iCT Conference 2014 – www.3dct.at 149

An innovative use of CT method in light metals development Christoph Angermeier1, Amir M. Horr1

1LKR Leichtmetallkompetenzzentrum Ranshofen GmbH, Austrian Institute of Technology, Postfach 26, 5282 Ranshofen, Austria, e-mail: christoph.angermeier@ait.ac.at, amir.horr@ait.ac.at

Abstract During the past two decades, the macrostructural characteriziation and its effects on material properties have been considered by many authors, and special attention has been devoted to the material strength and mechanical design. Varios methods have been proposed to investigate the material defects using microstructure characteriziation, e.g. ultrasonic scanning, macro etching, light microscopy, scanning electron microscopy and X-ray computer tomography (CT). Conventionally, determining component porosity would require destructive testing. However, industrial CT scanning can detect internal features and flaws without destroying the part. It can detect flaws inside a part such as porosity, an inclusion or a crack before a failure can occur. Studying porosity variation in different metals and alloys, while varying the percentage composition of the material (especially under different compressive and tensile loads), is an important consideration for the mechanical strength and design of metal parts. The industrial CT, depends on its resolution, can even detect and localized small defects in metals and casting parts in three dimensions. In recent years, in order to investigate component defects, the use of industrial CT has been emphasized in the Leichtmetallkompetenzzentrum Ranshofen (LKR). The macrostructural characterization and its effects on material strength are carried out using detection of material defects (in particular pores) by CT. There have been various projects where the CT has been used to investigate the material characterization, e.g., the characterization of cell shape and cell distribution in metal foams (aluminium foams) and also the microstructure defects like gas pores and shrinkage porosity of casting parts (in the high pressure casting process). The three dimensional scanning of samples and the possibility of porosity analysis opened up an opportunity to measure voids volume fraction and the size of the inclusions. The subsequent damage modelling would establish the material strength and durability of parts for their design life time. One of the main contributions of this paper is to show the advantages of using CT in material characterization and damage modelling. The squeeze casting process (used to cast aluminium step plates) has been modified in order to achieve a high velocity/pressure casting quality parts using an atomized flow characteristic (atomized jet at inlet). Typical flow (melt) velocity of about 50m/s has been achieved beyond the gate where a jet front is formed immediately after the inlet. This has been accomplished by scaling down the gating of a step plate mold where the melt has been injected with high velocity. The geometry of the mold itself has not been modified which resulted in casting plate being identical (in terms of size and thickness) to squeeze casting ones. This has opened up an opportunity to carry out extensive comparative investigations on the effect of casting process and its conditions on the quality of the plates. Further experimental and simulation tasks have also been carried out to investigate the variation of porosity and its effects on accumulation of damage in light metal alloys using CT results.

Keywords: Computer Tomography, Porosity, Damage Model, Mechanical Strength, Aluminium

1 Introduction CT is an innovative and ascending technology in non-destructive material testing. Many research studies about CT possible applications in material science have been carried out recently to deal with

150

casting process optimization, fracture mechanics, fatigue life, damage modeling and also microstructural characterization. These investigations were concern with different technological approaches of CT scanning. Outstanding results [1] have already been gained from the use of a synchrotron CT scanning in light weight alloys (e.g., TiB phase in a TiB-reinforced Ti-6Al-4V was detected down to a size of 200nm). There have also been some in-debt investigations for use of CT scanning during solidification [2] and also as an effective tool for detection of the structure of pores to generate input for Finite Element (FE) analyses. These FE models have then been used to calculate fatigue critical stress concentrations at the pores [3]. Furthermore there have been some efforts to use CT technology to quantify casting defects for reference part test piece [4] where an innovative new approach has been developed. More recently, developments of advanced CT technology have focused on the synchrotron accuracy in standard laboratory scale. The so called sub-µCT technology is already available in some research centers and is going to be further developed. A comparison of synchrotron and sub-µCT was made by Kastner with a focus on different intermetallic in aluminum-silicon alloy [5]. While the scientific research works improve the application potential of the CT technology, the casting industry uses the technology mainly in pore detection to avoid rejections in a serial production [6].The research work herein is concern with the well-known advantage of CT in detecting pores and shrinkage voids in aluminum alloys. A brief description of past LKR experiences with CT technology has been given first. The presented work is using well-established feature of CT scanning on void detection of lightweight components to predict material behavior. In the recent research works [7-8], the flow regime of the melt, corresponding to the nozzle geometry, for High Pressure Die Casting (HPDC) mold have been investigated to detect porosity distribution. The same HPDC casting geometry has also been used in the study herein to open up the possibility of comprehensive comparative study. 2 Past CT Experiences in LKR As a well-established research center for light weight metals, many projects have been carried out in LKR using various technologies including CT. During the past decade, the investigation of light weight metal foam characterization has grown rapidly where LKR has been on forefront of this in-debt research field. The CT technology has been used to process the results of foaming techniques in light weight alloys. The target of one of the main research projects was to use the light weight metal foam formations as cores to tubular hollow sections which were cast as an integral entity in aluminum parts. Two cast processes were investigated, the Low Pressure Die Casting (LPDC) and the squeeze casting process. While no technical issues recorded with LPDC process, the higher melt pressure of the squeeze casting process led to infiltration of the metal foam cores, as can be seen in Figure 1. It was decided later that a certain type of coating needed to be applied on the cores to avoid infiltration effects. Another typical use of CT technology at LKR is its application in casting process development and part inspection. In particular for the concept of a wheel hub design (an ironless axial flux with permanent magnet drive), as shown in Figure 2, CT scanning was used to optimize the casting process. The wheel hub was cast in magnesium sand casting process at LKR. While the molds were produced by rapid prototyping, several versions of it were cast and examined. Figure 2 also shows a slice image of wheel hub scan, corresponding to an improved mold design. For lowering the scanning artifacts, 0.5mm thick copper plate was used as a pre-filter of the X-rays.

iCT Conference 2014 – www.3dct.at 151

Figure 1: CT slices of cast integral aluminum foam cores, remained stable (left) and infiltrated (right)

Figure 2: Exposition of wheel hub in Light Metal Technology (LMT) conference, Gmunden, 2012 and CT scan of

the magnesium wheel hub

3 HPDC and CT Application There has been some cast trails using LKR squeeze casting machine to investigate the possibilities of emulating HPDC process. The cast material was processed on a UBE HVSC 350 squeeze casting machine using a step plate mold. To emulate the HPDC process, the dimension of the nozzle was changed from squeeze casting thickness of 12mm to much smaller geometry (0.5 and 1mm), as can be seen in Figure 3. This enables the filling of the step plate mold with much higher melt velocity. While a nominal melt speed of 50m/s was adopted from HPDC, the previous investigations on the flow regime, shown an optimum gating widths of 0.5mm to 1.0mm [7]. The cast process parameters were calculated based on the new nozzle design to achieve 50m/s melt velocity. The plunger speed has also been adjusted to 0.5 to 1.0m/s accordingly. While the filling process in squeeze casting is naturally a low turbulence melt flow process, the adaptation of narrow gating geometries induces high turbulence and atomized flow regime. Hence, high porosity samples are expected to be cast from the adaptation of the new process parameters and also the nozzle design.

152

Figure 3: Details of new nozzle designs for step plates and UBE HVSC 350 Squeeze Casting machine at LKR

A common A226 high pressure die casting alloy was chosen for the casting trials. The actual composition was measured in a spark analysis (listed in Table 1). Degasing the melt with pure dry argon was done before the start of casting for 30 minutes to ensure a low level of hydrogen content. The melt temperature was controlled below 730°C and set to 720°C for casting. After casting the step plates, the gating and the overflow were removed and the samples cooled down using natural air cooling.

Legierung Si Cu Fe Zn Mn Mg Ni Ti Cr B

AlSi9Cu3 9.9 2.7 0.8 0.8 0.22 0.16 0.07 0.05 0.05 0.005

Table 1: Alloy composition, measured in spark analysis

4 Scanning Process A step plate which has been cast using the squeeze casting process, and also two step plates from emulated HPDC cast process (using new nozzle designs) were investigated by CT technology in the University of Applied Sciences Upper Austria, Wels Campus. An identical “Region Of Interest” (ROI) has been defined for all cast samples (with same cross section area). The samples have been scanned in two runs, each with three steps, in order to achieve a high resolution scanning. The pore detection procedure was also carried out by grey value analysis. The actual CT parameters for the analysis are shown in Figure 4 which defines a voxel size of 77µm along with ROIs regions and their corresponding porosity percentage. Grey value variation along the cross section of the ROIs was negligible due to their localization. Further investigations have also been performed in metallography group on micro-sections of step plates for both new nozzle designs (with 0.5 and 1mm dimensions) from the same casting trials. Additionally the microstructure of the step plates cast material was also investigated in light microscopy using the cast material from both new gating designs. The area percentage of pores was measured by intersect line method and the results compared with CT.

iCT Conference 2014 – www.3dct.at 153

Figure 4: ROIs, their porosity percentage(CT – measured in computed tomography; MT – measured in metallography) and CT scanning parameters for step plate investigations

5 Mechanical analyses – CT technique for damage modelling There are different mathematical formulations to explain the damage and failure of material under external loading condition. The failure of metals and alloys is a result of progressive damage under plastic deformation [9-10]. As the external load is increasing on the material, a significant plastic deformation may take place which would result in the ultimate failure of the cross section. The numerical models for damage evolution, fracture initiation and also its propagation can be formulated by the means of continuum/discrete damage techniques [11-12]. An initial investigation has been carried out herein to establish the effectiveness of some existing numerical damage models in predicting the evolution of damage and failure modes. These damage models need to be adapted for the material properties of aluminium alloys using parameters characterisation. The initial defects and void distribution (from casting process) parameters for damage models have been determined first, using CT scanning results. The volume fraction (the ratio of void volume over total volume of materials) and also average zonal void sizes can be extracted from CT scan results. The cast alloy component has been discretised into different zones where void volume fractions and their sizes have been averaged over the zone. The mechanical material properties (material card) can be defined by combing the CT initial defect data with elastic-plastic mechanical data (i.e., from experimental results) for each zone. A series of flat uniaxial specimens, notched specimens with different radius (varying tri-axiality) and also charpy tests have been carried out (see Figure 5) to determine the elasto-plastic mechanical properties. In the numerical damage modelling, the mechanical damage parameter needs to be defined carefully and the ductility diagram needs to be fitted into the different experimental results. The uniaxial smooth tensile specimens generally show higher ductility (larger plastic zone) under load, while the notched specimens usually show lower ductility. The stress tri-axiality status of materials under tension can affect their ductility and extend of plastic zone where it is proven that the material can have brittle fracture at higher tri-axiality.

154

Figure 5 – Experimental specimen, cast step plate and test machines

The tension tests have been carried out using a displacement control setup. During the tests, forces and displacements (strains) are measured with reasonable sampling rate. The displacements are applied slowly; hence the strain rate effects have been neglected. The standard Charpy impact test is also used to characterize material impact and fracture behaviours, such as impact and fracture resistance and also brittle-ductile transition behaviour. The 4mm thick V-shaped notch (with 2mm smooth notch) specimens were machined out of the cast alloy and the impact tests have been performed at room temperature.

6 Numerical Damage Simulation The results of some advanced simulation work for the damage evolution and failure in the aluminium alloy have been presented in this part of the paper. These simulation works use the same geometry as the experimental specimens. As stated in the previous sections, in the simulation of damage accumulation and failure in cast alloys different source of defects need to be considered. The defects in the cast alloy can be sourced back to the casting process (air entrapment, gas porosity, inclusions, shrinkage…) or it might be relate to macrostructure and secondary phases. The accurate prediction of the damage evolution, crack initiation and propagation and also failure of the material has to take into account these defects either explicitly or implicitly [13]. The generated defects during cast process can be quantified using either cast process simulation (using software tools like ProCast, Magmasoft, Novacast, Fluent…) or detection procedures (e.g., CT technology, Metallography…). The CT pores detection technique have gain wide spread popularities in recent years and the challenging issues of quantification of defects during cast process can now be addressed in a strait forward manner.

iCT Conference 2014 – www.3dct.at 155

As explain in previous sections, the X-Ray CT scanning technology has been used in recent years as an effective way to look at internal defects. It can generate high resolution 3D images of the cast part being scanned. Using industrial CT scan for cast alloy samples, even low-contrast defects (e.g., cracks, pores and blowholes), can be identified and measured in three dimensions. Post-processing of the 3D defects images can be carried out by either multi-positional 2D cross-section planes or the 3D “volume view”. Cast parts with complex geometries (even with inaccessible or hidden zones) can reliably be scanned for defects with CT. The cast process defect identification has been setup for the research work herein and a clear frame work has been defined to isolate and quantify major sources of defects in HPDC. The porosity quantification has been carried out using a CT scan results (From Wels scanning laboratory) where cast step-plates have been scanned for porosity defects. A CT scanner has been used in the investigation herein and the image processing techniques are used to post process the scan images for pores detection. To simulate the experimental specimens for damage and failure, a combined Gurson damage model [9] with power law plasticity has been employed where the initial porosity for the whole sample has been taken from the CT scan results. The full displacement control simulations have been carried out using the fine mesh (see Figure 6) in Ansys general purpose finite element software. The meshing has been carried out in Ansys Mechanical mesher module with 0.25mm mesh size. Mesh dependency of the results has not been taken into account for these runs and hence, no regularisation of the results has been carried out. The two dimensional quad elements are used for these simulation runs where a “hex dominated” mesh is used to discretised the model. The post-processed results of these simulation works show (Figure 6) a very promising damage prediction for these samples when they compared with experimental failures. The void growth, nucleation and coalescence are taken into account for all damage simulation runs and a combined Gurson-Chobache-power law damage model is used.

Figure 6: Geometry, mesh, stress Intensity contour, true stress-strain curves and material card

156

7 Concluding Remarks The use of CT scanning in numerical damage modelling has been presented in this paper, where the effects of casting defects have been investigated on the damage accumulation in aluminium alloys. In the first part of the paper, a brief review has been carried out to study CT technology and its application for detection of cast defects and also its subsequent numerical damage modelling. While in the second part of the paper, a description of a comprehensive set of experimental works along with novel damage simulation modelling have been discussed. The approach is somehow unique, as the conventional defect detection methods and the subsequent damage models appears to lack efficiency and accuracy. The comparative analyses between the experimental and simulation results show that the numerical models, with an initial porosity input from CT scanning, have generated reasonable accurate predictions for the damage and failures in light weight alloys. The overall affordable simulation time and also accuracy of accumulated damage and failure model would make the method quit attractive for practical use in industrial projects. An important point herein is that since the CT technology provides high resolution and detailed defect information, it can also be used to fully verify advanced cast process simulations results (distribution and sizes of voids…) for light weight alloys. It can be concluded that scanning power of the CT technology and the advanced numerical damage models can be combined together to create a powerful tool to assess the quality of cast components. It also streamlines the quality control procedures for the industrial HPDC productions. The opportunity to numerically investigate the more advanced damage accumulation and failure models for lightweight alloys would be taken and this would be the subject of subsequent research projects. The powerful detection features of the advanced CT technology would be tied to material characterisation, defects modelling and also composition effects in an automated manner. These investigations would be carried out with collaborations of our academic and also industrial partners to achieve practical and reliable simulation methods.

Acknowledgements The authors would like to thank Austrian Research Promotion Agency (FFG), the Federal Ministry for Transport, Innovation and Technology (bmvit), and the State of Upper Austria for sponsoring this research work in the framework of COMET. The financial support of LKR bei Georg Fischer Automotive AG is also gratefully acknowledged.

References [1] G. Requena, P. Cloetens, W. Altendorfer, C. Poletti, D. Tolnai, F. Warchomicka, and H. P.

Degischer, “Sub-micrometer synchrotron tomography of multiphase metals using Kirkpatrick–Baez optics,” Scr. Mater., vol. 61, no. 7, pp. 760–763, Oct. 2009.

[2] D. Tolnai, P. Townsend, G. Requena, L. Salvo, J. Lendvai, and H. P. Degischer, “In situ synchrotron tomographic investigation of the solidification of an AlMg4.7Si8 alloy,” Acta Mater., vol. 60, no. 6–7, pp. 2568–2577, Apr. 2012.

[3] G. Nicoletto, R. Konečná, and S. Fintova, “Characterization of microshrinkage casting defects of Al–Si alloys by X-ray computed tomography and metallography,” Int. J. Fatigue, vol. 41, pp. 39–46, Aug. 2012.

[4] A. Staude, M. Bartscher, K. Ehrig, J. Goebbels, M. Koch, U. Neuschaefer-Rube, and J. Nötel, “Quantification of the capability of micro-CT to detect defects in castings using a new test piece and a voxel-based comparison method,” NDT E Int., vol. 44, no. 6, pp. 531–536, Oct. 2011.

[5] J. Kastner, B. Harrer, G. Requena, and O. Brunke, “A comparative study of high resolution cone beam X-ray tomography and synchrotron tomography applied to Fe- and Al-alloys,” NDT E Int., vol. 43, no. 7, pp. 599–605, Oct. 2010.

iCT Conference 2014 – www.3dct.at 157

[6] Ambos, E., Neuber, D., Besser, W., Stuke, I., Teubner, S., Lux, H., and Brunke, O., “Einsatz der Schnellen Computertomographie zur Prorositätsbewertung an Druckgussteilen,” Gie, vol. 60, no. 1/2, pp. 14–22, 2013.

[7] Chimani, C. M., Kretz, R., and Angermeier, C., “Investigations On Microstructure Effect Of Changing Fluid Flow Characteristic In High Pressure Die Casting,” in 15th International Symposium on Metallography, Stara Lesna, 2013.

[8] Chimani, C. M., Kretz, R., Schneiderbauer, S., Puttinger, S., and Pirker, S., “Studies on flow characeristics at high-pressure die-casting,” in Light Metals 2012, 2012.

[9] Gurson, A.L., Continuum theory of ductile rupture by void nucleation and growth, Part I: Yield criteria and flow rule for porous ductile media. J. Eng. Mater. Technol., Vol. 99, pp. 1–15, 1977.

[10] Johnson, G. R. and Cook, W. H., Fracture characteristics of three metals subjected to various strains, strain rates, temperature and pressure, J. Eng. Fracture Mechanics, Vol. 21. No. 1, pp. 31-48, 1985.

[11] Needleman, A. & Tvergaard, V., Analysis of plastic flow localization in metals. Applied Mechanics Review, 45, pp. s3-s18, 1992.

[12] Neukamm F., Feucht M. and Haufe A., Considering damage history in crashworthiness simulation, Ls-Dyna Anwenderforum, 2009.

[13] Tresca, H., , Mémoire sur l'écoulement des corps solides soumis à de fortes pressions. C.R. Acad. Sci. Paris, vol. 59, pp. 754, 1864.