Formation of hot tear under controlled solidification conditions

Aluminum alloy 7050 is known for its superior mechanical properties, and thus finds its application in aerospace industry. Vertical direct-chill (DC) casting process is typically employed for producing such an alloy. Despite its advantages, AA7050 is considered as a "hard-to-cast" alloy because of its propensity to cold cracking. This type of cracks occurs catastrophically and is difficult to predict. Previous research suggested that such a crack could be initiated by undeveloped hot tears (microscopic hot tear) formed during the DC casting process if they reach a certain critical size. However, validation of such a hypothesis has not been done yet. Therefore, a method to produce a hot tear with a controlled size is needed as part of the verification studies. In the current study, we demonstrate a method that has a potential to control the size of the created hot tear in a small-scale solidification process. We found that by changing two variables, cooling rate and displacement compensation rate, the size of the hot tear during solidification can be modified in a controlled way. An X-ray microtomography characterization technique is utilized to quantify the created hot tear. We suggest that feeding and strain rate during DC casting are more important compared with the exerted force on the sample for the formation of a hot tear. In addition, we show that there are four different domains of hot-tear development in the explored experimental window-compression, microscopic hot tear, macroscopic hot tear, and failure. The samples produced in the current study will be used for subsequent experiments that simulate cold-cracking conditions to confirm the earlier proposed model.This research was carried out within the Materials innovation institute (www.m2i.nl) research framework, project no. M42.5.09340

Nanoindentation and cavitation-induced fragmentation study of primary Al3Zr intermetallics formed in al alloys

Mechanical properties of primary Al3Zr crystals and their in situ fragmentation behaviour under the influence of a single laser induced cavitation bubble have been investigated using nanoindentation and high-speed imaging techniques, respectively. Linear loading of 10 mN was applied to the intermetallics embedded in the Al matrix using a geometrically well-defined diamond nano-indenter to obtain the mechanical properties at room temperature conditions. Primary Al3Zr crystals were also extracted by dissolving the aluminium matrix of an Al-3wt% Zr alloy. The extracted primary crystals were also subjected to cavitation action in deionized water to image the fracture sequence in real time. Fragmentation of the studied intermetallics was recorded at 500,000 frames per second. Results showed that the intermetallic crystals fail by brittle fracture mode most likely due to the repeatedly-generated shock waves from the collapsing bubbles. The results were interpreted in terms of fracture mechanics using the nanoindentation results

On the governing fragmentation mechanism of primary intermetallics by induced cavitation

One of the main applications of ultrasonic melt treatment is the grain refinement of aluminium alloys. Among several suggested mechanisms, the fragmentation of primary intermetallics by acoustic cavitation is regarded as very efficient. However, the physical process causing this fragmentation has received little attention and is not yet well understood. In this study, we evaluate the mechanical properties of primary Al3Zr intermetallics by nano-indentation experiments and correlate those with in-situ high-speed imaging (of up to 1 Mfps) of their fragmentation process by laser-induced cavitation (single bubble) and by acoustic cavitation (cloud of bubbles) in water. Intermetallic crystals were chemically extracted from an Al-3 wt% Zr alloy matrix. Mechanical properties such as hardness, elastic modulus and fracture toughness of the extracted intermetallics were determined using a geometrically fixed Berkovich nano-diamond and cube corner indenter, under ambient temperature conditions. The studied crystals were then exposed to the two cavitation conditions mentioned. Results demonstrated for the first time that the governing fragmentation mechanism of the studied intermetallics was due to the emitted shock waves from the collapsing bubbles. The fragmentation caused by a single bubble collapse was found to be almost instantaneous. On the other hand, sono-fragmentation studies revealed that the intermetallic crystal initially underwent low cycle fatigue loading, followed by catastrophic brittle failure due to propagating shock waves. The observed fragmentation mechanism was supported by fracture mechanics and pressure measurements using a calibrated fibre optic hydrophone. Results showed that the acoustic pressures produced from shock wave emissions in the case of a single bubble collapse, and responsible for instantaneous fragmentation of the intermetallics, were in the range of 20–40 MPa. Whereas, the shock pressure generated from the acoustic cavitation cloud collapses surged up to 1.6 MPa inducing fatigue stresses within the crystal leading to eventual fragmentation

Investigation of mechanical properties of Al3Zr intermetallics at room and elevated temperatures using nanoindentation

This work deals with the measurement of mechanical properties of single and polycrystalline Al3Zr specimens from ambient to elevated temperatures using nano-indentation experiments. In this study, we employed three kinds of intermetallic specimens produced from Al3Zr crystals chemically extracted from an Al-3 wt% Zr alloy. The properties such as elastic modulus and hardness were determined under quasistatic loading conditions. Constant multicycle indentation testing (MCT) was further performed using a Vickers indenter to understand the fatigue response of intermetallics at high load low cycle conditions. The results showed that hardness and elastic modulus of Al3Zr intermetallics depended on the crystal structure/orientation, with polycrystalline samples showing higher elastic modulus than single crystal specimens at room temperature conditions. MCT experiments revealed that contact pressure of more than 7 GPa was needed to fracture a crack-free crystal under dynamic loading conditions. Consequently the properties of intermetallics at temperatures up to 700 ◦C were determined for the first time, using high-temperature nano-indentation technique. Elevated temperature measurements indicated that intermetallics had high creep resistance at low and intermediate temperatures, but exhibited significant plastic deformation and creep close to the melting point of pure aluminium

Effect of ultrasonic melt treatment on the sump profile and microstructure of a direct-chill cast AA6008 Aluminum Alloy

This work focuses on the effects of ultrasonic melt treatment (UST) during direct-chill (DC) casting on the temperature distribution across the billet, sump profile, and the resulting microstructure. Two AA6008 billets were cast; one was treated with UST in the hot top while the other was not. To determine the temperature distribution along the billet, multi-point temperature measurements were made across the radii of both billets. The sump profile was also analyzed through macrostructure analysis, after Zn was poured into the sump, while structure refinement was quantified through grain-size measurements. A numerical model of ultrasound-assisted DC casting is validated using the temperature measurements. As an outcome, this study provides information on the extent to which UST affects the sump profile and the corresponding changes in the microstructure. The knowledge gained from this study paves the way towards optimization of UST parameters in DC casting

Structure refinement upon ultrasonic melt treatment in a DC casting launder

This work focuses on ultrasonic melt treatment (UST) in a launder upon pilot-scale direct chill (DC) casting of 152-mm-diameter billets from an AA6XXX alloy with Zr addition. Two casting temperatures (650°C and 665°C) were used to assess their effect on the resulting microstructure (grain size, particle size, and number density). Structure refinement results show the feasibility of UST in the DC casting launder. This is quantified through the corresponding reduction of grain size by around 50% in the billet center, or more towards the billet surface, reduction of the average Al3Zr particle size, and increase in the particle number density. A higher Al3Zr particle density was obtained when the alloy was cast at 665°C. Numerical simulation results and suggestions on how to improve the treatment quality of UST in DC casting launder are also provided

Ultrafast synchrotron X-ray imaging and multiphysics modelling of liquid phase fatigue exfoliation of graphite under ultrasound

Ultrasound-assisted liquid phase exfoliation is a promising method for manufacturing of 2D materials in large scale and sustainable manner. A large number of studies using ex-situ nano/micro structural characterization techniques have been made to investigate the underlying mechanisms, aiming to understand the exfoliation dynamics. Due to the complex multiphysics and multi-length nature of the process, those ex-situ methods cannot provide the real-time and in-situ dynamic information for understanding how exactly layer exfoliation starts and grows under ultrasound. Here, we used the ultrafast synchrotron-X-ray phase-contrast imaging (a combined temporal resolution of 3.68 μs and a spatial resolution of 1.9 μm/pixel) to study the exfoliation dynamics in real time and operando condition. We revealed, for the first time, the fatigue exfoliation phenomenon at the graphite surface caused by the imploding ultrasonic bubbles occurring cyclically in line with the ultrasound frequency. A multiphysics numerical model was also developed to calculate the shock wave produced at bubble implosion and the resulting cyclic and impulsive tensile and shear stresses acting on the graphite surface. Our research reveals that the graphite layer exfoliation rate and efficiency are predominantly determined by the number of imploding bubbles inside the effective cavitation bubble zone. The findings are valuable for developing industrial upscaling strategies for ultrasound processing of 2D materials

Ultrasonic cavitation erosion mechanism of free-floating Al3Zr intermetallics

In this paper, we investigate the cavitation-induced erosion and breakdown mechanism of free floating Al3Zr crystals exposed to ultrasonic vibrations in water at different exposure times using in-situ high-speed imaging technique and scanning electron microscopy (SEM). The post-mortem microstructural examination of the damaged crystals shows that the micron-sized hierarchical crack network structure is initially formed in the outer layer of the crystals. Subsequently, the cracked surface undergoes delamination with subsequent layer-by-layer breakdown into micro-fragments in the range of 5-50 μm. This process is accelerated every time the fragment is dragged into the cavitation zone by the recirculating acoustic flow conditions

In-situ observations and acoustic measurements upon fragmentation of free-floating intermetallics under ultrasonic cavitation in water

Grain refinement in alloys is a well-known effect of ultrasonic melt processing. Fragmentation of primary crystals by cavitation-induced action in liquid metals is considered as one of the main driving mechanisms for producing finer and equiaxed grain structures. However, in-situ observations of the fragmentation process are generally complex and difficult to follow in opaque liquid metals, especially for the free-floating crystals. In the present study, we develop a transparent test rig to observe in real time the fragmentation potential of free-floating primary Al3Zr particles under ultrasonic excitation in water (an established analogue medium to liquid aluminium for cavitation studies). An effective treatment domain was identified and fragmentation time determined using acoustic pressure field mapping. For the first time, real-time high-speed imaging captured the dynamic interaction of shock waves from the collapsing bubbles with floating intermetallic particles that led to their fragmentation. The breakage sequence as well as the cavitation erosion pattern were studied by means of post-treatment microscopic characterisation of the fragments. Fragment size distribution and crack patterns on the fractured surface were then analysed and quantified. Application of ultrasound is shown to rapidly (<10 s) reduce intermetallic size (from 5 mm down to 10 μm), thereby increasing the number of potential nucleation sites for the grain refinement of aluminium alloys during melt treatment

The Effect of Zn Content on the Mechanical Properties of Mg-4Nd-xZn Alloys (x = 0, 3, 5 and 8 wt.%)

The mechanical properties of as-cast Mg-4Nd-xZn (x = 0, 3, 5 or 8 wt.%) alloys were investigated both in situ and ex situ in as-cast and solution-treated conditions. The additions of 3 or 5 wt.% Zn in the base Mg-4Nd alloy did not improve yield strength in comparison to the binary Mg-4Nd alloy. Mechanical properties were shown to improve only with the relatively high concentration of 8 wt.% Zn to Mg-4Nd. The change in intermetallic morphology from a continuous intermetallic to a lamella-like intermetallic was the primary reason for the decreased mechanical properties in Mg-4Nd-3Zn and Mg-4Nd-5Zn compared with Mg-4Nd and Mg-4Nd-8Zn. The dissolution of intermetallic at grain boundaries following heat treatment further indicated the importance of grain boundary reinforcement as shown in both in situ and ex situ compression testing. Azimuthal angle-time plots indicated little grain rotation most noticeably in Mg-4Nd, which also indicated the influence of a strong intermetallic network along the grain boundaries