Effect of phosphorus on the density and molar volume of Al–Si alloy without solidification shrinkage

The Al–Si alloys exhibit many unique properties, but not enough work has been dedicated to their thermophysical properties. In this work, the effect of phosphorus modifier on the density, molar volume and solidification shrinkage rate of Al-25% Si alloys was investigated by using the indirect Archimedes method. The results show that both density–temperature and molar volume–temperature curves show three inflection points: the liquidus temperature point, the eutectic transformation starting point and the finishing point. The density of the solidus linearly decreases and that of the liquidus linearly increases with phosphorus modifier content. Compared with Vegard’s law, the molar volumes show a negative deviation. Finally, the solidification shrinkage rate is calculated from the densities of solidus and liquidus.

Multiphase Model for the Prediction of Shrinkage Cavity, Inclusion and Macrosegregation in a 36-Ton Steel Ingot

A five-phase model consisting of a liquid phase, columnar dendrites, equiaxed grains, air, and inclusion (discrete phase) is developed to predict the shrinkage cavity, inclusion distribution and macrosegregation simultaneously during solidification of a 36-ton steel ingot. The air phase is introduced to feed the shrinkage cavity and no mass or species exchange with other phases occurs. The transport and entrapment of inclusions are simulated using a Lagrangian approach. The predicted results agree well with the experimental results. The characteristics of inclusion distribution are better understood. A thin layer of inclusions tends to form close to the mold wall, and more inclusions reside in the last solidified segregation channels. The inclusion is easy to aggregate near the riser neck, and it is dragged by the solidification shrinkage. The influence of the inclusion on macrosegregation is comparatively small, while the solidification shrinkage affects the formation of macrosegregation significantly and makes the simulation result more accurate.

Application of a Novel Chamfered Mold to Suppress Corner Transverse Cracking of Micro-Alloyed Steel Slabs

A novel chamfered mold is developed to solve the problem of corner transverse cracking in micro-alloyed steel slabs. The shape of the slab was changed from four corners and sides to eight corners and sides due to the use of a chamfered mold. Based on numerical simulation, the solidification and heat transfer of different steel grades in the mold are studied. The results reveal rapid solidification shrinkage of molten steel in the upper area of the mold and slow solidification shrinkage in the lower area; thus, a double-taper mold is designed according to these results. The first area of the variable taper falls in the range of 250–400 mm from the top of the mold, and the design method of double inclined water channels in the chamfered face is found to be the most helpful for the formation of a uniform initial shell and reducing hotspots of the mold. Actual production results show that the quality of the slab is better when the heat flux of the narrow face is larger than that of the broad face. Corner transverse cracking in micro-alloyed steels is greatly reduced from an incidence of 4.2% to less than 0.3%. In addition, the chamfered mold is applied in IF (interstitial free) steel production, and the edge quality of hot rolled sheets is found to also be dramatically improved.

Separation of the Formation Mechanisms of Residual Stresses in LPBF 316L

Rapid cooling rates and steep temperature gradients are characteristic of additively manufactured parts and important factors for the residual stress formation. This study examined the influence of heat accumulation on the distribution of residual stress in two prisms produced by Laser Powder Bed Fusion (LPBF) of austenitic stainless steel 316L. The layers of the prisms were exposed using two different border fill scan strategies: one scanned from the centre to the perimeter and the other from the perimeter to the centre. The goal was to reveal the effect of different heat inputs on samples featuring the same solidification shrinkage. Residual stress was characterised in one plane perpendicular to the building direction at the mid height using Neutron and Lab X-ray diffraction. Thermography data obtained during the build process were analysed in order to correlate the cooling rates and apparent surface temperatures with the residual stress results. Optical microscopy and micro computed tomography were used to correlate defect populations with the residual stress distribution. The two scanning strategies led to residual stress distributions that were typical for additively manufactured components: compressive stresses in the bulk and tensile stresses at the surface. However, due to the different heat accumulation, the maximum residual stress levels differed. We concluded that solidification shrinkage plays a major role in determining the shape of the residual stress distribution, while the temperature gradient mechanism appears to determine the magnitude of peak residual stresses.

Development of Finite Element Model for the Control of Solidification Shrinkage in Al-Si (A8011) Alloy Castings

The objective of this research work is to determine a realistic way of minimizing shrinkage in Aluminium-Silicon (Al-Si) alloy castings using finite element modelling. Finite Element method was used to discretize and solve the governing equations developed for the models using the commercial software, Comsol Multi-Physics. The models developed were validated from experimental data obtained from the foundry using six samples which were used to study the temperature profiles and nature of the solidification of the alloys. A comparison of the temperature profiles generated from the experiments and simulations show that in 64% of the processes, there were no significant differences between the experimental and simulated values. In comparing the Niyama values obtained from the experiments and those from the simulations, there were no significant differences in 46% of the processes. Threshold Niyama values of 0.143 (°C-s)1/2/mm was also established. Below these threshold values, it is predicted that shrinkage will occur in castings from these metals.Keywords— Aluminium alloy, Al-Si (A8011), Castings, Finite Element, Shrinkage, Solidification

Influence of Alloy Elements on Cracking in the Steel Ingot during Its Solidification

The average steepness of |dT/d(fs)1/2| on the T − (fs)1/2 curve were calculated during peritectic solidification, which was used to investigate the effect of alloying elements on surface longitudinal cracks of peritectic steels in the solidification process. The value of |dT/d(fs)1/2| indicates the liquid feeding capacity between interdendrites during solidification, where cracks can easily occur if there is poor capacity of liquid feeding, as in peritectic solidification shrinkage. The cracking tendency as a function of carbon content was well described by the |dT/d(fs)1/2| at the cooling rates of 0.5, 5, and 10 °C/s, and the influences of other solute elements on |dT/d(fs)1/2| were also calculated. The results indicate that the possibility of crack occurrence increased and the maximum average steepness |dT/d(fs)1/2| changed from 496.75 °C located near 0.09C wt.% to 622.14 °C near 0.11C wt.% with increasing cooling rate. The value of |dT/d(fs)1/2| on the T − (fs)1/2 curve during the peritectic solidification can be used to analyze the solidification crack for peritectic steels.