PDF《metal mould and lost foam casting》

? ? Microstructure and mechanical properties of Mg-Gd-Y-Zr alloy cast by metal mould and lost foam casting LI Ji-lin, CHEN Rong-shi, KE Wei State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China Received

25 September 2010;

accepted

20 December

2010 Abstract: The microstructure and mechanical properties of Mg-10.

1Gd-3.74Y-0.25Zr (mass fraction, %) alloy (GW104 alloy) cast by metal mould casting (MMC) and lost foam casting (LFC) were evaluated, respectively. It is revealed that different forming modes do not influence the phase composition of as-cast alloy. In the as-cast specimens, the microstructures are similar and composed of α-Mg solid solution, eutectic compound of α-Mg+Mg24(Gd, Y)5 and cuboid-shaped Mg5(Gd, Y) phase;

whereas the average grain size of the alloy produced by metal mould casting is smaller than that by lost foam casting. The eutectic compound of the alloy is completely dissolved after solution treatment at

525 °C for

6 h, while the Mg5(Gd, Y) phase still exists after solution treatment. After peak-ageing, the lost foam cast alloy exhibits the maximum ultimate tensile strength of

285 MPa, and metal mould cast specimen

325 MPa at room temperature, while the tensile yield strengths of them are comparable. It can be concluded that GW104 alloy cast by lost foam casting possesses similar microstructure and evidently lower mechanical strength compared with metal mould cast alloy, due to slow solidification rate and proneness to form shrinkage porosities during lost foam casting process. Key words: Mg-Gd-Y-Zr alloy;

lost foam casting;

metal mould casting;

microstructure;

mechanical property ? ?

1 Introduction Magnesium alloys containing heavy rare earth elements, such as Gd and Y, are of high strength and low density, which make them be very attractive as structural materials in the aerospace and racing automotive industries where mass saving is of great value[1?2]. The equilibrium solid solubility of Gd in Mg is relatively high (4.53% in mole fraction, or 23.49% in mass fraction at

548 °C) and decreases exponentially with the decrease of temperature (0.61% in mole fraction or 3.82% in mass fraction at

200 °C). Thus, the Mg-Gd based alloys are prone to form supersaturated solid solution during the solidification, and the new strengthening precipitates would readily form by suitable aging treatment, which makes Mg-Gd alloys be ideal systems for precipitation hardening[3]. While Y and Zr added to Mg-Gd binary alloy will help to increase aging effect and to refine the grains[4]. Previously developed Mg-Gd-Y-Zr alloys exhibit higher specific strength at both room and elevated temperatures and better creep resistance than WE54 and QE22[5]. Currently, casting is still the main industrial forming method for magnesium alloys. Although magnesium alloys have been applied widely in aerospace, automobile and tele-communication industries, the lag of research and development on casting technology is still a bottleneck for their further application[6]. As is well known, lost foam casting (LFC) is a cost-effective, environment-friendly vital option to the conventional casting process for production of near-net shape castings with high quality. And the LFC process is widely employed by the automotive industry for making engine components[7]. Up to now, research of LFC has primarily concentrated on aluminum alloys, cast iron and steel[8?11], while little has been reported on magnesium alloys. It is anticipated that the combination of the lightest structural magnesium alloys with LFC process will bring a bright future for magnesium applications, especially in forming components with thin-wall, complex geometry, tight tolerance and smooth as-cast surface. In the present work, a typical Mg-Gd-Y-Zr alloy (GW104) was cast by metal mould casting and lost foam casting, respectively. The as-cast microstructures, ageing Foundation item: Project (2007CB613704) supported by the National Basic Research Program of China Corresponding author: CHEN Rong-shi;

Tel: +86-24-23926646;

Fax: +86-24-23894149;

E-mail: [email protected] DOI:?10.1016/S1003-6326(11)60777-2 LI Ji-lin, et al/Trans. Nonferrous Met. Soc. China 21(2011) 761?766

762 response and mechanical properties of the alloys at room temperature were studied comparatively in order to expose the differences between the two casting processes and discuss the reliability of lost foam casting process on Mg-Gd-Y-Zr alloy castings production.

2 Experimental 2.1 EPS pattern preparation The foam material used in the lost-foam process is expanded polystyrene (EPS) with a density of

20 kg/m3 . The foam was cut into different shapes by a foam cutting machine, and then assembled into a block shaped model, as shown in Fig.1. The model was then coated with a hydrated graphite blacking and the coating thickness was approximately

1 mm. Vacuum (30 kPa) assistance was employed during the casting procedure. Fig.1 illustrates the experimental set up for vacuum assisted mould filling. Fig.1 Experimental set up for vacuum assisted mould filling: 1―Quartz sand;

2―Permeable coating;

3―EPS pattern;

4―Sandbox;

5―Vacuum sucker 2.2 Alloy smelting, casting and heat treatment The Mg-Gd-Y-Zr alloy was prepared by melting high pure Mg (>

99.95%), Gd (>

99%), Y (>

99%) and Mg-30Zr (mass fraction, %) master alloy in an electric resistance furnace at 780?800 °C under protection of an RJ6 flux. The melt was poured firstly into a block shaped EPS pattern at about

780 °C, and then poured into a mild steel mould preheated to

250 °C. The chemical composition of the ingots was determined to be Mg-10.10Gd-3.74Y-0.25Zr (mass fraction, %) or Mg-1.61Gd-1.11Y-0.07Zr (mole fraction, %) by using inductively coupled plasma atomic emission spectro........