Defects and Properties of Cast Metals: Porosity
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Solidification Committee
Program Organizers: Mark Jolly, Cranfield University; Charles Monroe, University of Alabama; Brian Thomas, Colorado School of Mines; Peter Lee, University of Manchester
Tuesday 2:00 PM
February 28, 2017
Location: San Diego Convention Ctr
Session Chair: Murat Tiryakioglu, University of North Florida; David Browne, University College Dublin
2:00 PM Introductory Comments
2:05 PM Keynote
Porosity Formation and Shrinkage Effects in Alloy Samples Solidified on Earth and in Space as Observed In-situ by X-ray Monitoring: David Browne1; 1University College Dublin
The development of gas pores and volumetric contraction have been observed during equiaxed solidification of thin samples of Al-20wt.%Cu grain-refined alloy, using a micro-focus X-ray source and high resolution solid state detector/CCD camera. Furnace design is a key element in achieving a field-of-view which is geometrically isothermal throughout solidification, ensuring an even and random distribution of nucleation sites. Experimental improvements to reduce gas porosity are explained, and the effects of sample orientation and gravity on equiaxed solidification, pre-coherency grain motion, and shrinkage contraction are presented. Total elimination of gravity effects were achieved on the Maser-13 sounding rocket flight, launched from the Swedish Space Corporation’s Esrange Space Center, northern Sweden, on 1 December 2016, resulting in the only grain movement being due to the volumetric contraction at high fraction solid and during final intergranular eutectic growth. The project was funded by the European Space Agency.
2:25 PM Invited
Influence of Fe-rich Intermetallics on the Formation of Solidification Defects: Chedtha Puncreobutr1; Surada Chuaypradit1; André Phillion2; Julie Fife3; Peter Lee4; 1Chulalongkorn University; 2McMaster University; 3Paul Scherrer Institut; 4The University of Manchester
The enrichment of iron during aluminium recycling increases the quantity of Fe-rich intermetallics formed, particularly plate-like β-Al5FeSi intermetallics. This often promotes porosity formation and hot tearing susceptibility, limiting the production yield during the casting of recycled aluminium. To better understand the influence of Fe-rich intermetallics on solidification defect formation, fast in situ synchrotron X-ray tomographic microscopy experiments were performed for a range of iron levels and grain refining conditions in a commercial Al-Si-Cu alloy. Direct observations during solidification and semi-solid deformation experiments, together with CFD simulations, reveal that β-intermetallics contribute to defect formation via several different mechanisms. The presence of β-intermetallics strongly blocks the interdendritic channels and induces porosity formation and hot-tearing tendency as a result of permeability reduction and stress concentration. These novel insights into influence of intermetallics on defect formation highlight the very complex effect of secondary phase formation on the rheology and mechanical behaviour of semi-solid microstructures.
Modelling of Defects in Aluminium Castings: Laurens Katgerman1; Mark Jolly2; 1Delft University; 2Cranfield University
Over the last 4 decades, remarkable progress has been made in the modelling of casting processes. This is reflected in the proceedings of the 14 MCWASP conferences that have been held since 1980. Computer simulations have enabled a better understanding of the physical phenomena involved during solidification. Modelling gives the opportunity to uncouple the physical processes. Furthermore, quantities that are difficult or impossible to measure experimentally can be calculated using computer simulations e.g. flow patterns and recalescence. However, when it comes to accurately predicting casting performance and in particular the occurrence of defects like cracks, segregation and porosity there is certainly some way to go. In this paper, the current understanding of the main mechanisms of structure and defect formation during shape casting and DC casting processes will be reviewed and requirements will be discussed to give a direction to making casting models more predictive and quantitative.
Quantification of Porosity in Electron Beam Welded Dissimilar Steel to Fe-Al Alloy Joints by X-ray Computed Tomography: Soumitra Dinda1; Gour Gopal Roy1; Prakash Srirangam2; 1Indian Institute of Technology, Kharagpur, India; 2University of Warwick
Dissimilar joining of steel to Fe-7%Al alloy was carried out using electron beam welding (EBW) process under three different weld conditions such as with beam oscillations, without beam oscillations and at higher welding speed. Laboratory based X-ray computed Tomography(XCT) was used for 3D visualisation and quantification of porosity in dissimilar weld joints. Quantitative analysis of porosity such as the average size and number of pores were evaluated from XCT results as a function of beam oscillation and welding speed. XCT results show that the application of beam oscillation results in decrease in average size of the pores as well as decrease in number of pores in the welded joints. Further, it was observed that increase in weld speed results in increase in porosity in the weld joint. These quantified porosity information from XCT results as well the methodology of EBW of dissimilar joints will be presented and discussed.
3:25 PM Break
Role of Grain Refiners on Porosity Formation in Directionally Solidifed Al-Si Alloys: Muhammet Uludag1; Derya Dispinar2; 1Selcuk University; 2Istanbul University
Mechanical properties of aluminum alloys can be improved by alloying element addition where the microstructure is modified to enhance properties. Grain refinement is one of the methods and Ti is the preferred choice in the form of AlTi5B1 master alloy. Disadvantage of Ti grain refinement is the fading effect to due sedimentation. Alternatively, Ti-free Al-B grain refiners are also used. In this work, A356, A380 and A413 alloys were used and directional solidification apparatus was established in order to investigate porosity distribution. Sampling was made along the cast part and both vertical and horizontal sections were analyzed. Pore size, number of pores and average pore length were statistically measured. It was found that porosity was decreased in all three Al-Si alloy when Al-B grain refiners were used.
Self-Healing Micro-Porosity in Ductile Iron by Controlling Graphite Nodule Solidification Kinetics: Simon Lekakh1; 1MST
The shrinkage micro-porosity in ductile iron castings is a common problem. Precipitation of graphite nodules during solidification increases a specific volume and control of nucleation rate can be used for self-healing micro-porosity and improving casting soundness. The experimental castings were produced with variations in inoculation and pouring temperature. Porosity was measured in the casting sections and an automated SEM/EDX system was used to determine the “true” graphite nodule volume distribution. A novel method of structural reconstruction was developed to link the graphite nodule distribution to nucleation rate and detected shrinkage porosity in a test casting. It is shown that the second nucleation wave in low temperature poured and inoculated ductile iron eliminated interdendritic micro-porosity. The suggested method could be used in industry to improve the soundness of SGI castings.
Theoretical Calculations for Pore Formation in Aluminum during Solidification: Pedram Yousefian1; Murat Tiryakioglu1; 1University of North Florida
Porosity is one of the main reasons of hot tearing, which is one of the most serious defects in casting parts that happens under uniaxial tension in a poorly fed metal casting products. In this paper, Theoretical calculations for homogeneous and heterogeneous nucleation of pores in aluminum during solidification with and without dissolved gas were considered. Critical radius of the embryo was calculated by using Fisher’s approach with and without considering effects of curvature energy on the surface tension. Meager calculated values for the probability of homogenous vacancy clusters proved there is not any chance for observing homogeneous pore formation in pure solidifying aluminum. By considering the possibility of homogeneous nucleation in the pure melt, beyond a shadow of doubt, hot tearing may only happen by heterogeneous nucleation at bifilms or oxide surfaces entrained in the liquid aluminum as nucleation sites.
3D Visualisation of Porosity in Cast Al-Si Alloys Using X-ray Tomography: Mario De Giovanni1; Jason Warnett1; Mark Williams1; Prakash Srirangam1; 1University of Warwick
Al-Si alloys are widely used in the automotive and aerospace industries mainly due to their excellent strength to weight ratio, castability and corrosion properties. Various elements can be added to these alloys in order to alter the eutectic Si morphology from flake to fibre-like thus improving mechanical properties. However the addition of some of these elements is known to causes increased porosity within the alloy, which deteriorates the mechanical properties. Therefore understanding the size and distribution of porosity content is essential to understand the mechanical properties of the alloy. In this study laboratory source sub-micron resolution X-ray computed tomography (XCT) was used to visualise and quantify the porosity in unmodified and modified alloys. For example, XCT results show a large increase in porosity, such as in the case of modification by 400ppm Sr addtion. The results obtained from this study will be presented and discussed.