Microstructure and Mechanical Properties of Mg-6Zn-2Sn-1Al ... ... method analysis, the...

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  • Microstructure and Mechanical Properties of Mg-6Zn-2Sn-1Al Alloy Designed by Numerical

    Analysis with Extrusion Process K.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 to participate in the extrusion behavior of Mg alloys, no specific information is available yet to clarify their roles in extrusion process of Mg alloys because of a wide variety of compositions. In this study, a good understanding the role of die in the extrusion process is expected to contribute to the improvement of processing efficiency for Mg-6Zn-2Sn-1Al alloy. To design Mg-6Zn-2Sn-1Al alloy, the parameters, such as temperature and angle of the designed materials, were determined using the commercial software DEFORM 3D. With this simulation model, the real-time extrusion temperature and angle of the die were adjusted according to the simulation results. Using the optimal extrusion process predicted by finite element method analysis, the Mg-6Zn-2Sn-1Al alloy was manufactured. Also, the extruded Mg-6Zn-2Sn-1Al alloys were 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 automobiles and airplanes, needs to be reduced to improve fuel consumption and help address some of the concerns regarding greenhouse emissions (Ref 1, 2). Magnesium alloys have the lowest density among existing common alloys. Unfortunately, such magne- sium alloys have limited use owing to their low ductility and corrosion resistance compared to other light materials. The relatively low strength and ductility of Mg alloys due to the hexagonal close-packed (HCP) structure with limited slip systems is a major impediment to their widespread application. Many limitations have been overcome with the advances in the alloy and plastic deformation technology (Ref 3-5).

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

    applications. Many studies have examined the age hardening properties of Mg-RE (RE = rare earth elements) casting alloys for the development of excellent elevated-temperature mechan- ical properties (Ref 7-9). On the other hand, only limited applications are expected from Mg-RE alloys due to the high cost of RE. Therefore, it is essential to explore promising magnesium alloys with excellent elevated-temperature mechan- ical properties that do not use rare earth elements. Many studies have been carried out to improve the elevated-temperature mechanical properties of AZ91 by alloying and micro-alloying over the past few years. On the other hand, no desirable results have been achieved.

    The aim of the study was to develop an efficient simulation model for the extrusion process to enhance the mechanical properties of Mg-6Zn-2Sn-1Al alloy using the hot-extruded and heat-treatment process. In this study, the mechanical properties and precipitate of Mg-6Zn-2Sn-1Al extruded alloys were examined by FEM (Finite Elements Method) analysis to help determine the optimal conditions to improve the mechanical properties at room and elevated temperatures. Based on the thermodynamic calculations, heat treatment was conducted at 150, 200, and 250 �C. This study is expected to contribute to the design of new Mg alloys by helping control the properties of the alloy. These alloys will also provide useful information on new Mg alloys to industries that utilize Mg through the appropriate combination of information.

    2. Numerical Models

    A finite element simulation of extrusion was performed using commercial DEFORM_3D designed for modeling the forging and extrusion processes. This package is used widely in the metal forming industry because of its accuracy and

    K.W. Lee, B.D. Lee, U.H. Baek, E.H. Kwon, and J.W. Han, Inha university, Incheom KS006, Republic of Korea. Contact e-mails: flyhigh78@gmail.com and jwhan@inha.ac.kr.

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

    Journal of Materials Engineering and Performance Volume 24(3) March 2015—1253

  • effectiveness. The system provides a thermo-mechanical inter- active simulation of metal flow where the flow stress is represented as a function of the strain, strain rate, and temperature in analytical or table form. The velocity of the ram was kept constant during the simulation for this extrusion. The simulation of extrusion was performed under the following conditions:

    • The as-cast ingots were extruded into the cylinder-shaped specimen. 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 friction coefficient used was 0.3. Table 1 lists the billet dimensions and process parameters used in the numerical simulation. Figure 1 shows 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 at approximately 740 �C under a mixed atmosphere of CO2 and SF6. The chemical components of the casted Mg alloys were 6.42 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 at 300 �C, and the diameter of the ingots was approximately 70 mm. Before extrusion, extrusion die and the Mg alloy were heated to 250, 300, and 350 �C for 60 min, respectively. And,

    Table 1 Extrusion process and simulation parameters

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

    1254—Volume 24(3) March 2015 Journal of Materials Engineering and Performance

  • 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

    Journal of Materials Engineering and Performance Volume 24(3) March 2015—1255

  • the Mg alloy was extruded into bars at 250, 300, and 350 �C with an extrusion ratio of 10:1. The extrusion rate was 10 mm/s for all trials, and the extruded Mg alloy bar was air cooled. The extruded samples were cut for microstructure analysis. Cross sections were prepared for optical microscopy, and the mean grain size was measured using an image analyzer in the computer program. The tensile test specimens were prepared from longitudinal axis of the extrusion direction. The prepared specimens were annealed for 1 h at 150, 200, and 250 �C.

    Depending on the annealing conditions, the mechanical properties were analyzed at room and elevated temperatures with a strain rate of 10�3 s�1. The temperature of the tensile test was 20, 100, and 200 �C. The microhardness was measured using a Vickers hardness (HV) tester. HV was measured at a minimum of ten points to reduce the error. The heat-treatment conditions were as follows: the as-extrudedMg alloy was solution-treated at 300 �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. The measurements for phase identification were conducted using a Rigaku DMAX 2500 x-ray powder diffractometer using Cu-Ka and operated at 40 mA and 40 kV. The XRD step scan was performed over the range, 20�-90� 2h with 3� increments and a

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

    4. Results and Discussion

    4.1 Thermodynamic Calculations

    The phase diagrams of the ternary Mg-xZn-2Sn alloy and quaternary Mg-xZn-2Sn-1Al alloy were calculated using the Pandat program. Figure 2(a) shows the phase transformation of the Mg-xZn-2Sn alloy and Mg-xZn-2Sn-1Al alloy as a function of temperature. The phase zone of a-Mg in the Mg-xZn-2Sn alloy and Mg-xZn-2Sn-1Al alloy was approximately 400 �C. a-Mg + Mg2Sn were observed when the temperature of the alloy was less than 300 �C. In the case of the Mg-xZn-2Sn alloy, a-Mg + Mg2Sn + MgZn phases were detected at tem- peratures

  • phase and a-Mg + Mg2Sn +