Grain Refinement Mechanism of Aluminum by Al-Ti-B Master Alloys
07 Feb 2016-pp 189-193
TL;DR: In this article, a dynamic nucleation theory for the grain refinement is proposed as a dynamic Al3Ti layer in the thickness of a couple of atoms existing on the surface of (Ti,Al)B2 and being responsible for nucleating aluminum grains.
Abstract: Al-Ti-B type master alloys are routinely added into aluminum for a grain refinement purpose. However, a consensus about the mechanism of grain refinement is still yet to reach. This work is aiming at further understanding the grain refinement mechanism through studying the stability of TiB2 in pure aluminum and TiB2/Al3Ti interfaces in aluminum by transmission electron microscopic (TEM) techniques. It is found that TiB2 reacts with aluminum forming (Ti,Al)B2 solid solution and releasing titanium into aluminum. A crystallographic orientation relationship exists between TiB2 and Al3Ti in Al-Ti-B alloys. A dynamic nucleation theory for the grain refinement is therefore proposed as a dynamic Al3Ti layer in the thickness of a couple of atoms existing on the surface of (Ti,Al)B2 and being responsible for nucleating aluminum grains.
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TL;DR: In this article, the microstructure, mechanical properties and thermal stability of AlxTi1−xN and Al1Ti1-xBN coatings grown by reactive high-power impulse magnetron sputtering (HiPIMS) have been analyzed as a function of Al/(Al+Ti) ratio (x) between 0.5 and 0.8.
Abstract: The microstructure, mechanical properties and thermal stability of AlxTi1−xN and Al1Ti1-xBN coatings grown by reactive high-power impulse magnetron sputtering (HiPIMS) have been analyzed as a function of Al/(Al + Ti) ratio (x) between 0.5 and 0.8. The coatings were predominantly formed by a face-centered cubic Ti(Al)N crystalline phase, both with and without B, even for x ratios as high as 0.6, which is higher than the ratio typically encountered for AlxTi1−xN coatings deposited by reactive magnetron sputtering. B doping, in combination with the highly energetic deposition conditions offered by HiPIMS, results in the suppression of the columnar grain morphology typically encountered in AlxTi1−xN coatings. On the contrary, the AlxTi1−xBN coatings grown by HiPIMS present a dense nanocomposite type microstructure, formed by nanocrystalline Ti(Al)N domains and amorphous regions composed of Ti(Al)B2 and BN. As a result, high-Al content (x ≈ 0.6) AlxTi1−xBN coatings grown by HiPIMS offer higher hardness, elasticity and fracture toughness than AlxTi1−xN coatings. Moreover, the thermal stability and the hot hardness are substantially enhanced, delaying the onset of formation of the detrimental hexagonal AlN phase from 850 °C in the case of Al0.6Ti0.4N, to 1000 °C in the case of Al0.6Ti0.4BN.
10 citations
17 Jun 2019
TL;DR: In this paper, the location of USP from the sump to the launder and applying it to the melt flow for continuous treatment is suggested to improve the efficiency of the process.
Abstract: Ultrasonic processing (USP) during direct-chill (DC) casting of light metal alloys is typically applied in the sump of a billet. This approach, though successful for structure refinement and modification, has two main drawbacks: (a) mixture of mechanisms that rely heavily on dendrite fragmentation and (b) a limited volume that can be processed by a single ultrasonic source. We suggest moving the location of USP from the sump to the launder and applying it to the melt flow for continuous treatment. The apparent benefits include: (a) degassing of the melt volume, (b) grain refinement through activation of non-metallic inclusions, fragmentation of primary crystals, and deagglomeration of grain refining substrates, and (c) a possibility to use a single ultrasonic source for processing large melt volumes. To optimize this process with regard to the acoustic intensity and melt residence time in the active cavitation zone, flow modification with baffles as well as informed location of the ultrasonic source are required. In this paper, we demonstrate the results of experimental trials where the degassing degree and grain refinement have been the indicators of the USP efficiency for two aluminium alloys, i.e. LM25 and AA7050. The results are supported by acoustic measurements and computer simulations.
4 citations
01 Jan 2019
TL;DR: In this paper, microstructure, microhardness, tensile properties, surface roughness and wear behavior of Al and both Al-0.15Ti and Al- 0.01B microalloys were investigated.
Abstract: Grain refinement, by adding master alloys, is an important industrial process for aluminum casting operations. In this work, microstructure, microhardness, tensile properties, surface roughness and wear behavior of Al and both Al-0.15Ti and Al-0.05Ti-0.01B microalloys were investigated. The addition of Ti and B to pure Al reduced the grain size by 83%. The grain refinement effect was due to the presence of Al3Ti and TiB2 particles, which activated the columnar-to-equiaxed transition and improved both microhardness and tensile properties. The presence of both Al3Ti and TiB2 particles was confirmed using thermodynamic calculations. Average microhardness values increased form 39 HV for pure Al to 95 and 76 HV for Al-Ti and Al-Ti-B microalloys, respectively, by solid solution hardening. The enhanced wear behaviour of Al was due to the coarse-grained structure where the plastic deformation mechanism took place. Whereas, grain pull-out dominated the wear behavior of fine-grained specimens. It was concluded that the material with a smooth surface has high friction coefficient and low wear rate.
3 citations
TL;DR: In this paper, a neural network model has been developed to control the grain size in aluminum casting alloys, where some of the important grain refinement parameters such as casting temperature, holding time and addition level have been evaluated as inputs for the model.
Abstract: Optimization of casting parameters is essential in terms of quality factors in foundries. Nowadays, to optimize process parameters, new approaches such as artificial neural networks method are being used. In this study, a neural network model has been developed to control the grain size in aluminum casting alloys. Some of the important grain refinement parameters such as casting temperature, holding time and addition level have been evaluated as inputs for the model. The network training architecture was optimized at 241 training cycles with quasi-Newton algorithm with a single hidden layer and 6 neurons. With modeling, mean absolute percent error was found at 0.99 between experimental measurements and model estimation. R2 value has been calculated as 99.2%. The minimum grain size was measured for the parameter of 680 °C casting temperature, 0.25% Ti, 25-min holding time. It was found that there was a good agreement between experimental measurements and artificial neural network predictions.
3 citations
29 Dec 2017
TL;DR: In this article, a finite element analysis for the combined rolling-extrusion process, the stress-strain state, force on the tool, and the moments on the rolls are calculated as a function of the tool temperature and the rotational speed of the rolls.
Abstract: Based on the finite element analysis for the combined rolling-extrusion process, the stress-strain state, force on the tool, and the moments on the rolls are calculated as a function of the tool temperature and the rotational speed of the rolls. The calculations are performed for an Al – Ti – B system alloy containing 5% titanium and 1% boron, widely used in the industry for melt modification when casting ingots of aluminum alloys. The authors proposed ligature rods from this alloy to be produced by the method of combined rolling-extruding (CRE), which has significant advantages in comparison with the traditional technologies of continuous casting-rolling and discrete extruding. Therefore, for the design of technology and equipment for combined processing, it is necessary to have preliminary design data on the temperature-velocity conditions and energy-force parameters of the metal deformation process. For 3D modeling in a software package SolidWorks® the model of the combined rolling-extruding process was created, which was imported into the package DEFORMTM. The simulation process of producing a rod diameter of 9.5 mm by installing rolls with diameters of 462 mm and a protrusion stream 394 mm with rolling reduction of 50% drawing ratio during extruding 6.2 at the rotation speed of 9 rpm data obtained by the temperature distribution metal, strain rates, normal contact stresses on the tool and internal stresses in the metal. In addition, graphs of the change in the forces and moments of rolling acting on the rolls are plotted, depending on the rotational speed of the rolls and the required power of the drive motor is calculated. The obtained data were used in the design of new industrial equipment for combined rollingextruding of aluminum alloys and experimental studies, which confirmed the adequacy of the obtained modeling results.
3 citations
References
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TL;DR: In this article, a numerical model is presented for the prediction of grain size in inoculated castings and is tested against measured grain sizes obtained in standard grain-refiner tests on aluminium alloys.
870 citations
TL;DR: A review of the literature on grain refinement by heterogeneous nucleation and alloying can be found in this paper, where the types of grain refiner, Al-Ti-B master alloys in particular, and their methods of manufacture are discussed.
Abstract: Grain refinement of aluminium and its alloys is common industrial practice. The field has been extensively investigated by many workers over the past 50 years, not only to develop efficient grain refiners for different aluminium alloys, but also to achieve an understanding of the mechanism of grain refinement. The present review confines itself to the literature on grain refinement by heterogeneous nucleation and alloying. Initially, the fundamentals of grain refinement by inoculants are outlined. The types of grain refiner, Al-Ti-B master alloys in particular, and their methods of manufacture are next discussed. The grain refining tests to assess the efficiency of the grain refiners and the grain refining behaviour of aluminium alloys are also discussed in brief. The performance of a grain refiner, as well as the response of an aluminium alloy to grain refinement, is influenced by the microstructure of the grain refiner as controlled by the process parameters involved in its preparation and the alloying elements present in the aluminium alloy. The roles of these factors, and particularly the roles of poisoning elements such as Si, Cr, Zr, Li, are reviewed. The paper also reviews the mechanisms of grain refinement, the fading and poisoning phenomena, and the trends in the development of new grain refiners for aluminium alloys containing poisoning elements.
700 citations
TL;DR: Grain structure is an important and readily observable feature in cast aluminium alloys as mentioned in this paper, and three types of grain morphology are possible, namely, columnar, twinned columnar and equiaxed.
Abstract: Grain structure is an important and readily observable feature in cast aluminium alloys. Three different types of grain morphology are possible, namely, columnar, twinned columnar, and equiaxed. Inoculants in the form of master alloys are used to promote the formation of a fully equiaxed grain structure and this is termed grain refinement. Initially, fundamental aspects of solidification are outlined in order that the principles of grain refining using master alloys can be understood. Techniques for the commercial production and testing of common Al–Ti-based master alloys are then discussed briefly. The exact mechanisms by which grain refinement occurs are not yet fully understood and experimental and theoretical studies on the problem are critically reviewed with particular emphasis on (a) the role of solute titanium, (b) the thermodynamics of Al–Ti-based alloy systems, and (c) the nature of heterogeneous nuclei. Finally, current and future trends in the use of grain refining alloys are summarised.
572 citations
TL;DR: In this article, the commercial grain refining practice of aluminium has been experimentally simulated by introducing synthetic TiB2 crystallites into melts by a specially developed technique, and experimental findings indicate that TiB 2 crystallites alone do not nucleate α-Al.
Abstract: Despite the commercial importance of grain refinement and the volume of scientific studies on this topic, its mechanism is still unclear. There are several theories as to how and why commercial grain refiners (Al-Ti and Al-Ti-B) work, but careful analysis shows that no clear consensus has yet emerged. In the present study, the commercial grain refining practice of aluminium has been experimentally simulated by introducing synthetic TiB2 crystallites into melts by a specially developed technique. Experimental findings indicate that TiB2 crystallites alone do not nucleate α-Al. On the other hand, in the presence of dissolved Ti even below the peritectic level, an interfacial TiAl3 layer is formed at the TiB2/melt interface which subsequently nucleates the α-Al. The theoretical and practical implications of grain refinement phenomena are discussed in the light of the present experimental findings. A ‘duplex’ nucleation mechanism is proposed based on solute segregation to the substrate/melt interface.
405 citations
TL;DR: In this paper, a high-resolution electron microscopy investigation has confirmed the existence of a Ti-rich monolayer on the surface of the (0.0, 0.1) TiB2 surface, which is most likely to be a (1 − 1 2 ) Al3Ti 2DC.
391 citations