Defects and Properties of Cast Metals: Properties II & Hot Tearing
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 8:30 AM
February 28, 2017
Location: San Diego Convention Ctr
Session Chair: Hongbiao Dong, University of Leicester; Daan Maijer, The University of British Columbia
8:30 AM Introductory Comments Properties II
8:35 AM Invited
Study of The Species Macro-segregation in A356 Wheel Casting: Pan Fan1; Andre Phillion2; Steven Cockcroft1; Daan Maijer1; Carl Reilly1; Lu Yao1; 1University of British Columbia; 2McMaster University
A numerical model has been developed to study macrosegregation associated with liquid feeding in the Low Pressure Die Casting of A356 aluminum alloy wheels. The model of the wheel casting process has been implemented within the commercial CFD software package, FLUENT, using a User Defined Function (subroutine) to account for Si rejection based on the Scheil approximation. The model has been validated against temperature and microstructural data taken from a commercially cast wheel. The amount of segregation in the wheel has been shown to be significant in a couple of key areas. The implications of accounting for segregation include an improved ability to predict shrinkage porosity, fatigue performance and potentially also hydrogen porosity.
8:55 AM Invited
The Mechanism of a Rapidly Solidified Structure in Spray Forming: Hani Henein1; 1University of Alberta
Spray forming generates a rapid solidification structure while producing ingots with large dimensions. In contrast with casting that generates ingots of similar dimensions to spray forming, the microstructures generated by the two processes are very much in contrast. The ingots from casting and from spray forming experience the same cooling rates yet their structures are markedly different. Mathematical modelling of the spray forming process using energy and the momentum equations has not been successful in explaining the difference in structure. A single fluid technique provides an ideal tool to study the effect of atomization variables on the structure evolution of a spray formed deposit. It will be shown based on Cu-6Sn, Al-Cu, Al-Fe and D2 tool steel alloys that the undercooling of second and subsequent phases in the deposit accounts for the rapid solidified structure in spray forming. The mechanism of rapid solidification will be explained using a slushy balloon analogy.
Update on Bifilms - The Fundamental Defect in Cast Metals.: John Campbell1; 1University of Birmingham
The concept of the bifilm is now about 15 years old. The realization of its influence in metals has significantly advanced over these years. It has been clearly shown to control structural defects such as porosity and hot tearing, plus mechanical properties such as ductility and fatigue. It appears to explain for the first time the structure of Al-Si alloys and cast irons. Furthermore, it appears to be the only crack initiating feature in metals, thereby constituting the only source of Griffith cracks. Thus it has control over crack initiation for metals. As a result, it seems that failure by fatigue and creep may be eliminated, together with invasive corrosion such as pitting, stress corrosion cracking and possibly other failure modes. The elimination of bifilms by improved melting and casting methods appears to be possible, and would deliver metals of extraordinary properties not seen so far.
4D Synchrotron X-ray Imaging of Magnetically Controlled Al Alloy Solidification: Biao Cai1; Andrew Kao2; K. Pericleous2; Peter Lee1; 1University of Manchester; 2University of Greenwich
The use of magnetic fields to influence solidification via damping, or inducing, fluid flow is well established; however, there are only a few studies that quantify the changes in grain growth and morphology in situ, and those all in 2D using radiography. In this study, we use high speed synchrotron tomography and a bespoke directional solidification furnace with an external static magnetic field to quantify the time resolved kinetics of the process in 4D (3D plus time). This allows us to directly compare the evolving microstructures of directionally solidifying Al alloys with and without an applied magnetic field. The findings reveal the potential mechanisms by which an imposed magnetic field can significantly modulate dendrite growth rates and morphology.
In-situ Synchrotron X-ray Imaging of Inter-dendritic Fluid Flow Using a Model Al-Pb Alloy: Enzo Liotti1; Andrew Lui1; André Phillion2; Patrick Grant1; 1University of Oxford; 2McMaster University
Inter-dendritic fluid flow is a key factor controlling the formation of solidification defects such as macro-segregation, hot tearing and shrinkage porosity. Castings are designed to facilitate the fluid flow and feeding of residual liquid into solidifying and shrinking regions to minimize these defects. However, these designs are optimized using empirical rules or numerical models that rely on experimentally fitted parameters because little is known quantitatively about the nature of fluid flow within the mushy zone. We present preliminary results of direct measurements of inter-dendritic flow using in-situ X-ray radiography of a model Al-Pb alloy. Foil samples with compositions that formed tiny liquid Pb droplets within inter-dendritic channels were directionally solidified and the resulting videos analysed to produce time-resolved vectors maps of the fluid flow under different conditions. Measurements and arising insights are compared with those from simulations, and future directions for this technique are discussed.
10:15 AM Break
10:35 AM Introductory Comments Hot Tearing
10:40 AM Keynote
Prediction of Hot Tearing in Steel and Aluminum alloys: Andre Phillion1; 1McMaster University
Hot tearing defects commonly occur during the casting of industrial metals. Over the years, this defect has been extensively studied, from macroscopic criteria relating hot tearing susceptibility to the solidification interval, permeability, stress, strain and strain-rate, to microscopic examinations of grain coalescence. Recently, multi-physics or granular solidification models have been developed that combine fluid flow behaviour at the scale of the microstructure with stress/strain behaviour at the scale of the component. In this presentation, a comparison of recent advances in modelling of hot tearing in steel and aluminum alloys will be made identify common mechanisms as well as differences in behaviour. This will assist in identifying new directions of research with the goal of improve quantitative hot tearing prediction.
11:00 AM Keynote
X-ray Imaging of Solidification Cracking during Welding of Steel: Hongbiao Dong1; 1University of Leicester
Solidification cracking during high-solidification-rate manufacturing processes such as welding and additive manufacture of metals is one of the key phenomena in controlling defect formation. Direct experimental observation of cracking has been challenging for high speed solidification processes. To address the issue, a novel welding stage has been developed for use in high-speed, high-energy synchrotron X-ray radiography experiments. Our preliminary results reveal that it is possible to elucidate the initiation and growth mechanisms for solidification cracking under high-solidification-rate manufacturing process conditions.
Hot-tearing of Multicomponent Al-Cu Alloys Based on Casting Load Measurements in a Constrained Permanent Mold: Adrian Sabau1; Seyed Seyed Mirmiran2; Christopher Glaspie2; Shimin Li3; Diran Apelian3; Amit Shyam1; J. Haynes1; Andres Rodriguez4; 1Oak Ridge National Laboratory; 2Fiat Chrysler Automobiles North America; 3Worcester Polytechnic Institute; 4Nemak Monterrey
Hot-tearing is a major casting defect that is often difficult to characterize, especially for multicomponent Al alloys used for cylinder head castings. The susceptibility of multicomponent Al-Cu alloys to hot-tearing during permanent mold casting was investigated using a constrained permanent mold in which the load and displacement was measured. The experimental results for hot tearing susceptibility are compared with those obtained from a hot-tearing criterion based temperature range evaluated at fraction solids of 0.87 and 0.94. The Cu composition was varied from approximately 5 to 8 pct. (weight). Casting experiments were conducted without grain refining. The measured load during casting can be used to indicate the severity of hot tearing. However, when small hot-tears are present, the load variation cannot be used to detect and assess hot-tearing susceptibility.
11:40 AM Cancelled
Semi-solid Mechanical Behaviour and Hot-tearing of a 7050 Alloy: Experimental Analysis and Thermomechanical Modeling: Kjerstin Ellingsen1; Arne Nordmark1; Mohammed M'Hamdi1; 1SINTEF
Aluminum 7xxx alloys has excellent mechanical properties, but are hard to cast due to relatively low thermal conductivity and large solidification range. A hot-tearing experimental set-up allowing for investigation of the hot-tearing susceptibility of industrial aluminium alloys has been used to study the semi-solid behavior of a 7050 alloy. Load and temperature are registered during constrained solidification giving information on the mechanical behavior of the alloys during solidification. The experimental analysis have been combined with thermo-mechanical modelling of the experiment and a semi-ermpirical hot-tearing criteria based on stress values from tensile testing in the semi-solid is proposed.
The Nucleation and Growth of Hot Tearing during Strip Casting Steel: Wanqiang Xu1; Michael Ferry1; 1The University of New South Wales
Hot tearing is a severe defect formed on the surface of castings, ingot, bloom, billet and slab during the solidification of metals and alloys. It often causes expansive cost to fix or direct rejection to these casting products. Its formation process and mechanism, such as where and how it nucleates, and how it propagates after nucleation, is still unclear, although experimental and modelling works have been intensively carried out in this hot research area for many decades. Herein, we carefully carried out a experiment by systematic investigation into the effects of processing variables and casting environment on the defect formation during strip casting steel, and found out its nucleation and propagation process by symmetrically examining into the macro- and micro-structures of the defect.
Investigation of Hot Tearing A380.1 In “T Shape Mold”: Muhammet Uludag1; Remzi Cetin2; Derya Dispinar3; Murat Tiryakioglu4; 1Selcuk University; 2Halic University; 3Istanbul University; 4University of North Florida
In this study, A380.1 alloy was examined under six different conditions to evaluate hot tearing formation. Traditional T shape mold was used for hot tearing test. Grain refinement by AlTi5B1 addition and Al3B addition were the main parameters of this study. For each test, samples were cast before and after degassing of the melt. Hot tearing tendency value was calculated from T shape mold tests by taking into account crack size and its location. Relationships between this new proposed index was provided by including both bifilm calculations and porosity that occurred middle of the T zone of casting parts.