Abstract:Microstructural evolution of semi-solid 7075 Al alloy manufactured by strain-induced melt activation (SIMA) process was investigated. The effects of different processing parameters, such as isothermal temperature and holding time on the semi-solid microstructures (the liquid volume fraction, average grain size, and degree of spheroidization of the solid particles) during partial remelting have been investigated on 7075 Al alloy that was extruded by an extrusion ratio of 20 before remelting. Experiments of reme… Show more
“…Although, the coarsening kinetics of semisolid alloys are still a controversial problem, the mean diameter ( D ) of the grains in various alloys after time t at the elevating temperature have confirmed the classical LSW equation, and the power exponent is approximately3 though varying between 2 and 4 during the liquid–solid region heating 5 . An increasing number of experts believe that the exponent of metallic coarsening in solid–liquid region is generally 3 5 , 7 , 10 , 13 , 18 , however, the best fitting is n = 4 according to the calculated results, coarsening was controlled by grain boundary diffusion. Figure 8 Plot of mean grain size ( a ) squared, ( b ) cubed and ( c ) quadruplicated as a function of holding time for Al–Fe–Cu alloy reheated at 630 °C, ( d ) relationship between quartic power of equivalent diameter and holding time for the alloy in various temperatures, lines represent the best linear fitted to the data.…”
Section: Resultsmentioning
confidence: 98%
“…Each step of semisolid processing has a great effect on the following processes 4 : the coarsening of migrating grain boundary liquid films in which the coarsening rate is relevant to the solid volume fraction, where the coarsening rate decreases as the solid fraction increases before reaching a critical value and then decreases over this critical value 5 , 6 , coalescence for alloys of high solid fraction and aggregation for grains with low disorientation boundaries or to reduce the free energy, is primarily a lattice diffusion-controlled process 7 – 10 . For many diffusion-controlled coarsening systems, including solid–liquid mixtures, coarsening kinetics can be described by the Lifshitz, Slyozov and Wagner ( LSW ) theory as follows: where t is the isothermal holding time, D is the grain size after time t , D 0 is the initial grain size, K is the coarsening rate constant, and n is the power exponent 11 .…”
Section: Introductionmentioning
confidence: 99%
“…Subsequently, the grains were transformed into globular or near-globular shapes under the action of the solid–liquid interfacial tension 8 , 19 , 20 , and evolved or agglomerated into larger and irregular shapes with increasing holding time or heating temperature. The α-Al grains were quickly coarsened by connecting the secondary arms of the rosette or merging fine α-Al phases with a small quantity of the liquid, and slowly coarsened through diffusion at the stage of a large quantity of the liquid 9 , 10 . Some reinforced precipitations that are rich in copper, magnesium, and zinc in the 7075 alloy 21 , or rich in copper in the Al–5Fe–4Cu alloy 22 are low-melting intermetallics.…”
A hypereutectic Al–Fe–Cu alloy with a high-volume fraction ferro-aluminum second phase (AlFe phases for short) was reheated in the solid–liquid region and the microstructure evolution was investigated. During semisolid heating, the high-melting AlFe phases in the Al–Fe–Cu alloy were demonstrated to stunt the grain growth and to block the liquid coalescing and the solid moving. Consequently, the grain sizes in the alloy increased rapidly and then slowly with increasing holding time, and the grains increased gradually with increasing temperature. Smaller grain grew into the large grain but it did not continually grow into the larger grain with increasing temperature or holding time. The shape factor (SF) of the alloy increased gradually and then decreased quickly with increasing temperature or holding time. The major alloying elements in addition to magnesium in the hypereutectic Al–Fe–Cu alloy were finally enriched at the grain boundaries or around the AlFe phases. Besides dissolving in the grains or AlFe phases, copper also diffused between the grains or around AlFe phases, resulting in the formation of diverse Cu-enriched zones. Cu constituents in the inter-grains are outnumbered in the intra-grains. The coarsening kinetics of the alloy was controlled by grain boundary diffusion. The coarsening rate constants K in the initial stage of heating (5–20 min) were several times larger than that in the later stage of heating (20–60 min), indicating the blocking effect of AlFe phases on coarsened grain being obvious.
“…Although, the coarsening kinetics of semisolid alloys are still a controversial problem, the mean diameter ( D ) of the grains in various alloys after time t at the elevating temperature have confirmed the classical LSW equation, and the power exponent is approximately3 though varying between 2 and 4 during the liquid–solid region heating 5 . An increasing number of experts believe that the exponent of metallic coarsening in solid–liquid region is generally 3 5 , 7 , 10 , 13 , 18 , however, the best fitting is n = 4 according to the calculated results, coarsening was controlled by grain boundary diffusion. Figure 8 Plot of mean grain size ( a ) squared, ( b ) cubed and ( c ) quadruplicated as a function of holding time for Al–Fe–Cu alloy reheated at 630 °C, ( d ) relationship between quartic power of equivalent diameter and holding time for the alloy in various temperatures, lines represent the best linear fitted to the data.…”
Section: Resultsmentioning
confidence: 98%
“…Each step of semisolid processing has a great effect on the following processes 4 : the coarsening of migrating grain boundary liquid films in which the coarsening rate is relevant to the solid volume fraction, where the coarsening rate decreases as the solid fraction increases before reaching a critical value and then decreases over this critical value 5 , 6 , coalescence for alloys of high solid fraction and aggregation for grains with low disorientation boundaries or to reduce the free energy, is primarily a lattice diffusion-controlled process 7 – 10 . For many diffusion-controlled coarsening systems, including solid–liquid mixtures, coarsening kinetics can be described by the Lifshitz, Slyozov and Wagner ( LSW ) theory as follows: where t is the isothermal holding time, D is the grain size after time t , D 0 is the initial grain size, K is the coarsening rate constant, and n is the power exponent 11 .…”
Section: Introductionmentioning
confidence: 99%
“…Subsequently, the grains were transformed into globular or near-globular shapes under the action of the solid–liquid interfacial tension 8 , 19 , 20 , and evolved or agglomerated into larger and irregular shapes with increasing holding time or heating temperature. The α-Al grains were quickly coarsened by connecting the secondary arms of the rosette or merging fine α-Al phases with a small quantity of the liquid, and slowly coarsened through diffusion at the stage of a large quantity of the liquid 9 , 10 . Some reinforced precipitations that are rich in copper, magnesium, and zinc in the 7075 alloy 21 , or rich in copper in the Al–5Fe–4Cu alloy 22 are low-melting intermetallics.…”
A hypereutectic Al–Fe–Cu alloy with a high-volume fraction ferro-aluminum second phase (AlFe phases for short) was reheated in the solid–liquid region and the microstructure evolution was investigated. During semisolid heating, the high-melting AlFe phases in the Al–Fe–Cu alloy were demonstrated to stunt the grain growth and to block the liquid coalescing and the solid moving. Consequently, the grain sizes in the alloy increased rapidly and then slowly with increasing holding time, and the grains increased gradually with increasing temperature. Smaller grain grew into the large grain but it did not continually grow into the larger grain with increasing temperature or holding time. The shape factor (SF) of the alloy increased gradually and then decreased quickly with increasing temperature or holding time. The major alloying elements in addition to magnesium in the hypereutectic Al–Fe–Cu alloy were finally enriched at the grain boundaries or around the AlFe phases. Besides dissolving in the grains or AlFe phases, copper also diffused between the grains or around AlFe phases, resulting in the formation of diverse Cu-enriched zones. Cu constituents in the inter-grains are outnumbered in the intra-grains. The coarsening kinetics of the alloy was controlled by grain boundary diffusion. The coarsening rate constants K in the initial stage of heating (5–20 min) were several times larger than that in the later stage of heating (20–60 min), indicating the blocking effect of AlFe phases on coarsened grain being obvious.
“…Pioneering studies on the semisolid process of aluminum alloy aimed mainly at cast aluminum alloys such as A356 and A357. Only a few studies [3][4][5][6][7] focused on the semisolid wrought aluminum alloys such as Al-Zn-Mg-Cu (7XXX) which have been widely used in aerospace and automotive applications due to their high mechanical properties. Semisolid alloys slurries could be stirred to destroy the dendrite grains.…”
TiB 2 /7050 (3, 6, and 9 wt%) composites slurries with globular were synthesized by in situ reaction and serpentine tube pouring techniques. The results showed that the semisolid 7050 alloy and 3, 6, and 9 wt%TiB 2 /7050 composites with average grain diameter of 28, 25, 20, and 19 mm and shape factor of 0.77, 0.83, 0.90, and 0.93, respectively, can be obtained at 660 C pouring temperature. With increasing TiB 2 content and curves number, the a-Al grain size was decreased. The composite melts have an effect of ''self-stirring'' when they flow through the serpentine tube, which is beneficial to make the primary nuclei with globular grains. Moreover, the wear resistance of TiB 2 =7050 7050 composites improved obviously with increasing in situ TiB 2 particles content and that the wear rate of 9 wt%TiB 2 =7050 composite was 79% lower than that of 7050 matrix alloy under 100 N applied load, 30 min sliding time, and 0.15 m=s sliding velocity.
“…Vaneetveld et al [19] investigated the effects of heating parameters on the recrystallization in the RAP process and suggested using the high solid fraction to prevent hot tearing in the thixoforging process. Bolouri et al [20] studied the effect of cold work on the semi-solid microstructure by compression straining of the samples up to 40% and Mohammadi et al [21] investigated the effects of the isothermal treatment on the semi-solid microstructure of extruded samples.…”
Microstructural and mechanical behaviors of semi-solid 7075 aluminum alloy were investigated during semi-solid processing. The strain induced melt activation (SIMA) process consisted of applying uniaxial compression strain at ambient temperature and subsequent semi-solid treatment at 600-620˝C for 5-35 min. Microstructures were characterized by scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). During the isothermal heating, intermetallic precipitates were gradually dissolved through the phase transformations of α-Al + η (MgZn 2 ) Ñ liquid phase (L) and then α-Al + Al 2 CuMg (S) + Mg 2 Si Ñ liquid phase (L). However, Fe-rich precipitates appeared mainly as square particles at the grain boundaries at low heating temperatures. Cu and Si were enriched at the grain boundaries during the isothermal treatment while a significant depletion of Mg was also observed at the grain boundaries. The mechanical behavior of different SIMA processed samples in the semi-solid state were investigated by means of hot compression tests. The results indicated that the SIMA processed sample with near equiaxed microstructure exhibits the highest flow resistance during thixoforming which significantly decreases in the case of samples with globular microstructures. This was justified based on the governing deformation mechanisms for different thixoformed microstructures.
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