Quality Mapping of Aluminium Alloy Diecastings

Quality mapping of aluminium alloy diecastings

Giulio Timelli, Franco Bonollo Department of Ma nagement and Engineering, University of Padova, Vicenza, Italy

ABSTRACT RIASSUNTO The effect of casting de fects on mechanical Nel presente lavoro si è studiata l’influenza de i campioni di trazione. La valutazione della prope rtie s was investigate d for a high- dei difetti sulle caratteristiche meccaniche di qualità dei getti pressoco lati in lega pressure die-cast aluminium alloy. A series of getti pressocolati in lega d’alluminio. È stata d’alluminio ha permesso di evide nziare come U-shaped components were cast using a fully colata una serie di componenti con profilo ad le proprietà meccaniche finali si controlled cold chamber high- pressure die- U utilizzando una tecnologia di pressocolata distribuiscano sulla base dei parametri di casting machine with different process in camera fredda e una variazione processo utilizzati. parame te rs. Defects existe d in gas and sistematica dei parametri di processo. I difetti shrinkage pores as well as oxide inclusions. più comuni riscontrati sono state cavità da An X- ray equipment was used for a ritiro o porosità dovute a intrappolamento di preliminary quality control but showed its gas, ed inclusioni di ossidi. Per le analisi limits in detecting defects. The castings were preliminari di tipo qualitativo sui componenti sectioned and tensile bars extracted from co lati, è stato utilizza to un impianto different locations in the castings in order to radioscopico che ha però rivelato i propri map the distribution of the mechanical limiti nell’individuazione dei difetti presenti. prope rtie s. The decrease in mechanical Dive rse zone del getto hanno fornito properties correlated with the area fractions campioni per prove statiche a trazione; lo of defects revealed on the fracture surface of studio di questi ha permesso di redigere una KEYWORDS the te nsile specimens. A quality mapping mappatura della distribuzione delle approach showed how the final properties caratteristiche meccaniche all’interno del High-pressure die- casting; Aluminium depend on the position of the casting and the getto stesso. La diminuzione delle proprietà alloy; Defects; Oxide inclusions; Porosity; process parameters adopted. meccaniche è propo rzionale all’area Fractograp hy; X- ray radiography; occupata dai difetti sulla superficie di frattura Me chanical properties.

INTRODUCTION High-pressure die-casting (HPDC) is widely researchers, with the common conclusion how the mechanical properties decrease used for the possibility of obtaining net to that defects can make the tensile behaviour monoto n i c a l ly with an increase in the are a shape components of complex geometry of casting alloys unpredictable. Castings f raction of defects revealed on the fra c t u re and thin wall thickness at high production with thin sections, like those produced by s u r face of gravity cast aluminium specimens. ra t es [1,2]. Howeve r, a number of HPDC technology, are vulnerable to the Avalle et al. [2] and Wang et al. [ 10 ,11 ] parameters exists, which, if not adequately effect of defects since a single macrodefect studied the effect of defects on fa t i g u e d e te r mined and adjusted, result in a can cover a significant fraction of the cross- b e h aviour of cast aluminium alloys show i n g decadence of quality of the die cast part. section area. Even high integrity castings h ow casting defects have a det ri m e n tal effe c t Common defects in manufactured parts are are expected to contain defects and thus it by shortening not only fatigue cra ck shrinkage cavities, cold fills, oxide films, is important to predict their effect on final p ro p a gation, but also initiation period, which dross, entrapped air bubbles. One of the m e c hanical pro p e r ties of the mate r i a l . is influenced by defe c t s’ size. They also major source of defects in HPDC is air Campbell [5] and Dai et al. [6,7] showed s h owed how castings with defects have at least entrapment in the melt during the filling that improper design of the filling system an order of magnitude lower fatigue life stage of the die, but a detrimental effect results in higher turbulence so that oxide c o mp a red to defe c t - f ree mate rials and how can also come from defects generated in films are generated and entrapped into the p o rosity is more det ri m e n tal to fatigue life th a n the shot sleeve befo re and during th e bulk liquid metal. Inga t e ve l o c i t i e s oxide films. In lite ra t u re data, static st re n g th injection process [3,4]. The influence of s i g n i f i c a n t l y affect the number and a p p e a rs to be modest ly influenced by cast i n g c a s ting defects on the mech a n i c a l d i s t r ibution of the oxide film defe c t s d e fects comp a red to the elongation to fra c t u re properties of cast aluminium alloys has generated from filling stage. Cáceres et al. [ 2 , 3 , 8 , 9 ] . been inve s t i ga t ed by a number of [8] and Gokhale et al. [9] demonstrated G e n e r a l ly, the effect of porosity on

2 Metallurgical Science and Technology Vol. 26-1 - Ed. 2008 mechanical properties is studied through detect cold flakes, especially the oxide shots, commutation points, injection and the measurement of the defect vo l u m e l aye r s, bet t er than X- r ay ra d i o g ra p hy u p s e t pre s s u re. There fo r e, a qu a l i t y f ractions or X- ray analysis, but th e s e [12,14]. A better correlation and reduced mapping approach could be the right path m ethods have the disadva n ta ge to not scatter is then obtained if the decrease in to lead to the qu a n t i t a t i ve ex te n d e d reveal the presence of detrimental oxide tensile strength and elongation to fracture prediction of HPDC properties. films [12]. There fo re, qu a n t i t a t i ve are plotted against the projected area of In this wo r k a HPDC component wa s fractography became a useful tool for such defects on the fracture surface, instead of analysed and a combination of injection study because the fra c t u re surface of defect volume fractions. p a r a m e te r s and pouring te mp e ra t u r e s tensile bars contains the part of th e Almost all the above mentioned researches we re adopted in order to ge n e ra t e microstructure and defects that affect and we re based on gravity cast te s t i n g d i f fe r ent type and amount of cast i n g g ove r n the fra c t u re mechanism. If th e specimens separately poured in sand/chill defects. The material was an EN AC-46000 fracture surface is large enough, there is a mold, where the amount of defects was aluminium alloy, widely used in load likely to be a discernible pattern of chevron generated avoiding degassing, vigorously bearing components in automotive field. X- marks that leads to the point at which stirring the melt, or controlling the feeding ray inspection was preliminary carried out fracture was initiated [13]. Examination of and pouring temperature. On industrial in different zones of the casting and in this initiation point or region can reveal if HPDC production it is more difficult to tensile bars extracted from the components, f ra c t u re was initiated from a st r u c t u ra l control the amount and type of defects due while fractography analysis was carried defect or a microstructural feature. Non- to the complexity of the casting shape or to out to detect the projected area of defects destructive methods are also adopted, such va r iable para m ete rs such as the mold on fracture surface to quantify their effect as acoustic microscope, which are able to temperature, dosing volume, slow and fast on the mechanical behaviour.

EXPERIMENTAL PROCEDURE CASTING EXPERIMENT For R&D purposes, a die for casting U-shaped of aluminium alloys has been made. The CAD model of aluminium casting with runners, gating and overflow s ystem is displayed in fi g u re 1a. The U-shaped casting was coupled with ribs, which are generally a location with high defect concentration. Figure 1b shows an example of an interrupted filling of the die with the arrow indicating a location where A) B) two metal flows meet. The die-castings, with wall th i c kness ranging from 2.5 mm Fig. 1: a) The U-shaped CAD geometry (Length: 260 mm, height: 110 mm, width: 70 mm, outer wall through 6 mm, were produced in a Müller- thickness: 2.5 mm, rib thickness: 4-6 mm); b) Interrupted filling of the die. The arrow indicates a location Weingarten 750 ton cold chamber die- where two metal flows meet. c a s ting machine, with a shot ch a m b e r length of 580 mm and diameter of 70 mm. The fill fraction of the shot sleeve was kept properly the die material and aid the release of the casting after complete solidification. c o n s tant at 0.56 and eve r y shot wa s After-pressure was also adopted to ensure the proper ejection of the parts. d o c u m e n ted with its shot pro f i l e . In the present work an EN AC-46000 aluminium alloy (European designation, equivalent Te m p e ra t u re measurements we re to the US designation A380), according UNI EN 1706, with composition listened in table p e r fo r med re g u l a r ly, both using an 1, was cast. This alloy has a liquidus-solidus temperature range of approximately 593- infrared camera and temperature probes 538°C [15]. and the furnace temperature was checked The U-shaped castings were produced with the process parameters indicated in table 2. frequently. Manual spraying and blowing The shot profiles can also be seen in figure 2. Throughout this paper, the different process process were used in order to cool down parameters will be termed P1,..., P4 and T2 respectively.

Table 1. Chemical composition of the casting alloy (wt.%)

Alloy Si Mg Cu Fe Mn Zn Ni Cr Ti Al. EN AC-46000 9.87 0.22 2.441 0.758 0.216 0.467 0.059 0.018 0.07 bal.

Vol. 26-1 - Ed. 2008 Metallurgical Science and Technology 3 In the profile P1, the plunger speed was Table 2. HPDC process parameters kept constant in the first phase and a rapid acceleration was applied in the second Plunger Plunger Co mmutation Co mmutation phase, i.e. at the beginning of die filling. velocity velocity point point Ingate Pouring Shot slow shot fast shot slow-fast to upset velocity temperature The same profile was adopted for T2 but profile (ms-1) (ms-1) shot (mm) pressure (mm) (ms-1) (°C) the pouring temperature was 50°C lower. P1 0.4 3 428 552 48.9 690 For the other shot profiles the plunger was P2 0.4-0.6 3 447 562 48.9 690 accelerated slowly also in the first phase, trying however to minimize air entrapment P3 0.4-0.7 2 373 544 32.9 690 in the slow shot. The main diffe re n c e s P4 0.4-0.6 2 451 547 32.9 690 regarded the variations of the switch point T2 0.4 3 428 552 48.9 640

3,5

P1 3 P2

2,5 P4 P3 2

1,5

1

0,5

Fig. 3: Thermal management of the die. 0 500 600 700 800 900 1000 1100 1200 1300 1400 TIMES (ms)

Fig. 2: Different shot profiles used in the present work. between the first and second phase, where length examined, pre and after the tensile the commutation point was anticipated (P3) testing, with a micro-focus X-ray equipment, and postponed (P2 and P4), and th e which can magnify an image several times reduction of the plunger velocity in the while still offering a better definition than a second phase for the profiles P3 and P4. conventional X-ray tube. Throughout this The shot sleeve and the die were preheated paper, the different locations will be termed but re a c hed a qu a s i - s teady sta t e zones 1,...,8 respectively. The locations and temperature after some shots, thus the first the main dimensions of the te n s i l e several castings were scrapped. Figure 3 specimens are indicated in figure 4 and s h ows the cooling system adopted to table 3. The X- ray images we re th e n stabilise the temperature of the die and of a n a lysed using a commercial image - Fig. 4: U-shaped die casting with investigated the shot sleeve. analysis software package. The defect area locations. of each image was evaluated by counting the image pixels in each defect (figure 5). X-RAY, TENSILE TESTS AND Every defect was counted indifferently. MICROSTRUCTURAL INVESTIGATIONS Table 3. Dimensions of tensile bars The U-shaped die-castings were mapped th roughout with a macro - f ocus X- ray zone 1,2 zone 3,4,7 zone 5 zone 6 zone 8 equipment for a preliminary analysis and (mm) (mm) (mm) (mm) (mm) comparison. Gauge length 60 30 26 60 30 Tensile test bars with a rectangular cross Thickness 2.5 3.1 6 4.1 2.4 section were extracted from eight different Width 10 10 10 10 10 locations of the castings and the gauge

4 Metallurgical Science and Technology Vol. 26-1 - Ed. 2008 The tensile strength, UTS (MPa), and the engineering elongation to fracture, sf, of each sample were converted to true tensile strength (σf) and true elongation to fracture (εf) through equations [16, 17]:

σf =UTS(1+ sf) (1) A) B) εf =1n(1+ sf) (2)

The true stress (σ)-true plastic strain (ε) flow Fig. 5: (a) X-ray image; (b) the same X-ray image binarized to determine the amount of defects. c u r ves of each specimen we re approximated with a constitutive equation of the form [16, 17]: The fractured surfaces after testing were the fractographic analysis. then observed under an opt i c a l At least three different castings from every σ =Kεn (3) ste re o m i c roscope at a magnification of shot profile were chosen to be analysed about x10. The acquired images were then according to the described procedure. where K is the alloy’s strength coefficient transferred into a single photograph and and n the strain hardening exponent. The the area of defects was measured by using Quality Index, Q (MPa), defined [16-18] as an image analyser. The tot al measured defect area was then divided by the initial . Q=UTS+ 0.4Klog(100 sf) (4) cross section of the sample to find the defect area fraction. A scanning electron was then considered. microscope was also used as support for

RESULTS AND DISCUSSION X-RAY ANALYSIS The X-ray images in figure 6 are taken from zone 2 of th ree consecutive U-shaped castings poured according the profile P4. The size and the distribution of defects is d i f fe rent in spite the same pro c e s s A) B) C) parameters were adopted. It is evident how the formation of defects is sensitive to small variations in the casting conditions and the Fig. 6: Macro-focus X-ray images taken from zone 2 of three different castings consecutively poured with causes cannot be only connected to the shot profile P4. p rocess pro f ile adopted, even if th i s variable results the main source of defects. Figure 7 shows t he X-ray pictures, taken with macro- and micro-focus, from a tensile bar extracted from zone 2 of a casting poured according to the profile P4. The m i c ro - f ocus pictures show defects and details not appreciable with conventional X-ray tube. The defects seem to result from local die filling condition, i.e. a vortex, as indicated by arrows, was generated in the zone 2, entrapping and dragging air bubbles. The results of the X-ray image analysis of zones 1 and 2 are shown in figures 8 and 9. While fi g u r e 8 shows the defe c t distribution in zones 1 and 2 of castings p o u r ed with diffe r ent shot pro f i l e s , Fig. 7: Macro- and micro-focus X-ray images taken in a tensile bar extracted from zone 2 of a casting poured with shot profile P4. evidencing the influence of pro c e s s

Vol. 26-1 - Ed. 2008 Metallurgical Science and Technology 5 parameters in the amount of defects, figure 12 9 demonstrates how, in three consecutive 1P1 castings poured with the process P4, the 1P2 defect reproducibility is sensitive to small 10 19P3 1P4 u n c o n t rolled va riations of the cast i n g 15T2 conditions, and how the amount of defects 8 cannot be controlled only by changing shot profiles. 6 The same X-ray analysis was then repeated 4 in the fractured tensile bars to characterize the position of the fracture into the whole 2 b a r. The analysis underlined how th e f r a c t u re developed in the presence of 0 d e f ects (fi g u re 10), but sometime th e Z1 Z2 fracture path took place in a zone where POSITION OF THE BARS the X-ray radiography didn’t show any d e f ect (fi g u re 11). In these cases, th e following fractographic inspection revealed Fig. 8: Distribution of defects in zones 1 (Z1) and 2 (Z2) of castings poured with different process parameters. a high amount of oxide inclusions, which represent surfaces of discontinuity in the material and point for crack initiation. The oxide films were not revealed by X-ray 30 inspection because the difference of density 1P4 3P4 b e t ween the aluminium alloy and th e 25 4P4 aluminium oxide skins is minimum and thus not detectable through conventional X-ray 20 technique [12]. 15

MECHANICAL PROPERTIES 10 From the tensile test results, it was possible 5 to observe how the amount of defects influenced considerably the UTS and the 0 elongation to fracture (sf), which ranged Z1 Z2 from 129 to 307 MPa and from 0.29% to POSITION OF THE BARS 2.89% respectively. If elastic properties of the material, as the Young modulus (E) and Fig. 9: Distribution of defects in zones 1 (Z1) and 2 (Z2) of three different castings consecutively poured the Yield St re n g t h (YS0 . 2 %), we re with shot profile P4.

A) B) C) A) B) C)

Fig. 10: X-ray images of a tensile bar (a) before and (b) after testing; (c) Fig. 11: X-ray images of a tensile bar (a) before and (b) after testing; (c) fracture fracture surface. The line in (a) represents the fracture path in the specimen. surface. The line in (a) represents the fracture path in the specimen. Defects Defects were detectable by X-ray inspection. were not detectable by X-ray inspection.

6 Metallurgical Science and Technology Vol. 26-1 - Ed. 2008 c o n s i d e red, all data arra n ged aro u n d constant values, (72 GPa and 180 MPa 350 350 respectively) independently from the zone of extraction, the process parameters, but 300 300 above all the amount of defects. Plotting the 250 Mean flow 250 true stress-true plastic strain flow curves of curve each specimen and the mean flow curve (figure 12), according eq. (3), it is possible 200 200 to observe how all the curves followed a similar trend and the difference consisted in 150 150 the break point, i.e. the fra c t u re . 100 Fit Results 100 Fit Results Considering the same graph with the mean Equation Y=755*pow(X,0.23) Equation Y=755*pow(X,0.23) Coef of determination, R-squared=0.95 Coef of determination, R-squared=0.95 fl ow curve but plotting the true te n s i l e 50 50 strength (σ ) and the true elongation to 0 0.004 0.008 0.012 0.016 0.02 0 0.004 0.008 0.012 0.016 0.02 f TRUE PLASTIC STRAIN TRUE PLASTIC STRAIN fracture (εf) values obtained according eqs. (1) and (2), a bet t er view of what Fig. 12: The true stress-true plastic strain flow curves Fig. 13: The true tensile strength ( ) and true previously said is shown (figure 13). The σf of each specimen and the mean fl ow curve , elongation to fracture (εf) values of each sample scattering of data from the mean flow curve according eq. (3). obtained according eqs. (1) and (2) are plotted σ = 755ε0.23 (5) together with the mean flow curve. is low, as shown by the coefficient of determination, R2, equal to 0.95. 350 350 T h e r e fo r e, eq. (5) can be considere d constitutive of the uniform plastic behaviour 300 300 of the alloy analysed. To understand the correlation between the fracture and the 250 250 amount of defects, the bubble plot in figure 14 can be considered where each bubble 200 Increasing area 200 Increasing fraction of defecs Quality Index represents the true tensile strength (σf) and 150 150 the true elongation to fracture (εf) and the diameter of the bubble is proportional to 100 Fit Results 100 Fit Results Equation Y=755*pow(X,0.23) Equation Y=755*pow(X,0.23) the defect area fraction. Increasing the Coef of determination, R-squared=0.95 Coef of determination, R-squared=0.95 distance from the origin, the diameter of 50 50 the bubbles and therefore the defect area 0 0.004 0.008 0.012 0.016 0.02 0 0.004 0.008 0.012 0.016 0.02 TRUE PLASTIC STRAIN TRUE PLASTIC STRAIN f raction decreases, indicating th e fundamental role of defects on fracture mechanism. A similar graph can be seen in Fig. 14: Bubble plot of the true tensile strength (σf) Fig. 15: Bubble plot of the true tensile strength (σf) fi g u re 15 where the diameter of each and true elongation to fracture (εf) values of each and true elongation to fracture (εf) values of each bubble is instead pro p o r tional to th e sample where the diameter of each bubble is sample where the diameter of each bubble is Quality Index, defined by eq. (4). Quality proportional to the defect area fraction. proportional to the Quality Index.

325 2,5%

300 2,0% 275 1,5% 250

225 1,0% 200 0,5% 175 P1 P2 P3 P4 T2 P1 P2 P3 P4 T2 150 0,0% Z1 Z8 Z1 Z8 Z1 Z8 Z1 Z8 Z1 Z8 Z1 Z8 Z1 Z8 Z1 Z8 Z1 Z8 Z1 Z8 POSITION POSITION

Fig. 16. The mean UTS, together with the scattering bands, is plotted versus the Fig. 17: The mean engineering plastic st rain, to gether with the scatte ring bands, is different positions (Z1,...,Z8) and process parameters adopted. p l ot ted ve rsus the diffe rent positions (Z1,...,Z8) and process para m ete rs adopte d .

Vol. 26-1 - Ed. 2008 Metallurgical Science and Technology 7 Index has the advantage to consider both scattering bands (minimum and maximum change with changing location. Therefore, tensile and strain properties of the material values), are plotted versus the different from these considerations it is possible to at the same time. As expected, the general positions (Z1,...,Z8) and pro c e s s understand the difficulty in optimising the t rend of the graph is opposite to th e parameters adopted. In general, it can be whole casting and the HPDC pro c e s s p revious one, showing thus the st r i c t seen how, considering the same position, parameters. relationship with the defect amount. the UTS and the engineering plastic strain The final quality maps can be seen in show different values changing the process fi g u res 16 and 17 where the mean p a ra m e te r s; while, fixing the pro c e s s mechanical properties, together with the va riables, the mechanical pro p e r t i e s

CONCLUSIONS The primary objective of this work has been 3. An X-ray technique reveals its limit in 6. The defect area fraction, detected on to characterize the influence of casting d etecting the presence of ox i d e the fra c t u re surface of tensile te s t defects on mechanical properties of a high- inclusions. specimens, can be used to establish the pressure aluminium U-shaped casting. From 4. The amount of defects infl u e n c e s f ra c t u re point along the const i t u t i ve the ex p e r i m e n t al findings it can be considerably the plastic properties of equation. concluded that: the mate r ial but not the elast i c 7. A quality mapping approach can be 1. The amount and type of defe c t s characteristics. used in high-pre s s u re die-casting to ch a n g es by changing the pro c e s s 5. A constitutive equation, σ =Kεn, can be demonstrate how the amount and type parameters. c o n s i d e red as re p re s e n ta t i ve of th e of defects, and the mech a n i c a l 2. The size and distribution of defects are uniform plastic behaviour of the alloy, properties are distributed in a casting different inside the casting in spite the i n d e p e n d e n t ly of the presence of by changing the process parameters. same process parameters are adopted. defects.

ACKNOWLEDGMENTS REFERENCES This wo r k was developed inside IDEAL [1] G.O. VERRAN, R.P.K. MENDES and [11] Q.G. WANG, D. APELIAN and D.A. ( I n t e gra t ed Development Ro u tes fo r M . A . ROSSI, J. Mate r. Pro c e s s . LADOS, J. Light Met. 1, (2001), pp. 85- Optimised Cast Aluminium Components – Technol., (2006), article in press. 97. C o n t ract n. GRD2-2001 - 5 0 042) and [2] M. AVALLE, G. BELINGARDI, M.P. [12] S. FOX and J. CAMPBELL, Scri p ta NADIA (New Au t o m ot i ve comp o n e n t s CAVATORTA and R. DOGLIONE, Int. J. Mater. 43, (2000), pp. 881-886. Fatigue 24, (2002), pp. 1-9. Designed for and manufa c t u red by [13] J.A. FRANCIS and G.M.D. CANTIN, Mater. Sci. Eng. A407, (2005), pp. 322- I n t e l l i g ent processing of light Alloys – [3] F. FAU R A , J. LÓPEZ and J . H E R NÁNDEZ, Int. J. Mach. To o l s 329. Contract n. 026563-2) Projects, supported Manuf. 41, (2001), pp. 173-191. [14] A.K.M. AZIZ AHAMED, H. KATO, K. by European Union. The authors would like [4] H.I. LAUKLI, PhD thesis, No r we g i a n KAGEYAMA and T. KOMAZAKI, Mater. to thank Dr. M. Sadocco for helping with University Of Science and Technology Sci. Eng., (2006), article in press. the pre p a ration and conduction of th e (NTNU) (Trondheim), 2004. [15] A m e rican Society of Metals (ASM), experimental activities. Many thanks are [5] J. CAMPBELL, Casting. Elsevier Science Casting-Nonferrous casting alloys. ASM also due to Dr. E. Blümcke and Dr. S. Nisslé Ltd., Oxford (2003). Handbook, vol.15, ASM International, for their help in the experimental work as [6] X. DAI, X. YANG, J. CAMPBELL and J. Materials Park OH, (1991). well as to Audi GmbH. WOOD, Mate r. Sci. Eng. A354 , [16] C.H. CÁCERES, Int. J. Cast Metals Res. (2003), pp. 315-325. 12, (2000), pp. 367-375. [7] X. DAI, X. YANG, J. CAMPBELL and J. [17] C.H. CÁCERES, Int. J. Cast Metals Res. WOOD, Mate r. Sci. Te c hnol. 20, 12, (2000), pp. 385-391. (2004), pp. 505-513. [18] M. DROUZY, S. JACOB, M. RICHARD, [8] C.H. CÁCERES and B.I. SELLING , AFS Int. Cast Metals J. 5, (1980), pp. Mater. Sci. Eng. A220, (1996), pp. 43-50. 109-116. [9] A.M. GOKHALE and G.R. PAT E L , Scripta Mater. 52, (2005), pp.237-241. [10] Q.G. WANG, D. APELIAN and D.A. LADOS, J. Light Met. 1, (2001), pp. 73- 84.

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