Effects of Al–5Ti–1B on the structure and hardness of a super high strength aluminum alloy produced by strain-induced melt activation process
Highlights
► Effects of Al–5Ti–1B on the aluminum alloy produced by SIMA process were studied. ► Al–5Ti–1B reduced the grain size of the alloy. ► During partial remelting, with increasing of the predeformation, the average grain size decreased.
Introduction
Al–Zn–Mg–Cu alloys are used in many industrial applications because of their low density and high strength [1]. These alloys are heat treatable and show attractive properties where good combination of strength and stiffness are obtained particularly after T6 condition [2], [3], [4]. Super high strength aluminum alloys have been extensively studied after mechanical deformation for several decades [5], [6], [7], but little attention has been made on the alloy in as-cast condition and semi-solid state. As-cast structures of these alloys have a significant influence on their mechanical properties and the quality of finished products [8]. The structure of such materials can be controlled by some important factors such as: changing the composition, adding grain refining agents, minimizing inclusions and applying thermomechanical treatments [9]. The use of high concentrations of alloying elements results inhomogeneity in the microstructure and severe segregation of second phases. In casting products, the mechanical properties may vary from location to location due to the variations of grain size, the amount of eutectic phases and the amount of precipitates. Much attention has been made to reduce the segregation of the alloying elements during solidification period of high-alloyed Al alloys [5], [10].
Strain-induced melt activation (SIMA) process has been used to enhance the mechanical properties of Al alloy in recent years. A conventional SIMA process produces the desired structures by deformation and following heat treatment in the mushy zone. Parameters such as heating time, temperature and the degree of cold working are critical factors in controlling the semi-solid microstructures in SIMA process [11], [12], [13], [14], [15]. It has been shown that the microstructure of an alloy prepared in the semi-solid state depends on its microstructure prior to partial remelting, so it is important to study the preliminary microstructure and subsequent evolution process during partial melting.
The main objective of this investigation is to study the effect of Al–5Ti–1B and modified SIMA process on the microstructure and hardness of the Al–12Zn–3Mg–2.5Cu alloy. A modified SIMA process includes homogenization and warm deformation instead of cold working in the convectional SIMA process [15]. Fig. 1 shows schematically the modified SIMA process.
Section snippets
Experimental procedure
Industrially pure Al (99.8%), Mg (99.9%), Zn (99.9) and Cu (99.9%) were used as starting materials to prepare the primary ingots of Al–12Zn–3Mg–2.5Cu aluminum alloy. An electrical resistance furnace (with a 10 kg SiC crucible) was applied for heating the parent materials and preparing the alloy ingots. Table 1 shows the chemical composition of Al–12Zn–3Mg–2.5Cu alloy. In order to prepare alloys with different Ti concentrations, the parent alloy was remelted in a small electrical resistance
Structural characterization in as-cast condition
Fig. 3 shows the effect of various amounts of Al–5Ti–1B grain refiner on the average grain size of the cast specimens. It can be seen that the increase of Al–5Ti–1B master alloy from 0.1 to 2 wt.% in the alloy can result in a fine microstructure and almost significant reduction of the average grain size. However, by further addition of grain refiner (>2 wt.%) to the alloy, the average grain size almost remains constant and the excess addition of the grain refiner does not have a considerable
Conclusions
- 1.
Adding 2 wt.% Al–5Ti–1B master alloy to Al–12Zn–3Mg–2.5Cu alloy reduced its grain size from 480 μm to 40 μm.
- 2.
Increasing of the holding temperature in SIMA process led to the coarsening of the grains for the same amounts of predeformation and holding time.
- 3.
A dominant globular structure was developed by 40% predeformation. Further increase of the holding time altered the globularization of the microstructure.
- 4.
The results indicated that with the increase in holding time, sphericity of particles
Acknowledgment
The authors would like to thank University of Tehran for financial support of this research. The first author also thanks to the wife and this paper I presented to my wife.
References (30)
- et al.
The microstructural evolution of an Al–Zn–Mg–Cu during homogenization
Mater Lett
(2006) - et al.
Secondary ageing in an aluminum alloy 7050
Mater Sci Eng A
(2008) - et al.
Structure-property correlations in Al 7050 and Al 7055 high-strength aluminum alloys
Mater Sci Eng A
(2008) - et al.
A new way to cast high-alloyed Al–Zn– Mg–Cu–Zr for super-high strength and toughness
J Mater Process Technol
(2006) - et al.
The constituents in Al–10Zn–2.5Mg–2.5Cu aluminum alloy
Mater Sci Eng A
(2005) - et al.
Microstructure and mechanical properties of spray-deposited Al–10.8Zn–2.8Mg–1.9Cu alloy after two-step aging treatment at 110 and 150 °C
Mater Charact
(2007) - et al.
Effects of low-frequency electromagnetic field on microstructures and macrosegregation of continuous casting 7075 aluminum alloy
Mater Sci Eng A
(2003) - et al.
Semi solid forming of aluminum alloys by direct forging and lateral extrusion
J Mater Process Technol
(1994) - et al.
Characterization of Al 7075 alloys after cold working and heating in the semi-solid temperature range
J Mater Process Technol
(2001) - et al.
Effects of Al–5Ti–1B and Al–5Zr master alloys on the structure, hardness and tensile properties of a highly alloyed aluminum alloy
Int Mater Rev Mater Des
(2010)