Evolution of secondary phase particles during deformation of Al-5Ti-1B master alloy and their effect on α-Al grain refinement

Addition of Al-5Ti-1B alloy to molten aluminum alloys can refine α-Al grains effectively and thereby improve their strength and toughness. TiAl₃ and TiB₂ in Al-5Ti-1B alloy are the main secondary-phase particles for refinement, while the understanding on the effect of their sizes on α-Al grain refinement continues to be fragmented. Therefore, Al-5Ti-1B alloys with various sizes and morphologies of the secondary-phase particles were prepared by equal channel angular pressing (ECAP). Evolution of the secondary-phase particles during ECAP process and their impact on α-Al grain refinement were studied by X-ray diffraction and scanning electron microscope (SEM). Results show that during the ECAP process, micro-cracks firstly appeared inside TiAl₃ particles and then gradually expanded, which resulted in continuous refinement of TiAl₃ particles. In addition, micro-distribution uniformity of TiB₂ particles was improved due to the impingement of TiAl₃ particles to TiB₂ clusters during deformation. Excessively large sizes of TiAl₃ particles would reduce the number of effective heterogeneous nucleus and thus resulted in poor grain refinement effectiveness. Moreover, excessively small TiAl₃ particles would reduce inhibitory factors for grain growth Q and weaken grain refinement effectiveness. Therefore, an optimal size range of 18–22 μm for TiAl₃ particles was suggested.     

Relationships of diboride phases in Al–Ti(Zr)–B alloys

Abstract

The relationships of diboride phases in Al–Ti(Zr)–B alloys with a variable Ti/B ratio close to the stoichiometry of TiB₂ were studied. The formation of diboride solid solutions was confirmed. A grain refinement mechanism is proposed as that diboride particles in the Al–Ti–B master alloys reacting with aluminium upon adding into an aluminium melt and release titanium into the melt through forming a (Ti,Al)B₂ solid solution and maintain a thin dynamic Ti rich layer on the surfaces of the (Ti,Al)B₂ particles, which nucleates α-Al grains in solidification. The poisoning effect of zirconium on grain refinement of aluminium by Al–Ti–B master alloys is also discussed.     

Microstructure and Wear Resistance of Semisolid TiB2/7050 Composites Produced by Serpentine Tube Pouring Technique

TiB₂/7050 (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%TiB₂/7050 composites with average grain diameter of 28, 25, 20, and 19 µm and shape factor of 0.77, 0.83, 0.90, and 0.93, respectively, can be obtained at 660 °C pouring temperature. With increasing TiB₂ content and curves number, the α-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 TiB₂/7050 composites improved obviously with increasing in situ TiB₂ particles content and that the wear rate of 9 wt%TiB₂/7050 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.     

Effects of Sc(Zr) on the microstructure and mechanical properties of as-cast Al–Mg alloys

Both the addition of 0.6% Sc and simultaneous addition of 0.2% Sc and 0.1% Zr exerted a remarkable effect on grain refinement of as-cast Al–Mg alloys, changing typical dendritic microstructure into fine equiaxed grains. Such effect was found to be related to the formation of primary particles, which acted as heterogeneous nucleation sites for α-Al matrix during solidification. Primary particles formed in Al–Mg–Sc–Zr alloy could be identified as the eutectic structure consisting of multilayer of ‘Al₃(Sc,Zr)?+?α-Al?+?Al₃(Sc,Zr)’, with a ‘cellular-dendritic’ mode of growth. In addition, an attractive comprehensive property of as-cast Al–5Mg alloy due to the addition of 0.2% Sc and 0.1% Zr was obtained.     

The formation mechanism of non-dendritic primary α–Al phase in semi-solid AlSi7Mg Alloy

In this paper, a process to make non-dendritic semi-solid AlSi₇Mg alloy by electromagnetic stirring and the temperature field of the stirred melt cooled is continuously investigated. It is proposed that a new kinetic factor for primary α-Al nucleation is that alow thermal gradient exists in the electromagnetically stirred melt, for which the primary dendrite arms and secondary dendrite arms are refined. The results also show that the root remelting of the secondary dendrite arms is an important mechanism of the primary α-Al refinement. Strong electromagnetic stirring greatly reduces the composition supercooling in the melt and eliminates the preferred growth of primary dendrite arms, therefore, many rosettes or spherical primary α-Al phase particles form finally.     

Performance of AlTi5B1, AlTi3B3 and AlB3 master alloys in refining grain structure of aluminium foundry alloys

Abstract

The capacity of AlTi5B1, AlTi3B3 and AlB3 grain refiners to refine the grain structures of AlSi7Mg and AlSi11Cu2 foundry alloys was investigated. The performance of AlTi5B1, well established to be the best grain refiner for wrought aluminium alloys, is not nearly as good with the AlSi7Mg and AlSi11Cu2 alloys. Relatively smaller grains are obtained with the AlTi3B3 grain refiner in both alloys. The AlB3 grain refiner, on the other hand, improves the grain structure only as much as the AlTi5B1 grain refiner does. With as much as 0·04–0·1 wt-%Ti, the commercial alloys cannot enjoy the outstanding potency of the AlB₂ particles since the B supply is readily transformed to TiB₂ particles. However, the grains of the Ti free AlSi7Mg and AlSi11Cu2 alloys (～0·005 wt-%Ti) are very small and nearly globular for the entire range of holding times when inoculated with AlB3, implying not only a remarkable grain refining capacity but also a strong resistance to fading of the grain refinement effect. The lack of Ti in the melt allows the entire B to form AlB₂ particles, the perfect substrates to promote the nucleation of α-Al crystals. Aluminium castings can enjoy grain sizes well below 200 μm, with an addition of 0·02 wt-%B, provided that they are Ti free. That the potent substrates are made available just before the nucleation of α-Al crystals avoids fading effects and is a further advantage of the AlB3 grain refiner in recycling operations.     

Microstructural characterisation of novel 6351 Al–Al4SiC4 in-situ composite

Abstract

6351 Al–Al₄SiC₄ composite has been developed through stir casting route by incorporation of fine TiC powder in 6351 Al melt. Simultaneous effects of the generation of in-situ particles (Al₄SiC₄ and Al₃Ti) and grain refinement were observed. The in-situ generated Al₄SiC₄ particles were found to act as nucleation sites for primary α (causing grain refinement) along with engulfment effect promoting uniform particle distribution. As the volume fraction of Al₄SiC₄ particles increased, the dendritic solidification was suppressed (more equiaxed grains appeared) and overall grain size of the matrix decreased. Besides, the precipitation of Al₃Ti occurred at the dislocation enriched region. Accordingly, hardness of the composite was improved with increasing content of Al₄SiC₄ particles.     

Understanding mechanisms of grain refinement of aluminium alloys by inoculation

Abstract

For over half a century, grain refinement of aluminium alloys has been achieved by chemical inoculation; current grain-refinement practice involves the addition of master alloys (e.g. Al – Ti – B, Al – Ti – C) before casting, introducing inoculant particles to the melt. These particles act as nucleation points for α-Al grains, resulting in a uniformly fine, equiaxed as cast microstructure. Despite the ubiquity of this process, its underpinning science was not fully understood, hindering development of the area. From the 1950s onwards, the phase responsible for nucleation in alloys refined by Al – Ti – B was fiercely disputed. The debate focused so closely on this issue that other important factors were frequently ignored. During the 1990s, this debate was resolved through careful thermodynamic reasoning and novel experiments that derived their inspiration far from the foundry. This review focuses on subsequent experimental work and modelling: the expansion of the current understanding of grain refinement to include effects relating to the release of latent heat, the size distribution of inoculant particles and the alloying elements present in the melt (through poisoning and growth-restriction effects). The current contention regarding the nature of the interface between the nucleant phase (TiB₂) and the melt is also discussed. These recent advancements have lead to improvements in grain refining practice resulting in savings in the foundry and development of alternative master alloys for aluminium systems. The ideas have also been successfully applied to other metallic systems, notably magnesium and zinc alloys.     

Formation of TiB2 particles during dissolution of TiAl3 in Al–TiB2 metal matrix composite using an in situ technique

A new technique has been developed in an aluminum based metal matrix composite in order to reveal the mechanism of formation of TiB₂ particles by mixing molten master alloys i.e., Al–8Ti and Al–4B in the Ti:B weight ratio of 5:2. A composite containing fine TiB₂ particles produced by this technique. In this approach, the progress of In situ formation of TiB₂ was carried out using hot stage microscope, energy dispersive X-ray analysis, scanning electron microscopy and X-ray diffraction analysis. From the experimental observations obtained; it was proposed that the formation of TiB₂ particles occurred via diffusion of Boron atoms through TiAl₃ particles interface, thereby reacting to form fine TiB₂ particles. Studies indicate that since the primary TiB₂ particles on the surface of TiAl₃ are appreciably free and movable and because of boron diffusion across boundary layer towards TiAl₃, TiB₂ particles produced during growth with the primary ones formed agglomeration rings. A model was schematically developed to explain the formation of TiB₂.     

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.     

Effect of TiAl<Subscript>3</Subscript> particles size and distribution on their settling and dissolution behaviour in aluminium

The effect of TiAl₃ particle size and distribution on their settling and dissolution behaviour in molten aluminium during grain refinement has been studied. For this purpose Al–5Ti master alloys containing blocky TiAl₃ particles of different size and distribution are synthesised at reaction temperatures 750, 800 and 850 °C for 60 min and used for grain refinement. The extent of fading and the recovery due to stirring is calculated from the measured grain size and used to judge the dissolution and settling behaviour of TiAl₃ in molten Al, which is greatly attributed to its size and distribution in Al–5Ti master alloy. Fine TiAl₃ particle dissolve faster in the melt and cause fading. Larger size TiAl₃ particles exist for longer time in molten Al and act as a nucleating site even when added in hypoperitectic concentration (0.05 wt% Ti).     

Impurity effects on the nucleation and growth of primary Al3(Sc,Zr) phase in Al alloys

Impurity effects on the nucleation and growth of primary Al₃(Sc,Zr) phase have been investigated in high purity Al alloys and commercial purity Al alloys, respectively. In the case of high purity Al alloys, primary Al₃(Sc,Zr) phases were found to be pushed to grain boundaries ahead of the solidification front. Such type of primary Al₃(Sc,Zr) phase did not contribute to the heterogeneous nucleation, and thereby the grain refinement of Al alloys. In the case of commercial purity Al alloys, the presence of Fe, Si, Cu, Mg, Ti, and other impurities significantly enhanced the heterogeneous nucleation of primary Al₃(Sc,Zr) phase. Most primary Al₃(Sc,Zr) phases were found to be located within the α-Al matrix, and kept an identical orientation relationship with the α-Al matrix. Furthermore, the presence of the impurities also changed the growth mode on the primary Al₃(Sc,Zr) phase. In the case of commercial purity Al alloys, a peritectic to eutectic reaction was induced due to the presence of the impurities. A layered growth was observed leading to a narrow particle size distribution. In contrast, in the case of high purity Al alloys, a featureless structure was observed. This investigation demonstrates that impurities and their concentrations are important factors affecting the nucleation and growth of primary Al₃(Sc,Zr) phases, and thereby for the successful grain refinement in Al-based alloys.     

Tribological behaviour of hot extruded Al–Cu–Mg–Ag alloy reinforced by TiB2 particles

In the present investigation, tribological behaviour of the hot extruded Al–Cu–Mg–Ag (matrix) alloy and the effect of Ti and TiB₂ addition in matrix alloy have been studied. Hot extrusion was introduced to eliminate cast defects like porosity, voids and micro cracks. Addition of Ti and TiB₂ particles increased the hardness of the matrix by grain refinement and dispersion hardening, respectively. It has been observed that the increase in hardness had significantly improved the wear resistance of the material. Detail study of the wear surfaces and debris were carried out to understand the wear mechanism of the samples. It revealed a complex mechanism of micro-cutting, plastic deformation, abrasion and delamination of the wear samples.     

Electroforming of TiB2-Reinforced Copper Matrix Electrode for EDM

TiB₂ particle-reinforced copper matrix composite is electroformed in copper sulfate on stainless steel plate. The impact of the particle content in electroforming solution on the surface morphology, hardness, and electrical conductivity of the electroformed composite are studied, and the influence of electroforming current density on the effective content of particles in the composite is also analyzed. The results show that when the content of particles in electroforming solution is 25 g/L, the current density is 4 A/dm², particles in the electroformed composite are well-distributed, and the average grain diameter can be reduced to 20 µm. The microhardness of Cu/TiB₂ composite reinforced by particles with diameter of 3 µm is 25% higher than that of electroformed copper, and its conductivity remains 86% of the copper.     

Effect of TiB2 particle size on the material transfer behaviour of Cu–TiB2 composites

Cu-5.8vol.-%TiB₂ composites reinforced with different sized TiB₂ particles were prepared by spark plasma sintering (SPS). The arc erosion resistance was examined using a JF04C electrical contact instrument. The results indicate that the TiB₂ particle size and contact currents have important effects on the material transfer mode. Both lower mass loss and relative mass transfer was observed in the Cu-5.8vol.-%TiB₂ composites reinforced with fine TiB₂ particles (10?um, 30?um) compared to those with coarse TiB₂ particles (70?um, 110?um). The investigation and analysis of the microstructure and arc erosion mechanism show uniform distribution of fine TiB₂ particles in the copper matrix can significantly improve the viscosity of the molten pool and decrease the splashing of molten Cu.     

Microstructure modification for 2219 Al alloy through ultrasonic treatment and fast cooling

The combined effects of ultrasonic treatment and fast cooling on the bulk hydrogen content, the refinement of α-Al grains and θ-Al₂Cu precipitated phase in the 2219 Al alloy were investigated in this work. Results showed that under the combined effects of ultrasonic treatment and liquid nitrogen cooling, the bulk hydrogen content was dramatically reduced. Metallography and scanning electron microscopy results indicated that both the α-Al grains and θ-Al₂Cu precipitated phase were greatly refined under the combined effects of ultrasonic treatment and liquid nitrogen cooling. Correspondingly, the 2219 Al alloy sample exhibited excellent mechanical properties due to the refinement of α-Al grains and θ-Al₂Cu precipitated phase as well as the low bulk hydrogen content.     

Preparation of Al–Ti–C–Sr master alloys and their refining efficiency on A356 alloy

Al–Ti–C–Sr master alloys with various amounts of Sr were prepared through a method of liquid solidification reactions. The as-prepared Al–Ti–C–Sr master alloys were then used as grain refiners to modify A356 alloy. The microstructures of the Al–5Ti–0.25C–2Sr, Al–5Ti–0.25C–8Sr alloys and modified A356 alloy were investigated. The results showed that the Al–5Ti–0.25C–2Sr alloy consisted of phases of α-Al, lath-shaped or tiny blocky TiAl₃, granular TiC, and blocky or rim AlTiSr, while the Al–5Ti–0.25C–8Sr alloy contained an irregular blocky Al₄Sr phase besides the above-mentioned phases. Satisfactory grain refining and modifying effects were obtained by the addition of Al–Ti–C–Sr alloys (0.5 wt.%) to the A356 alloy. Meanwhile, the sizes of the α-Al dendrites / SDAS(40 µm) decreased to 32.7 µm (or 30 µm).The morphology of eutectic silicon was changed from needle-/platelike form to fibrous/globular form. The grain refinement and modification effects of Al–Ti–C–Sr alloys on A356 alloys were mutually promoted. Compared with the Al–5Ti–0.25C–2Sr alloy, the Al–5Ti–0.25C–8Sr alloy possessed higher efficiency in grain refinement and modification of the A356 alloys.     

Nucleation behaviour of TiB2 particles in pure AI and effect of elemental additions

Abstract

Particles of TiB₂ have been introduced into pure AI, AI-Cu, AI-Si, and AI-Ti alloys and their ability to encourage grain refinement has been studied. There was little or no reduction in the grain size in pure Al but the addition of 0.025 wt-%Ti, 1.5 wt-%Cu, or 0.8 wt-%Si enables the TiB₂ particles to become potent grain nuclei. When the effect of these alloying additions on the restriction of the crystal growth rate is assessed, it is found that the above Ti, Cu, and Si additions are roughly equivalent. It is concluded that hypoperitectic Ti additions enable TiB₂ particles to become effective grain nuclei through crystal growth rate restriction rather than through the formation of a TiAI₃ layer on the particle surfaces.     

In situ TiB2 particulate reinforced near eutectic Al–Si alloy composites

In situ TiB₂ particulate reinforced near eutectic Al–Si alloy composites fabricated by the melt reaction composing (MRC) methods have been investigated. It has been shown that minute TiB₂ particles (less than 1 μm) uniformly distribute in the eutectic structure and they are interlaced with the coralline-like eutectic Si, while there are very few TiB₂ particles in α-Al. It has been also shown that in situ TiB₂ particles can enhance the tensile strength of the Al–Si alloy matrix. The strengthening effect increases with increasing TiB₂ content. The ultimate tensile strength (UTS) at room temperature of as-cast 6%TiB₂/Al–Si–Mg composite is 296 MPa, that is a 14.7% increase over the matrix, and its elongation at fracture is 5.5%. After heat-treatment (T6), the UTS of the composites reaches 384 MPa. The strengthening mechanism has been discussed.     

Room-temperature synthesis of nanometric α-Bi2O3

Nanometric Bi₂O₃ powder was successfully synthesized by applying the method based on self-propagating room temperature reaction (SPRT) between bismuth nitrates and sodium hydroxide. X-ray powder diffraction (XRPD) and Rietveld's structure refinement method were applied to characterize prepared powder. It revealed that synthesized material is a single phase monoclinic α-Bi₂O₃ (space group P2₁/c with cell parameters a = 5.84605(4)Å, b = 8.16339(6) Å, c = 7.50788(6) Å and β = 112.9883(8)). Powder particles were of nanometric size (about 50 nm). Raman spectral studies conformed that the obtained powder is single phase α-Bi₂O₃. Specific surface area of obtained powder was measured by Brunauer-Emmet-Teller (BET) method.