Combined Deformation and Solidification-Driven Porosity Formation in Aluminum Alloys

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 onedimensional 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.

Progress on In Situ and Operando X-ray Imaging of Solidification Processes

“…Image processing steps such as registration, filtering, segmentation, and quantification, are performed using a number of tools and software applications (refer to [ 13 , 22 , 27 , 32 , 61 , 67 , 68 ] for more examples). Some of the recent quantification methods combined with machine learning-based trainable algorithms for feature recognition (such as weka segmentation (e.g., [ 29 , 69 ]), and Seo et al, [ 70 ] are proven to be extremely helpful in working with noisy data to identify features on large data-sets ([ 71 ]). A single in situ tomographic experiment often results in few tens of 3D scans, and thus, robust image processing algorithms are crucial for processing a large number of data sets collected over several experimental cases.…”

“…The in situ cells for such experiments have several complexities and challenges, particularly in developing precision heating, motion and rotation mechanisms, and integration with beamline control for specific needs. Over the last few years, the focus has been on exploring even more challenging processes, such as AM [ 25 , 26 , 27 ], high-pressure die casting (HPDC) [ 28 , 29 ], and inertial welding [ 30 ] where the cooling rates are over 10 3 K/s, and requires nearly sub-micron spatial resolutions, thus requiring frame rates of about 10 6 Hz.…”

“…During solidification, mechanical deformation is known to originate from imposed loading as well as weak thermal contractions, that are sufficient to cause hot-tearing (e.g., [ 36 ]). The dilation of the intergranular regions during deformation is known as Reynold’s dilatancy, and such dilatant shear bands accommodate hot-tears and shrinkage porosity (e.g., [ 29 ]). Various microstructures that form during solidification define the characteristics of the semi-solid region, where freely growing grains lead to granular behaviour and provide unintuitive responses to weak loads.…”

Heat Transfer Studies on Solidification of Casting Process

“…The superheated metal which is poured in the liquid state has more specific volume than the liquid metal in the cavity [2]. This liquid metal occupies the mold cavity and is in superheated state and comes in complete contact with the mold surface.…”

Effect of weld travel speed on solidification cracking behavior. Part 3: modeling

“…Stress based models applied solid-state fracture mechanics to liquid film rupture, taking into account surface energy effects and a modified Griffith criterion [65]. A pressure based model assumed a microscopic gas pore to grow as a pore [69,70] or a crack [51] [71][72][73] if the sum of pressures contributing to its growth (liquid pressure drop and dissolved gas pressure) exceeds the pressures contributing to its shrinkage (metallostatic pressure, capillary pressure, atmospheric pressure). Simplified mass balance approaches [36,67,68] consider the crack opening due to transverse deformation to be compensated by advancement of the crack and feeding of liquid.…”