Scholarly article on topic 'Isothermal Precision forging of magnesium alloy components with high performance'

Isothermal Precision forging of magnesium alloy components with high performance Academic research paper on "Materials engineering"

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{"Mg-Gd-Y-Zn-Zr magnesium alloy" / "isothermal forging process" / "mechanical properties"}

Abstract of research paper on Materials engineering, author of scientific article — Yang Wu, Qiang Chen, Xiangsheng Xia

Abstract Because of hexagonal crystal structure, magnesium alloys, especially magnesium alloys with addition of rare earths, exhibit poor formability at room temperature. The isothermal precision forging process at elevated temperature can improve the formability of magnesium alloys. In this paper, the isothermal precision forging process of the complex shape case of Mg-Gd-Y-Zn-Zr magnesium alloy was simulated based on FORGE Nxt. A two-stage isothermal precision forging process was determined. According to the simulation results, isothermal precision forging experiments of magnesium parts with high performance and complex shape were successfully carried out. After isothermal precision forging process, the cases of magnesium alloy were subjected to heat treatment. Moreover, the mechanical properties of parts were also determined.

Academic research paper on topic "Isothermal Precision forging of magnesium alloy components with high performance"

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Procedia Engineering 207 (2017) 896-901

www.elsevier.com/locate/procedia

International Conference on the Technology of Plasticity, ICTP 2017, 17-22 September 2017,

Cambridge, United Kingdom

Isothermal Precision forging of magnesium alloy components with

high performance

Yang Wua,b, Qiang Chena,b*, Xiangsheng Xiaa,b

aSouthwest Technology and Engineering Research Institute, Chongqing 400039, PR China bPrecision Formins Integrated Manufacturidg Technology of Callaborative Innhvatiqn Centeo, Chongqing 400039, PR China

Abstract

Because of hexagonal crystal structure, magnesium alloys, especially magnesium alloys with addition of rare earths, exhibit poog formability tit room temperature. The isothermal precision forging process ait elevated temperature can miprove the formability of magnesium alloys. In this paper, the iiorhfrmal precision forging process of the nomplex shape case of Mg-Gd-Y-Zn-Zr magnesium alloy was simulated based on FORGE Nxt. A two-stage isothermal precision fotging process was determined. According to the simulation results, isothermAl precision forging exp eriments of magnesigm parts with high performancf and complex al-iape were sucgestrally parried out. After isothermal pre cision forging process , the cases of magnes ium alloy wipi subje cted to heat rreatment. Moreover, the mechanical propertres o f parts were al so detegerined. dick here and insert your abstract text.

© 20S7 The Authors. Pushed by Elsevier Ltd.

Peeg-review under responsibiKty of ahe sc^rifim eommitree of the International Conference on the Technology of Plasticity. Keygords:Mg-Ge-Y-Zn-Zs magnesium alloy, isothermal forging process, mechanical properties.

1. Introduction

Magnesium alloys have provoked much interest in the aero space and automotive industries for weight reduction [1, 2]. However, thn applications of magnesium alloy s have bmen greatly limited by the ir low strength and p oor formability [3].Magnesium alloys containing rare earth (RE) elements such as Mg-Gd [4], Mg-Y [5] and Mg-Sn [0]

* Corresponding author. Tel.: +86 23 68792286; fax: +86 23 68792100. E-mail address:2009chenqiang@163.com (Q. Chen)

1877-7058 © 2017 The Authors. Published by Elsevier Ltd.

Peer-review under responsibility of the scientific committee of the International Conference on the Technology of Plasticity. 10.1016/j.proeng.2017.10.848

have the potential to improve mechanical properties of Mg alloys without obvious increase of the weight of components. Precipitates containing RE elements precipitation from the magnesium matrix appear stably at grain boundaries, which can improve the strengths of Mg alloy because of pinning the grain boundary and inhibiting the movement of dislocations [7]. With the increase of strength, the formability of Mg alloys with addition of RE elements could reduce gradually at room temperature.

Mg and its alloys have the lack of easily-activated slips at room temperature, which leads to a poor cold working capability. The start of non-basal slip systems in a large amount generally happens at a temperature over 220 oC [8]. So, traditional forging method for the parts with complex shape is casting. However, the parts fabricated by casting have low mechanical properties due to coarse dendritic structure and some solidification defects such as porosity and segregation [9]. The mechanical properties of forging Mg alloys are usually higher than that of as-cast Mg alloys. Otherwise, magnesium alloys are sensitive to temperature, have high thermal conductivity, and exhibit high viscosity, which leads to a poor formability during routinely forging process. However, these problems are not important in the isothermal precision forging process, as preheating the tools rather than cold tools, because the formability of Mg alloys can be improved by means of preheating the tools during forging at elevated temperature. The isothermal precision forging is a new process that can realize the near-net shape forging as well as the precision forming. Its characteristics are as follows: the moulds and the billet have the same temperature in the process of die pressing, and the speed of the press machine can be adjusted automatically according to the flow stress of billet at a preselected low pressure. The varying range of temperature can be decreased to the minimum by this process because the deformation rate is small and the billet can be separated from the environment for a long time[10]. Therefore, isothermal precision forging process is also an appropriate method to prepare those Mg alloy components with excellent complex shape and mechanical properties.

In the present study, the isothermal precision forging process of the Mg-Gd-Y-Zn-Zr magnesium alloy complex shape part was simulated by a finite element (FE) software and the forging process was optimized according to the simulation results. A Mg alloy part with excellent complex shape was successfully fabricated by two-stage isothermal precision forging process. After forging process, Mg alloy components were subjected to heat treatment. Moreover, mechanical properties of the as-forging Mg part were systematically studied. The homogeneous of mechanical properties were also determined.

2. FEM simulation and experiments

2.1. FEM simulation

In order to analyze the distribution of effective strains, effective strength and load during the isothermal precision forging process, the commercial FE software, FORGE Nxt, was used to simulate this forging process of the Mg-Gd-Y-Zn-Zr Mg alloy complex shape part. In the present simulations, the interface friction between the billet and the die is set as 0.25 and the punch speed is 10 mm/s. The FE model of the complex shape part is shown in Fig. 1. This Mg alloy component is a typical cylindrical shell part of complex shape with deep blind hole and four lugs in the spherical shape bottom. The two-stage isothermal precision forging process is shown in Fig. 1.

Fig. 1. Three-dimensional model of the (a) billet, (b) preforming part and (c) finish forging component.

2.2. Experiments

Mg-Gd-Y-Zn-Zr magnesium alloy in the as-extruded and homogenized condition were used for the fabrication of the complex shape part. The Mg alloy billets and forging tools were kept in furnace at 400 oC for 2 h to complete the solid solution, and immediately forged at 400 oC using the two-stage isothermal precision forging process. After forging process, the Mg alloy parts were aged at different conditions. Specimens for tensile test of the Mg alloy were cut from the as-forged part at different positions. Tensile test along the forging direction at room temperature were tested on a Shimadzu mechanical testing system using a strain of 10-3 s-1. Each mechanical test was repeated three times.

3. Results and discussion

3.1. Simulation results of isothermal forging process

Forming pressure is very important for design of dies, choice of equipment and determination of process. Too large load during forging process will not only affect of the dies filled completely, but decrease the die life and increase the cost of the component. In order to decrease the forging pressure, a two-stage isothermal precision forging process was designed. The processing method includes preforming process and finish forging process, as shown in Fig. 1. The calculated curve of the load-stroke during two-stage isothermal forging process was shown in Fig. 2. It can be seen that the curve consists of two stages, which correspond to preforming process and finish forging process, respectively. In the early of isothermal precision forging process, with increasing deformations the load increases slowly and most of metal flows to the impression die. At the finish forging process, the forging pressure increases rapidly until the end of isothermal forging process due to the greater flow resistance of complete filling of die cavity. After finish forging process, there appears a jump of forging pressure because of the final die closure.

0 10 20 30 40 50

Stroke/mm

Fig. 2. The curve of load and stroke at the top die during two-stage isothermal precision forging process

The simulation results of two-stage isothermal forging process are also shown in Fig. 3, and the distribution of equivalent strain and effective strength during the forging process should be systematically analyzed. The results show that the maximum effective strength is about 37 MPa and the maximum equivalent strain is about 1.71, which

appears in the spherical shape bottom. Preforming of four lugs in the spherical shape bottom could effectively reduce the forging press and the stress concentration in the complex shape Mg alloy component fabricated by two-stage isothermal precision forging process. Therefore, the two-stage isothermal precision forging process is chosen as the processing method for the complex shape Mg alloy part in the present work.

Fig. 3. Distribution of (a) effective strength and (b) equivalent strain during the two-stage isothermal precision forging process 3.2. Forging experiments and mechanical behavior

Since it is very difficult to forge the complex shape of Mg alloy part, a male die and two female dies were designed in the experiment, as shown in Fig. 4a. According to the FE simulation results, the forging dies were optimized in order to fill die cavity and decrease die pressure during two-stage isothermal forging process. Experiments were performed on a YX32-200 type hydraulic press with the maximum load of 200 t and speed of 10 mm/s. The dies and billets were kept in a resistance furnace at 400 oC for 2 h to complete the solid solution of Mg-Gd-Y-Zn-Zr magnesium alloy, and immediately forged at 400 oC. Fig. 4b presents the photo of the complex shape Mg alloy parts fabricated by two-stage isothermal precision forging process. The complex shape Mg alloy components were successfully prepared by this forging process. The difficult forging positions of four lugs in the spherical shape bottom are filled completely.

Fig. 4. Photos of (a) dies for the two-stage isothermal precision forging process and (b) Mg alloy complex shape parts.

The mechanical properties evolution of isothermal precision-forged Mg alloy parts during aging treatment at 160 oC with different time are shown in Fig. 5. With the increasing aging time, the ultimate strength and elongation increase gradually at the beginning of the aging treatment, until 22 h and then decrease slowly. The secondary phases precipitate including Mg24(GdY)5, Mg12Zn(YGd) and Mg5(GdY) [11] from the a-Mg matrix during the aging treatment, which contributes to the high strength of the Mg alloy. However, strengthening phases will dissolve into the matrix and grow to coarse precipitates at elevated temperature or through long-time aging treatment, which is the condition of over aging for the Mg alloy and leads to decrease the mechanical properties of the RE magnesium alloy[12]. After aging at 160 oC for 20 h, the optimal ultimate strength and elongation of the parts along the forging direction are 400 MPa and 8.5%, respectively. It indicates that this RE intensifying magnesium alloy could be applied in the aerospace component with high strength and complex shape.

Aging Time/h

Fig. 5. Aging time effective the ultimate strength and elongation.

After aging at the optimal condition, the mechanical properties in four regions around the cylinder of the part, marked '0o', '90o', '180o' and '270o', were examined. The results are shown in Table 1. It can be seen that mechanical properties of this complex shape Mg alloy part in different positions are homogeneous, which is conducive to meeting the requirements during industry applications.

Table 1. Mechanical properties of the part at different positions.

Positions UTS(MPa) Elongation(%)

0o 400 8.5

90o 410 8

180o 405 8.5

270o 400 9

4. Conclusion

In the present study, FE simulation of two-stage isothermal precision forging process has been researched. According to the FE simulation results, a complex shape Mg alloy part was successfully fabricated, and the evolution of mechanical properties during aging treatment was addressed, too. Several conclusions are reached as follows:

(1) According to the analysis of FE simulation results, the preforming part was designed, and the reasonable dies of two-stage isothermal precision forging process were successfully prepared.

(2) The maximum effective strength is about 37 MPa and the maximum equivalent strain is about 1.71, which appears in the spherical shape bottom. In the isothermal-forged Mg alloy part, four lugs in the spherical shape bottom are filled completely, which is in good agreement with the calculated result.

(3) The optimal mechanical tensile properties along the forging direction were obtained after aging treatment at 160 oC for 20 h with an ultimate strength of 400 MPa and 8.5% elongation at room temperature, and mechanical properties of this complex shape Mg alloy part in different positions are homogeneous.

Acknowledgements

This work was supported by the Chong Qing science and technology talent training plan Grant no. cstc2014kjrc-qnrc50004 and the Chong Qing Natural Science Foundation under Grant no. cstc2013jcyjjq70001.

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