Cooling rate inference in aluminum alloy squeeze casting

Cooling rate inference in aluminum alloy squeeze castingA.Boschetto a ,G.Costanza b ,F.Quadrini b ,M.E.Tata b,⁎a Department of Mechanics and Aeronautics,University of Rome La Sapienza,Via Eudossiana 18,00184Rome,Italy bDepartment of Mechanical Engineering,University of Rome Tor Vergata,Via del Politecnico 1,00133Rome,ItalyReceived 1August 2006;accepted 19October 2006Available online 7November 2006AbstractAn indirect method for the evaluation of cooling rate of a squeeze cast aluminum alloy is proposed.Cooling rate is inferred by simple local mechanical tests such as microhardness or indentation.A predictive model is used to correlate cooling rate and mechanical properties.An aluminum alloy (EN-AB46000)was squeeze cast.Four cylindrical samples were fabricated at different values of the squeezing pressure (from 0to 100MPa).Samples were cut longitudinally at the middle height and microhardness was measured along the sample radius as well as dendrite size.Moreover yielding stress was evaluated by means of FIMEC test in the centre of the sample section.Mechanical properties can be related to dendrite size and used for cooling rate inference.©2006Elsevier B.V .All rights reserved.Keywords:Metals and alloys;Solidification;Microstructure;Mechanical properties1.IntroductionCooling rate evaluation is a very difficult task generally for all the casting processes.This is particularly true for aluminum alloy squeeze casting as cooling rate can reach very high values and a direct temperature measurement is practically impossible due to the mold thermal inertia.It is well known that squeeze pressure decreased the percent-age of porosity and increased density as well as grain size.It is often observed that grain size depends on solidification rate during casting.In fact the combination of high pressures and metal molds leads to high heat transfer coefficients which in turn leads to a refinement of microstructure [1–3].Cooling rate,the most important parameter influencing casting mechanical properties,is never directly measured except for some cases when thermocouples are inserted inside casting.Lee et al.studied the effect of the gap distance on the cooling behavior and the microstructure of an indirect squeeze cast and gravity die cast aluminum alloy.They inserted thermocouples through the die and located at the centre of each casting.How-ever a numerical model was used to calculate temperature profile and was preferred for the cooling rate extraction due to the intrinsic dispersion of measured cooling curves [4].In fact too many factors affect the temperature profile during casting from melt turbulence in injection to mold transient phase.Britnell and Neailey placed thermocouples inside the die near the mold –melt interface to study the macrosegregation in thin walled squeeze cast samples but thermal traces can be used just for a qualitative analysis [5].If cooling rate is already difficult to extract from laboratory tests,it seems really impossible in industrial processes.How-ever local mechanical testing can be always performed on cast parts.As mechanical properties are correlated to cooling rate,the latter can be inferred from the former.Costanza et al.defined a predictive model which correlates mechanical properties and cooling rate.In this study the authors employ the same law to infer in small cast parts the cooling rate after mechanical testing.This information can be used for process optimization as well as for the validation of process numerical simulations [6].2.The predictive modelA simple law which correlates mechanical properties (yield strength and Vickers hardness)to cooling rate is presentinMaterials Letters 61(2007)2969–2972/locate/matlet⁎Corresponding author.Tel.:+390672597181;fax:+39062021351.E-mail address:elisa.tata@uniroma2.it (M.E.Tata).0167-577X/$-see front matter ©2006Elsevier B.V .All rights reserved.doi:10.1016/j.matlet.2006.10.048scientific literature [6].The average grain size can be correlated to the mechanical properties by Hall –Petch:r Y ¼r 0þK Y k −1=2ð1ÞHV ¼HV 0þK H k −1=2:ð2ÞFurthermore a relationship between the cooling rate (ε)and the average dendrite cell size (λ)is:k ¼B e n :ð3ÞCombining Eqs.(1),(2)and (3)a direct correlation between the cooling rate and both the final yield strength and hardness is obtained:r Y ¼r 0þC Y e m where C Y ¼K Y B −1=2m ¼−n28<:ð4ÞHV ¼HV 0þC H e m where C H ¼K H B −1=2:ð5ÞMaterial constants of Eqs.(1–5)were extracted from labo-ratory tests on small specimens for the EN-AB46000aluminum alloy [6].3.Materials and experimental methodsThe examined material,aluminum alloy EN-AB46000,has nominal composition (wt.%):8.0–11.0Si,0.6–1.1Fe,2.0–4.0Cu,0.55Mn,0.15–0.55Mg,0.15Cr,0.55Ni,1.2Zn,0.35Pb,0.25Sn,0.2Ti,and Al to balance.It is a hypoeutectic Al –Si alloy,generally employed in die casting processes.A laboratory squeeze casting machine was constructed on purpose to fabricate specimens for the experimentation.A single cylindrical sample was fabricated at once (100mm in length and 30mm in radius).The die was designed without mold separation to avoid flash during squeezing.Alloy melting was performed in a muffle kiln inside a crucible.Subsequently the molten metal was poured in the cylindrical mold.The die was preheated by means of a kantal resistance kiln.Pressure was applied directly on the solidifying cast via a punch.After solidification,part extraction was performed by means of a steel pin connected to the punch.Four different squeezing conditions were used during the experimentation.The melt temperature was 750°C,the mold temperature 350°C,the applied pressure was 0MPa (A),50MPa (B),75MPa (C),and 100MPa (D).A delay time of 15s between the melt injection and the squeeze beginning was kept.Pressure was applied for 30s and the sample was ex-tracted after its complete solidification.Finally,the external surface roughness (Ra)was measured longitudinally on each sample.After casting,all the specimens were cut longitudinally to observe the circular section in the middle height.Each specimen section was preliminarily subjected to metallographic prepara-tion,etched with 1%HF aqueous solution to reveal the micro-structure and observed with an optical microscope.FIMEC (Flat-top Cylinder Indenter for Mechanical Characterization)and microhardness Vickers tests were performed in different points of the samplesections.Fig.1.Micrographs of metallographic sections of squeezed samples:a)sample A (0MPa)in the middle zone;b)sample A (0MPa)at the edge;c)sample D (100MPa)in the middle zone;d)sample D (100MPa)at the edge.2970 A.Boschetto et al./Materials Letters 61(2007)2969–2972The microhardness was measured along the radius as well as the alloy dendrite size whereas the FIMEC test was performed at the centre of the section.The FIMEC test is based on the penetration,at constant rate,of a flat punch of small size (2mm diameter and 1.5mm height)[7].During the test the applied load and the depth of penetration are measured.A limit load is reached after an initial linear stage.The yielding stress can be directly extracted from this limit load.A WC indenter,a 10N pre-load,a 0.1mm/min penetration rate and a 1mm depth were employed for testing.4.Experimental resultsA comparison among different microstructure aspects is provided in Fig.1for the maximum and minimum pressure in two different points (the centre and the edge).Fig.2shows the experimental curves obtained both from microhardness test (Fig.2-a)and FIMEC test (Fig.2-b).The higher the pressure,the higher the microhardness mean values and the lower the data scattering.The squeezing pressure has a significant effect also on FIMEC curves as they shift toward higher force values,increasing pressure.Finally,Table 1summarizes the mechanical,morphological and microstructural data.5.DiscussionThe effects of squeezing phase on cast properties are evident on all the material characteristics (Table 1).The surface aspect is clearly dependent on pressure due to the better metal-mold matching during squeezing.Roughness is strongly dependent on pressure not only in terms of mean value over the surface but also of dispersion.Also the microstructure is strongly affected by pressure (Fig.1).All the microstructures show primary aluminum dendrites,particles and eutectic constituent but a different dendrite size distribution is observed across the sample.The dendrite size appears to be comparable for all the pressures near the sample skin.The microhardness is directly correlated to the microstructure.A similar value is measured near the skin for all the squeezing pressures whereas a sig-nificant difference is observed toward the centre.Moreover a higher data scattering around is observed at lower pressure values.At 100MPa the trend tends to be a horizontal line and a lower dispersion is measured.The microhardness values and dendrite size ones of Table 1are evaluated in the same points along the radius.Instead theTable 1Mechanical,microstructural and morphological data extracted from squeezed samples in different points SampleFIMEC testRoughness (Ra)Distance from centre Micrographs MicrohardnessYielding stress in the centre Mean value Dispersion Dendrite cell size Local Vickers microhardness [MPa][μm][μm][mm][μm][HV]A120.43.080.82150.180.26.535.795.2831.398.11318103.4B 130.50.320.13135.782.7627.591.41315.9103C 144.00.360.07128.684.8625.895.61316.3111.6D 204.50.390.05129.694727.11031317.6103.4Fig.2.Mechanical tests for all the squeezing pressures:a)microhardness profiles along the sample radius;b)FIMEC tests at the centre.2971A.Boschetto et al./Materials Letters 61(2007)2969–2972yielding stress is always evaluated at the centre of the samples.Experimental data and model ones for yielding stress and microhardness are reported in Fig.3-a together with the theoretical trends extracted respectively from Eqs.(1)and (2).The semi-empirical models were fitted using the constants of Ref.[6].Costanza et al.evaluated these constants performing squeezing tests on small specimens of the same alloy.There is a good agreement between the experimental data and pre-dicted ones.A higher distance is observable for yielding stress and is probably due to the higher dimension of the zone interested to the indentation.As experimental data are in agreement with prediction of Eqs.(1)and (2),it is expected that Eqs.(4)and (5)can be used for the same alloy to infer cooling rate during solidification.In Fig.3-b microhardness values of Table 1were used to infer cooling rate at the different squeezing conditions in three sample positions.It is evident that the cooling rate is similar near the edge for all the squeezing pressures.6.ConclusionCooling rate can be extracted from cast parts by means of a simple indentation test.This is an important operation not only to provide correct information for process numerical simulation but above all to optimize the same casting process.In fact it is difficult to define a correct correlation among process para-meters and cycle time.Moreover the cycle time is strongly dependent on cooling rate.During die casting optimization,performing simple indentation tests on the first cast parts could allow to converge rapidly toward the process optimum.Material Eqs.(4)and (5)were fitted once by laboratory specimens but they are applicable on cast products for every casting condition.It would be possible to characterize all the interesting alloys and to use the results to calibrate industrial processes.Nomenclature σY Yield strength σU Ultimate tensile strength HV Vickers hardnessσ0,K Y Constants of Hall –Petch equation for yield strength HV 0,K H Constants of Hall –Petch equation for hardness λAverage dendrite cell size εCooling rate B ,n Alloy specific constants in the relationship between λand εC Y ,m Constants in the relationship between σY and εC H ,m Constants in the relationship between HV and εReferences[1]M.T.Abou El-khair,Materials Letters 59(2005)894–900.[2]P.Vijian,V .P.Arunachalam,Journal of Materials Processing Technology170(2005)32–36.[3]M.S.Yong,A.J.Clegg,Journal of Materials Processing Technology 145(2004)134–141.[4]J.H.Lee,H.S.Kim, C.W.Won, B.Cantor,Materials Science &Engineering.A,Structural Materials:Properties,Microstructure and Processing 338(2002)182–190.[5]D.J.Britnell,K.Neailey,Journal of Materials Processing Technology 138(2003)306–310.[6]G.Costanza,F.Quadrini,M.E.Tata,International Journal of Materials &Product Technology 20(5/6)(2004)345–357.[7]A.Donato,P.Gondi,R.Montanari,L.Moreschi,A.Sili,S.Storai,Journalof Nuclear Materials 258–263(1998)446–451.Fig.3.Theoretical predictions:a)comparison between experimental and theoretical data for mechanical properties;b)cooling rate inference by microhardness values at different squeezing pressures.2972 A.Boschetto et al./Materials Letters 61(2007)2969–2972。