钕与旋锻对AZ71镁合金力学性能的影响
Trans. Nonferrous Met. Soc. China 25(2015) 3223?3231
Effects of Nd and rotary forging on mechanical properties of AZ71 Mg alloys
Jhewn-kuang CHEN1, Yu-cheng CHEN1, Hsien-tsung LI2, Kam-shau CHAN2, Chun-jung CHANG2
1. Institute of Materials Science and Engineering, National Taipei University of Technology, Taipei 10608;
2. Foxconn Technology Group, New Taipei 23680
Received 21 November 2014; accepted 18 March 2015
Abstract: The effects of Nd addition on the mechanical properties and plastic deformability of AZ71 Mg alloys were investigated.
0.5%?2.0% (mass fraction) Nd was added to AZ71 Mg alloys. The grain size and the amount of brittle β-Mg17Al12 phase reduce with increasing the Nd addition, while nanosized Al x Nd y precipitates form. In combination with 32% rotary forging and subsequent annealing, the grain size of Nd-added AZ71 Mg alloys reduces greatly from over 350 μm to below 30 μm. Both tensile strength and ductility increase with the Nd addition up to 1.0%. The addition of Nd beyond 1.0% leads to the aggregations of rod-shaped Al11Nd3 and blocky Al2Nd precipitates, thereby deteriorating both strength and ductility. The 1.0% Nd-added AZ71 Mg alloy shows tensile strength up to 253 MPa and elongation of 10.7%. It is concluded that adding 1.0% Nd to AZ71 Mg alloy yields the optimum toughness, whether under as-cast or rotary forging and annealing conditions.
Key words: AZ71 Mg alloys; Nd; rotary forging; mechanical properties; recrystallization
1 Introduction
Mg alloy, renowned as the 21st century green
engineering material, has emerged as the third metallic
structural material, following steels and Al alloys [1?3].
Despite increasing emphasis on energy conservation and
environmental issues, development and application of
Mg and Mg alloys are still limited due to the following
concerns. 1) Mg alloys have few slip systems and usually
have poor ductility and thus inferior forming ability.
Furthermore, the formation of brittle β-Mg17Al12phase
largely impairs their ductility [4]. 2) The difference in
corrosion potentials of α-Mg and β-Mg17Al12phases
causes severe galvanic corrosion and poor corrosion
resistance properties of Mg alloys. 3) The low melting
point of β-Mg17Al12 phase tends to soften the Mg alloys
and demonstrates poor high temperature properties.
Two routes were employed to resolve the above-
mentioned issues in Mg alloys. Firstly, ZHANG et al [5]
proposed to refine grain size to below 10 μm by
combining severe plastic deformation (SPD) and heat
treatment. The fine grain size can give rise to
superplastic properties at high temperature, leading to
significant enhancement of high-temperature forming
ability. AZ61 Mg alloy was shown to produce grains
with size of 3.5 μm via repetitive upsetting?extrusion
process [6]. CHAPMAN and WILSON [7] showed that
when the grain size is smaller than 2 μm, the ductile?
brittle transition temperature of Mg alloys reduces to
room temperature, thereby enhancing forming ability for
cold forging. Secondly, by the addition of strengthening
elements, intermetallic compounds with high melting
point can form to improve mechanical properties of Mg
alloys at both room and high temperatures [8,9]. And by
eliminating the formation of β-Mg17Al12phase, the
corrosion resistance of Mg alloys can also be improved.
The present study chooses a less studied AZ71 Mg
alloy with medium combination of strength and ductility
compared with the deformable AZ31 and the high
strength AZ91 Mg alloys. Two routes are pursued to
improve the properties of AZ71 Mg alloy: adding
neodymium (Nd) and plastic deformation using rotary
forging. Nd is selected since its electronegativity (1.14)
is closer to that of Mg (1.31) [10] than that of Al (1.61).
This gives rise to higher Nd solid solubility in Mg than
Al. It can also reduce the formation of β-Mg17Al12
phase [11]. On the other hand, Nd has been reported to
have great effect on the grain refinement in Mg?4Al [12]
and AZ80 Mg alloys [13], which improves both strength
and ductility. The Nd-containing Mg alloys also showed
improved high temperature deformability [14].
The current study uses rotary forging as the plastic
deformation to assist grain size reduction, which has
Corresponding author: Jhewn-kuang CHEN; E-mail: jkchen@https://www.wendangku.net/doc/f1205309.html,.tw
DOI: 10.1016/S1003-6326(15)63955-3