350 �C were similar.

Fig. 6 Microstructure of the alloy after extrusion (a) as-extrusion (b) heat-treatment at 150 �C (c) heat-treatment at 200 �C (d) heat-treatment at250 �C

Fig. 7 X-ray diffraction patterns of experimental alloys after hot-extruded

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4.3 Application of Extrusion Process

Based on the simulation results, extrusion of the alloy wasperformed at 300 �C with a die angle of 10�. Figure 6(a) to(d) shows optical micrographs of a hot-extruded alloy and heat-treated alloy after hot extrusion along the extruded direction. Thegrain sizes increased significantly with increasing heat-treatmenttemperature. The grain size of the as-extrusion sample and heat-treated alloy at 150, 200, and 250 �C after extrusion wasapproximately 18, 14, 12, and 12 lm, respectively.

These results are due to the growth of the recrystallizedgrains with increased temperature after being extrusion. Inaddition, by increasing heat-treatment temperature, the

increased precipitate fraction inhibited the grain growth. Thisis consistent with calculated precipitate fraction and XRDresults.

Figure 7 shows XRD patterns of the as-extruded alloys andthe heat-treated alloy after hot extrusion. XRD was conductedto confirm the crystallographic identity of the phases in theMg-6Zn-2Sn-1Al alloy. The alloys were composed mainly ofa-Mg, MgAlZn, Mg2Sn, and MgZn phases in the heat-treatedalloy after hot extrusion. The amount of observed phases suchas MgZn and Mg2Sn is small quantities. These results areconsistent with the thermodynamic results. The simulationresults are indicative, and this study can be used as an indicator

Fig. 8 The SEM image of alloy with different heat-treatment temperature after hot-extruded (a) as-extrusion (b) heat-treatment at 150 �C (c)heat-treatment at 200 �C (d) heat-treatment at 250 �C

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for specific trends, such as the heat-treatment temperature andalloy design. Therefore, to precipitate an appropriate secondaryphases, it is important to help simplify the experimental processbased on the results of the thermodynamic calculation. Manystudies have reported on the effect of secondary phases that themechanical properties of the alloys with Mg, Zn, and Snelements are improved by the formation of intermetalliccompounds, such as Mg2Sn and MgZn phases (Ref 10-13).

Figure 8 shows SEM images of the Mg-6Zn-2Sn-1Al alloy.The results revealed needle-like phases as well as some clusterphases in the alloys. This suggests that the size of the MgZnphase became coarser with increasing heat-treatment temper-ature (Ref 5, 6, 14), indicating that the solubility of Zn in Mgvaried with the heat-treatment temperature, as shown inFig. 8(b). In particular, after heat treatment at 250 �C, thephases were relatively coarse. In addition, the phases in theheat-treated alloy formed around the grain boundary. EDSanalysis was conducted to identify the composition of thesephases.

4.4 Mechanical Properties

Figure 9(a) shows the hardness of the alloys depending onthe heat-treatment conditions. The results showed high hardnesswith increasing temperature. The hardness was highest after heattreatment at 200 �C. Figure 9(b) to (d) shows the tensileproperties of the Mg-6Zn-2Sn-1Al alloy. The results suggestthat compared to the different heat-treatment conditions, theyield strength of the alloy after heat treatment at 200 �C wasslightly higher, which corresponds to the hardness, as shown inFig. 9(c). In addition, the tensile strength of the alloy after heattreatment at 200 �C was slightly higher in the tensile test at RT,

100 and 200 �C, as shown in Fig. 9(b). On the other hand, theelongation of the alloy after heat treatment at 200 �C was thelowest, whereas the elongation of Mg-6Zn-2Sn-1Al alloyextruded without heat treatment was the highest, as shown inFig. 9(d). The others in Fig. 9(d) were similar to the elongation.Therefore, some heat-treatment conditions on the extrudedalloys are more effective on the mechanical properties at thehigh temperature range than the room temperature range. On theother hand, the elongation was smaller with the heat-treatedalloy than the extruded alloy. Therefore, the heat-treatmenttemperature of the Mg-6Zn-2Sn-1Al alloy must be limited to acertain range. These results were attributed to the precipitatephase, such as the MgZn andMg2Sn phase with thermally stablecharacteristics. Therefore, to precipitate an appropriate second-ary phase, it is important to help simplify the experimentalprocess based on the results of thermodynamic calculations.

5. Conclusions

In this study, the extrusion behavior of the Mg-6Zn-2Sn-1Alalloy was investigated using simulation programs to determinethe effect of extrusion on the mechanical properties. The mainconclusions of this study are summarized below:

(1) Favorable conditions for the extrusion of Mg-6Zn-2Sn-1Al alloy were determined using the flow stress andtemperature data obtained for a series of hot extrusionbased on FEM simulations. The possibility of crack for-mation was quite high under some hot-extrusion condi-tions. Based on the present extrusion process modeling,

Fig. 9 Mechanical properties (a) hardness (Hv) (b) Tensile strength (c) yield strength (d) elongation

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the extruded material of the Mg-6Zn-2Sn-1Al alloy wasfabricated.

(2) The Mg-6Zn-2Sn-1Al alloy heat treated at 200 �C afterextrusion showed the best mechanical properties at roomtemperature and elevated temperatures.

(3) The use of an appropriate thermodynamic calculation isimportant for simplifying the experimental process andimproving the mechanical properties.

Acknowledgment

This work was supported by INHA UNIVERSITY ResearchGrant.

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