Microstructure and grain refining performance of Al–5Ti–1B master alloy prepared under high-intensity ultrasound

The microstructure and grain refining performance of an Al–5Ti–1B master alloy prepared under high-intensity ultrasound were investigated. With applying continuous high-intensity ultrasound vibrations in the reaction, the Al–5Ti–1B master alloy is successfully manufactured in 4 min. Compared with conventional Al–5Ti–1B master alloys, the mean size and the size spread of TiB₂ particles in the prepared master alloy are evidently decreased. The narrower particle size spread significantly improves the grain refining performance of the master alloy, which proves the calculation predictions by Greer. Consequently, the limiting grain size of commercial purity aluminium refined by the new master alloy can reach 45 μm.     

Effect of eutectic particles on the grain size control and the superplasticity of aluminium alloys

Aluminium alloys containing eutectic particles of the Al-Ni, Al-Mg-Si, Al-Ni-Ce and Al-Cu-Ce systems are investigated. The particles which control grain growth and stimulate grain nucleation are studied. The Zener-Smith law about dependence between grain size and particle parameters is confirmed and experimental coefficients are found. Experimental coefficients of the Zener-Smith equation obtained in this study depend on the particle size and differ from theoretical coefficients proposed by Zener and Smith. Some alloys with grain size about 3 μm demonstrate very good superplasticity indicators, namely: the strain rate sensitivity index m = 0.5-0.6 and the elongation over 400% at constant strain rate 5 × 10⁻³ s⁻¹.     

The effect of chemical grain refinement and low superheat pouring on the structure of NRC castings of aluminium alloy Al–7Si–0.4Mg

The effect of the addition of Al–5Ti–1B (wt.%) chemical grain refiners on the nuclei generation for a range of superheats during pouring in new rheocasting (NRC) of aluminium alloy Al–7Si–0.4Mg (wt.%) has been investigated. The contributions to the grain density by the grain refiner additions and impurity particles were quantified and it was found that the addition of grain refiner provides increasing number of nucleation sites as the superheat is decreased from 105 to 35 °C. However, at superheats of 15 °C, which are more typical of NRC, the grain density is similar in the alloy both with and without grain refiner additions. At this superheat, the equiaxed grain morphology is globular rather than dendritic and it is postulated that the grain size is dependant upon grain coarsening mechanisms rather than the number of heterogeneous nucleation events. In agreement with previous studies on semi-solid processing, it was found that the achievement of a fine globular grain structure led to a more homogeneous casting being produced. The mechanism of the macrosegregation observed in these castings is discussed and explained by the ‘sponge effect’.     

Effects and mechanisms of grain refinement in aluminium alloys

Grain refinement plays a crucial role in improving characteristics and properties of cast and wrought aluminium alloys. Generally Al-Ti and Al-Ti-B master alloys are added to the aluminium alloys to grain refine the solidified product. The mechanism of grain refinement is of considerable controversy in the scientific literature. The nucleant effects i.e. which particle and its characteristics nucleate α-Al, has been the subject of intensive research. Lately the solute effect i.e. the effect of dissolved titanium on grain refinement, has come into forefront of grain refinement research. The present paper attempts to review the literature on the nucleant effects and solute effects on grain refinement and addresses the importance of dissolved titanium in promoting nucleation of α-Al on nucleant particles.     

Microstructural refinement and tensile properties enhancement of Mg-3Al alloy using charcoal additions

Significant grain refinement in Mg-3Al alloy is achieved with the addition of charcoal due to the formation of Al₄C₃ particles, which act as effective nuclei for magnesium grains. Addition of 0.5 wt% charcoal has lead to reduced grain size of Mg-3Al alloy from 500 to 80 μm and no substantial grain refinement is obtained on further addition of charcoal. The results further reveal that the prolonged holding of the melt after the addition of charcoal has not affected the grain refining efficiency of Al₄C₃. Steady increase in tensile properties observed with increasing amount of charcoal addition has been attributed to the grain refinement and the presence of fine Al₄C₃ particles. The strengthening mechanisms due to charcoal addition are discussed in terms of Hall-Petch relation and dispersion strengthening. The predicted values are in good agreement with experimental results.     

Understanding grain refining and anti Si-poisoning effect in Al-10Si/Al-5Nb-B system

Grain refinement is critical for fabricating high-quality Al-Si casting components in the application of automobile and aerospace industries,while the well-known Si-poisoning effect makes it difficult.Nbbased refiners offer an effective method to refine Al-Si casting alloys,but their anti Si-poisoning capability is far from being understood.In this work,the grain refining mechanism and the anti Si-poisoning effect in the Al-10 Si/Al-5 Nb-B system were systematically investigated by combining transmission electron microscope,first-principles calculations,and thermodynamic calculations.It is revealed that NbB₂provides the main nucleation site in the Al-10 Si ingot inoculated by 0.1 wt.%Nb Al-5 Nb-B refiner.The exposed Nb atoms on the(0001)NbB₂and(1-100)NbB₂surface can be substituted by Al to form(Al,Nb)B₂intermedia layers.In addition,a layer of NbAl₃-like compound(NbAl₃')can cover the surface of NbB₂with the orientation relation of(1-100)[11-20]NbB_2//(110)[110]NbAl₃'.Both of the(Al,Nb)B₂and NbAl₃'intermedia layers contribute to enhancing the nucleation potency of NbB₂particles.These discoveries provide fundamental insight to the grain refining mechanism of the Nb-B based refiners for Al-Si casting alloys and are expected to guide the future development of stronger refiners for Al-Si casting alloys.     

Effects of processing parameters on aluminide morphology in aluminium grain refining master alloys

Abstract

An investigation has been made of the effects of processing parameters on the microstructure of Al–Ti and Al–Zr alloys produced by the reduction of potassium fluorotitanate and potassium fluorozirconate by aluminium. Blocky aluminides (TiAl₃ and ZrAl₃ respectively) are produced when the reduction is performed in the liquid+solid phase region and flaky aluminides are produced when it is performed in the liquid phase region. The use of Na₂TiF₆ rather than K₂TiF₆ has no apparent effect on the alloy structure. The microstructure and the size of aluminide blocks or flakes produced are further influenced by the subsequent cooling rate experienced by the alloy. Post-fabrication heat treatment of Al–5Ti–0·2B (wt-%) alloys (produced by combined reduction of potassium fluqrotitanate and potassium fluoroborate by aluminium) causes boride particles to precipitate on the aluminide surfaces. These borides grow in size and decrease in number with heat treatment time.

MST/1355     

Effects of calcium on the microstructures and tensile properties of Mg-5Li-3Al alloys

The as-cast Mg-5Li-3Al-xCa (x = 0, 0.5, 1, 1.5 wt.%) was prepared with vacuum induction melting furnace, then processed by hot extrusion. The microstructures and tensile properties were investigated. The results show that the grains of as-cast alloys were refined gradually with the increase of Ca content from 0.5 wt.% to 1 wt.%, while the Ca content increases to 1.5 wt.%, the grain size increases. The microstructures of investigated alloys were further refined after hot extrusion. Both as-cast and as-extruded Mg-5Li-3Al-0.5Ca alloys have the highest mechanical properties, which is mainly attributed to the grain refinement caused by the addition of Ca and the formation of strengthening phase, Al₄Ca. When the addition of Ca is up to 1-1.5 wt.%, the tensile properties of alloys are worsened due to the excessive (Mg, Al)₂Ca eutectic phase forming at grain boundary.     

Ultrasonic processing of molten and solidifying aluminium alloys: overview and outlook

Ultrasonic melt processing attracts since the 1930 a lot of interest both from academic researchers and industry. In the last 10 years the interest to ultrasonic melt processing grew with regard to understanding the underlying mechanisms of previously established effects, developing numerical models of ultrasonic cavitation and the development of nanocomposite technology. This review paper summarises the mechanisms involved in the ultrasonic melt processing, including cavitation, flows, nucleation, activation, fragmentation and their consequences for degassing, structure refinement and particle dispersion. Some typical mistakes made by researchers in performing experiments and in interpretation of the results are discussed. New advanced methods of studying ultrasonic treatment and phenomena are considered. The paper also gives an outlook to future developments and challenges.

This paper is part of a Themed Issue on Aluminium-based materials: processing, microstructure, properties, and recycling.     

Improved prediction of the grain size of aluminum alloys that includes the effect of cooling rate

A comprehensive study of the effect of cooling rate on the grain size of a range of grain refined wrought aluminum alloys was carried out under quiescent solidification conditions where nucleation occurs predominantly by a constitutional undercooling mechanism. Increasing the cooling rate reduced the grain size by increasing the number of particles that nucleate grains and by affecting the development of constitutional undercooling. Both effects are represented using simple analytical relationships. By coupling these results with earlier work, an empirical relationship is developed between grain size, density of nucleant particles, cooling rate, nucleant potency and alloy composition that allows prediction of grain size across a wide range of alloys and cooling rates.     

Nucleation and growth of Fe-rich phases in Al-5Ti-1B modified Al-Fe alloys investigated using synchrotron X-ray imaging and electron microscopy

Plate-like Fe-rich intermetallic phases directly influence the mechanical properties of recycled Al alloys;thus,many attempts have been made to modify the morphology of these phases.Through synchrotron X-ray imaging and electron microscopy,the underlying nucleation and growth mechanisms of Fe-rich phases during the solidification of Al-5Ti-1B-modified Al-2Fe alloys were revealed in this study.The results showed that the Al-5Ti-1 B grain refiner as well as the applied pressure both resulted in reduction of the size and number of primary Al3Fe phases and promoted the formation of eutectic Al6Fe phases.The tomography results demonstrated that Al-5Ti-1B changed the three-dimensional (3D) morphology of primary Fe-rich phases from rod-like to branched plate-like,while a reduction in their thickness and size was also observed.This was attributed to the fact that Ti-containing solutes in the melts inhibit the diffusion of Fe atoms and the Al3Fe twins produce re-entrant corner on the twin boundaries along the growth direction.Moreover,the TiB2 provides possible nucleation sites for Al6Fe phases.The nucleation mechanism of Fe-rich phases is discussed in terms of experimental observations and crystallography calculations.The decrease in the lattice mismatch between TiB2 and Al6Fe phases was suggested,which promoted the transformation of Al3Fe to Al6Fe phases.     

Effects of Al–5Ti–1B and Al–5Zr master alloys on the structure, hardness and tensile properties of a highly alloyed aluminum alloy

This study was undertaken to investigate the influence of Al–5Ti–1B and Al–5Zr master alloys on the structural characteristics and tensile properties of Al–12Zn–3 Mg–2.5Cu aluminum alloy. The optimum amount for Ti and Zr containing master alloys was selected as 1 wt.% and 6 wt.%, respectively. The results also showed that Ti containing master alloy is more effective in reducing average grain size of the alloy. T6 heat treatment was applied for all specimens before tensile testing. In heat treated condition, the average tensile strength of 505 MPa was found to be increased to 621 MPa for sample refined with 1 wt.% Al–5Ti–1B (0.05 wt.% Ti). SEM fractography of the fractured faces of several castings showed an overall macroscopically brittle appearance at low magnifications. At higher magnifications, unrefined specimens showed cracking along the grains, whereas Ti-refined specimens showed cracks in individual intermetallic compounds.     

Electrochemical behaviour of aluminium alloys containing indium and tin in NaCl solution

The electrochemical behaviour of Al, Al–In, Al–Sn and Al–Sn–In alloys in 2 M NaCl solution has been studied using an open circuit potential, potentiodynamic polarization and ac impedance measurements as well as by optical microscopy examination. The addition of alloying components to aluminium produced in all cases a considerable activation of aluminium. The activation is manifested by shifting the open corrosion potential and the pitting potential in the negative direction (for about 0.6 V) and significant reducing of the passive potential region. The degree of activation depended on alloying element and it was found that there is an increase in the order: Al < Al–In < Al–Sn ≈ Al–Sn–In. The anodic dissolution of the Al–Sn and Al–Sn–In alloys started at open circuit potential which is only 0.45 V more positive than the thermodynamic Al³⁺/Al potential. The ac impedance measurements performed at different potentials in wide potential range (corresponding to passive and active state of each examined samples) confirmed the great activity of Al–Sn and Al–Sn–In alloys compared to aluminium.     

Investigation of the grain refining performance of Al-5Ti-1B master alloy on the recycling process of A356 alloy

In this paper, the grain refining performance of Al-5Ti-1B master alloy on the recycled A356 alloy is investigated using the macrostructure examination and chemical analysis. Results show that Al-5Ti-1B is composed of the dispersion of blocky TiAl₃ particles and mixtures of small TiAl₃ and TiB₂ particles. Both particles refine grain structures of A356 alloy. As the initial recycling process proceeds, A356 alloy still exhibits fine structures. However, during the subsequent recycling process, the grain size of A356 alloy become larger. The concentration of titanium and boron decreases with increasing the number of recycling, especially in the subsequent recycling process. It is proposed that recycling of refined A356 can best be conducted in the initial recycling process and then additional grain refiner needs to be added to maintain the grain refining performance for the continuous recycling process of A356 alloy.     

Texture effect on fatigue crack propagation in aluminium alloys: an overview

Texture or grain orientation was of crucial importance to fatigue crack propagation (FCP) in aluminium alloys due to boundary character between neighbouring grains and crack closure effect. The current understanding of the relationship among texture, grain size, slipping and crack propagation at fatigue stage I to III was reviewed and discussed. The recommendations for improving FCP resistance were proposed. Intensifying Goss, P and Q textures and moderating these grains were an effective method to improve FCP resistance in Paris regime. However, in stage I, due to the predominated crack closure effect, large grain is beneficial for improving the threshold value of crack propagation. Principally, excellent FCP resistance could be obtained at a balance of crack deflection and crack closure.     

Beta grain growth behaviour of TG6 and Ti17 titanium alloys

The grain growth kinetics of TG6 and Ti17 alloys in the β phase have been studied. The grain size parameters have been obtained by means of the image analysis software for different heat treatment temperatures and times. The growth time exponents and activation energies have also been determined. The activation energies for grain growth of both alloys increase with the increase in soaking time. This trend might be due to the different solute drag effect. The uniformity of grain size under each heat treatment has also been studied and the variations resulted from the combined action of nucleation rate of β phase and β grain growth velocity.     

Microstructural study of cold forged and annealed Mg alloys

Cold forging deformations (3 and 8%) were carried out on extruded AZ31 and AZ61 Mg alloys, and subsequent annealing is performed at 573?K for different durations. The microstructure evolution is investigated and further analysed. The results show that the thick lenticular twinning is mainly formed at initial deformation stage and subsequently transforms into narrow band twinning. Compared to AZ31, AZ61 produces broader twins with lower volume fraction in response to 3% cold forging due to Al–Mn phase hindering the twinning activity and plastic deformation. Twin boundary plays an important role in refining grains as it provides the nucleation sites of static recrystallisation. Moreover, as for AZ61, the starting and ending times of static recrystallisation are much later than those of AZ31.     

The effect of grain refining and oxide inclusion on the fluidity of Al–4.5Cu–0.6Mn and A356 alloys

The effect of grain refinement and oxide inclusion on the fluidity of Al alloy was investigated with a test casting with eight thin flow channels. Pouring in air increased the amount of oxide in the A356 melt. The fluidity compared between normal A356 melt and contaminated melt. The amount of oxide was evaluated qualitatively by ultrasonic treatment. The flow length varied linearly with the pouring temperature. By adding Ti and Al–5Ti–B, fluidity increased. The grain size decreased by adding grain refiner. The fluidity depended on the degree of grain refining. It was noticed that pouring in air increased the amount of oxides in the melt by ultrasonic treatment. The fluidity of contaminated melt was decreased comparing to the normal one especially in lower temperature.     

Effect of melt treatment on microstructure and impact properties of Al-7Si and Al-7Si-2.5Cu cast alloys

The microstructures and impact toughness of Al-7Si and Al-7Si-2.5Cu cast alloys were studied after various melt treatments like grain refinement and modification. The results indicate that combined grain refined and modified Al-7Si-2.5Cu alloys have microstructures consisting of uniformly distributed α-Al grains, interdendritic network of fine eutectic silicon and fine CuAl₂ particles in the interdendritic region. These alloys exhibited improved impact toughness in as cast condition when compared to those treated by individual addition of grain refiner or modifier. The improved impact toughness of Al-7Si-2.5Cu alloys are related to breakage of the large aluminum grains and uniform distribution of eutectic silicon and fine CuAl₂ particles in the interdendritic region resulting from combined refinement and modification. This paper attempts to investigate the influence of microstructural changes in the Al-7Si and Al-7Si-2.5Cu cast alloys by grain refinement, modification and combined action of both on the impact toughness.     

Influence of constitutional undercooling on grain refinement in titanium aluminide alloys

In this present work, the influence of constitutional undercooling on the grain size of titanium aluminide alloys and the dependence on boron content have been investigated. By means of the TTTIAL1 thermodynamic database of the Thermo-Calc programme, the values of the growth restriction factor for a series of alloys have been calculated. It can be shown that grain size decreases with increasing growth restriction factor. A comparison of calculated and published experimental results shows that the constitutional undercooling significantly influences the grain size in titanium aluminide alloys, but most likely, it is not the unique mechanism of grain refinement alone, in so far as not all of the experimental results may be explained by the influence of constitutional undercooling.