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Instruments / Technical Area	Publication Details	Published
I22-Small angle scattering & Diffraction	Characterising precipitate evolution in multi-component cast aluminium alloys using small-angle X-ray scattering Panagiotis Panagos , Y. Wang , D. G. Mccartney , M. Li , B. Ghaffari , J. W. Zindel , J. Miao , S. Makineni , J. E. Allison , O. Shebanova , J. D. Robson , Peter D. Lee Journal Of Alloys And Compounds DOI: 10.1016/j.jallcom.2017.01.293 Diamond Proposal Number(s): [13238] Show Abstract ▼ Abstract: Aluminium alloys can be strengthened significantly by nano-scale precipitates that restrict dislocation movement. In this study, the evolution of inhomogenously distributed trialuminide precipitates in two multi-component alloys was characterised by synchrotron small-angle X-ray scattering (SAXS). The appropriate selection of reference sample and data treatment required to successfully characterise a low volume fraction of precipitates in multi-component alloys via SAXS was investigated. The resulting SAXS study allowed the analysis of statistically significant numbers of precipitates (billions) as compared to electron microscopy (hundreds). Two cast aluminium alloys with different volume fractions of Al3ZrxV1-x precipitates were studied. Data analysis was conducted using direct evaluation methods on SAXS spectra and the results compared with those from transmission electron microscopy (TEM). Precipitates were found to attain a spherical structure with homogeneous chemical composition. Precipitate evolution was quantified, including size, size distribution, volume fraction and number density. The results provide evidence that these multi-component alloys have a short nucleation stage, with coarsening dominating precipitate size. The coarsening rate constant was calculated and compared to similar precipitate behaviour.	Jan 2017
I12-JEEP: Joint Engineering, Environmental and Processing	Combined deformation and solidification-driven porosity formation in aluminum alloys S. Bhagavath , B. Cai , R. Atwood , M. Li , B. Ghaffari , P. D. Lee , S. Karagadde Metallurgical And Materials Transactions A 49 DOI: 10.1007/s11661-019-05378-8 Diamond Proposal Number(s): [16188] Show Abstract ▼ Abstract: In die-casting processes, the high cooling rates and pressures affect the alloy solidification and deformation behavior, and thereby impact the final mechanical properties of cast components. In this study, isothermal semi-solid compression and subsequent cooling of aluminum die-cast alloy specimens were characterized using fast synchrotron tomography. This enabled the investigation and quantification of gas and shrinkage porosity evolution during deformation and solidification. The analysis of the 4D images (3D plus time) revealed two distinct mechanisms by which porosity formed; (i) deformation-induced growth due to the enrichment of local hydrogen content by the advective hydrogen transport, as well as a pressure drop in the dilatant shear bands, and (ii) diffusion-controlled growth during the solidification. The rates of pore growth were quantified throughout the process, and a Gaussian distribution function was found to represent the variation in the pore growth rate in both regimes. Using a one-dimensional diffusion model for hydrogen pore growth, the hydrogen flux required for driving pore growth during these regimes was estimated, providing a new insight into the role of advective transport associated with the deformation in the mushy region.	Aug 2019
I13-2-Diamond Manchester Imaging	Role of the local stress systems on microstructural inhomogeneity during semisolid injection S. Bhagavath , Z. Gong , T. Wigger , S. Shah , B. Ghaffari , M. Li , S. Marathe , P. D. Lee , S. Karagadde Acta Materialia 392 DOI: 10.1016/j.actamat.2021.117015 Diamond Proposal Number(s): [22053] Show Abstract ▼ Abstract: High pressure metal die casting is an extremely dynamic process with widely ranging cooling rates and intensifying pressures, resulting in a wide range of solid fractions and deformation rates simultaneously existing in the same casting. These process parameters and their complex interplay dictate the formation of microstructural solidification defects. In this study, fast synchrotron X-ray imaging experiments simulating high pressure die casting of aluminium alloys were conducted to investigate the effect of solid fraction, loading conditions and semisolid flow on local microstructural inhomogeneity. While most of the existing literature in this field reports speeds up to 10 µm/s for in situ deformation, the present work captures much faster filling and solidification, at speeds closer to 100 µm/s and at different solid fractions. Semisolid deformation of low solid fractions reveals two typical microstructural features: (i) coarser grains in the middle and finer ones near the walls, and (ii) remelting near the solid-liquid interface due to Cu enrichment in the liquid by the flow. Ex situ scans and digital image correlation analysis of the higher solid fraction samples reveal a porosity formation mechanism based on the local state of stresses, microstructure and feeding. Four different characteristics were identified: (i) plug flow, (ii) dead zone (densified mush), (iii) shear and (iv) bulk zones. These insights will be used to develop zone-specific strategies for the numerical modelling of defect formation during die casting.	May 2021
I13-2-Diamond Manchester Imaging	Mechanisms of gas and shrinkage porosity formation in solidifying shear bands Shishira Bhagavath , Zhixuan Gong , Tim Wigger , Saurabh Shah , Bita Ghaffari , Mei Li , Shashidhara Marathe , Shyamprasad Karagadde , Peter D. Lee Journal Of Materials Processing Technology 26 DOI: 10.1016/j.jmatprotec.2021.117338 Diamond Proposal Number(s): [22053] Show Abstract ▼ Abstract: In specialised solidification processing techniques such as High Pressure Die Casting, Twin-Roll Casting and others, an additional external deformation load is applied to achieve the required shape, leading to the formation of microstructural features such as shear bands. The mechanism for forming these features is believed to be dependent on dynamically evolving strain fields, which are dependent on the local solid fraction, applied strain rates and casting geometry. To investigate this, a semisolid ( 50 % solid fraction) Al-10 wt.% Cu alloy is isothermally injected into a bespoke die using a custom-designed thermo-mechanical rig. The semisolid deformation, formation of Cu-rich dilatant bands and subsequent pore nucleation and growth are captured using fast synchrotron X-ray radiography. The local normal and shear strains acting on the mush are quantified using Digital image correlation to identify the dilatant shear bands and the dominant local strain component. Correlating the radiographs with strain maps reveals that gas pores within the dilated interstices grow, while those in compressed regions are squeezed out. A linear correlation between accumulated volumetric strain and porosity volume fraction demonstrates that higher dilations give rise to a local increase in both gas and shrinkage porosity.	Aug 2021

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