Die-casting dies are exposed to high thermal and mechanical loads. Thermal fatigue cracking of dies due to thermal cycling may importantly shorten the life-time of the die. Cracks degrade the surface quality of dies and consequently the surface of castings. In this study, thermal fatigue cracking of dies was analyzed during the process of die casting aluminium alloys. During the process cracks were observed and measured and their location and size were determined. Thermal and mechanical loads cause high local stresses and consequently surface cracks. First cracks occur as early as after 2000 cycles and propagate progressively with cycles.Kalupi (alati) za lijevanje pod tlakom su izloženi velikim toplinskim i mehaničkim opterećenjima. Pukotine uslijed toplinskog umora su zbog izmjene toplinskih ciklusa važan ograničavajući mehanizam životnoga vijeka kalupa. Pukotine smanjuju (degradiraju) kvalitetu površine kalupa i posljedično površinu odljevka. U ovom radu su analizirane pukotine nastale na kalupu zbog toplinskog umora tokom tlačnog lijevanja aluminija. Tokom procesa su promatranjem i mjerenjem utvrđene lokacije i veličine nastalih pukotina na kalupu. Toplinska i mehanička opterećenja uzrokuju velika lokalna naprezanja i posljedično pukotine na površini kalupa. Prva pukotina pojavljuje se već nakon 2000 ciklusa i progresivno se širi povećanjem broja ciklusa

D. Klobčar

F. Kosel

J. Tušek

M. Muhič

M. Pleterski

English

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M. MUHI^, J. TU[EK, F. KOSEL, D. KLOB^AR, M. PLETERSKITHERMAL FATIGUE CRACKING OF DIE-CASTING DIESReceived – Prispjelo: 2008-12-22Accepted – Prihva}eno: 2009-03-17Original Scientific Paper – Izvorni znanstveni radINTRODUCTIONDie casting is the process of forcing molten underhigh pressure into mold cavities (which are machinedinto dies). The process of die casting is a cost-efficientmethod of making castings from aluminium, zinc, mag-nesium and copper-based alloys 1–3. Dies are exposedto high mechanical, thermal and chemical loads whichcause surface defects. These defects are thermal fatiguecracks, erosion of the melt flow, corrosion and solderingof aluminium to the die surface, deformation of contactsurfaces of dies and gross fracture (Figure 1) 1-7.These defects are then reflected on castings. Figure 2shows surface defects on aluminium castings. Figure 2ashows a worn-out surface, which is caused by the ero-sion of aluminium alloy melt flow. Figure 2b showsthermal fatigue cracks which occur due to thermal stre-sses during die casting. Figure 2c shows damage on analuminium casting due to corrosion and soldering of alu-minium to the die surface. The burr is caused by defor-mation of die parts (Figure 2d).Die-casting dies usually fail due to thermal fatiguecracking 5–11. Dies are exposed to high temperaturegradients and high pressure peaks of melt. Temperaturegradients between the surface and the die core are caus-ing thermal stresses, which can exceed die materialyield strength 8, 9. This leads to the accumulation oflocal plastic strains on the die surface and consequentlyto surface cracks 10.EXPERIMENTAL PROCEDUREThe material used in the investigation of thermal fa-tigue cracking was aluminium alloy AlSi9Cu3 (Table1). (Figure 3). Experiments were made on an 8 MN coldchamber machine in actual production runs. The tem-perature of the aluminium melt was 680 °C and the totalcycle time was 60 s during which the die was closed 25 sMETALURGIJA 49 (2010) 1, 9-12 9Die-casting dies are exposed to high thermal and mechanical loads. Thermal fatigue cracking of dies due tothermal cycling may importantly shorten the life-time of the die. Cracks degrade the surface quality of dies andconsequently the surface of castings. In this study, thermal fatigue cracking of dies was analyzed during theprocess of die casting aluminium alloys. During the process cracks were observed and measured and their loca-tion and size were determined. Thermal and mechanical loads cause high local stresses and consequently surfa-ce cracks. First cracks occur as early as after 2000 cycles and propagate progressively with cycles.Key words:Die casting, die defect, thermal fatigue, surface cracksPukotine na kalupima (alatima) za tla~no lijevanje uslijed toplinskog umora. Kalupi (alati) za lijevan-je pod tlakom su izlo`eni velikim toplinskim i mehani~kim optere}enjima. Pukotine uslijed toplinskog umora suzbog izmjene toplinskih ciklusa va`an ograni~avaju}i mehanizam `ivotnoga vijeka kalupa. Pukotine smanjuju(degradiraju) kvalitetu povr{ine kalupa i posljedi~no povr{inu odljevka. U ovom radu su analizirane pukotinenastale na kalupu zbog toplinskog umora tokom tla~nog lijevanja aluminija. Tokom procesa su promatranjem imjerenjem utvr|ene lokacije i veli~ine nastalih pukotina na kalupu. Toplinska i mehani~ka optere}enja uzrokujuvelika lokalna naprezanja i posljedi~no pukotine na povr{ini kalupa. Prva pukotina pojavljuje se ve} nakon 2000ciklusa i progresivno se {iri pove}anjem broja ciklusa.Klju~ne rije~i: Tla~no lijevanje, defekti na kalupu, toplinski umor, povr{inske pukotineISSN 0543-5846METABK 49(1) 9-12 (2010)UDC – UDK 669.018.258:621.74.043:620.178.38=111M. Muhi~, TKC d.o.o., Ljubljana, SloveniaJ. Tu{ek, F. Kosel, D. Klob~ar, M. Pleterski, Faculty of Mechanical Engi-neering, Ljubljana, SloveniaFigure 1. Schematic of defects occurrence on adie-casting die.and open 35 s. Water at 200 °C was continuously circu-lating through cooling channels in the tool and the toolsurfaces were lubricated. The weight of each castingwas 332 g, the casting pressure 50 MPa, filling time 23ms and the entrance velocity of the melt was about 52m/s. The tool in this study was preheated to a tempera-ture about 200 °C.The die was made of Cr-Mo-V hot work tool steelAISI H11 (DIN X38CrMoV5.1, W. No.: 1.2343) (Table1). The tool was hardened at 1000 °C and tempered at600 °C to get the hardness of 45 HRc  450 HV. The diesurface was also nitrided to improve the wear resistanceand prolong the in-service die life.During the analysis thermal fatigue cracks weremeasured using thin wires of a diameter between 0,05 –0,1 mm, by inserting them into cracks and measuringtheir approximate depth. The surface cracks length wasdetermined with a string with a diameter of 0,2 mm byplacing it into the cracks and measuring their length.The measurements were performed at every 1000 cyclesup to 35000 cycles at four equal dies to get the relation-ship between crack size depth, length and number of cy-cles performed by the die.RESULTS AND DISCUSIONFigure 4 presents the locations of the surface defectson the dies and detailed photos of these defects. Theidentified local surface defects did not appear at thesame time, i. e. at the same number of cycles.They are numbered according to the time of occur-rence; the first was observed at the edge at location 1 andthe last defect was observed at location 12. The majorityof these surface defects generally occur on edges andcorners. They tend to occur first at corners and edgeswith small radiuses. Locations closer to the gate werealso subjected to higher temperature gradients and con-sequently higher stresses.Figure 5 shows the locations and number of cycles atwhich the first thermal fatigue cracks occurred. At loca-tion 1 the first thermal crack occurred at around 2500 cy-cles, at location 2 at 3000 cycles and at locations 3 at4000. Before 5000 cycles the cracks occurred at loca-tions 1, 2, 3. The cracks at locations 4, 5, 6 were first de-tected after 6000, 7000 and 8000 cycles, respectively.The cracks at locations 7 and 8 were first observed at16000 and 17000 cycles, at locations 9 and 10 at 21000and 23000 cycles, and location 11 and 12 at 26000 and at32000 cycles, respectively.Thermal fatigue cracks occurred at all locations. Atlocations 3 and 4 the neighbouring cracks were united,10 METALURGIJA 49 (2010) 1, 9-12M. MUHI^ et al.: THERMAL FATIGUE CRACKING OF DIE-CASTING DIESFigure 2. Surface defects of castings: a) erosion, b) ther-mal cracks, c) corrosion and soldering, d) de-formation.Figure 3. Die-casting die (a) and aluminium casting (b).Table 1. Chemical composition (mas.%) of the aluminum alloy AlSi9Cu3 and the Cr-Mo-V alloy hot work tool steelAISI H11 (DIN X38CrMoV5.1, W. No.: 1.2343) used in the experiment.Alloy Chemical composition / mas. %Si Cu Mn Mg Fe Zn Ni Al C Cr Mo VAlSi9Cu3 9,5 3,0 0,4 0,5 0,9 1,0 0,3 Rest / / / /H11 1,00 / 0,40 / / / / / 0,38 5,00 1,30 0,40so that the material between two cracks fell off and thecrack widened.Figure 6 shows mean and maximum crack depth atdifferent locations. Mean values of depth vary from 0,4mm to 1,9 mm, and the maximum depths from 0,7 mmto 2,7 mm. The deepest cracks occurred at locations 3, 1,6 and 2 with the maximal depths measured at 2,7 mm,2,4 mm, 2,3 and 2,2 mm, respectively. The deepestcracks showed a tendency to occur near the die gate,where higher temperature gradients caused higher ther-mal stresses. The local stress concentrators (cornerswith small radiuses) additionally raised these stresses,which caused the local accumulation of plastic strain.Figure 7 shows mean and maximum length of cracksand their locations. Mean values vary from 7 mm to 45mm and the maximum values from 9 mm to 75 mm. Thelongest cracks occurred at locations 3, 4, 5 and 6 withthe maximal length measured at 75 mm, 55 mm, 38 and23 mm, respectively.The longest cracks also tend to occur near the diegate due to higher temperature gradients, which causehigher thermal stresses.Figure 8 shows typical cross-sections of thermal fa-tigue cracks (found near the die gate at location 3).Depth of cracks amounts from a few tenths of a millime-ter to a few millimeters. The deepest crack reaches thedepth close to 3 mm. Width of cracks amounts approxi-mately to 0,1 mm. The width is narrower at the tip of thecrack (a few hundredths of a millimeter) and wider onthe surface (a few tenths of a millimeter). The largercrack width on the surface is a consequence of melt ero-sion that causes constant loss of die material.METALURGIJA 49 (2010) 1, 9-12 11M. MUHI^ et al.: THERMAL FATIGUE CRACKING OF DIE-CASTING DIESFigure 4. Locations of die surface defects and their detai-led photos.Figure 5. Die defect-crack occurrence (number of cycles).Figure 6. Mean and maximum crack depth and their loca-tions.Figure 7. Mean and maximum surface crack length of andtheir locations.The difference between the measured crack depthand its real depth varies from 30 % to 50 %. The reasonfor this deviation is in the diameter of measuring wiresbeing too large, thus not enabling the crack tips to bereached.Crack initiation is accelerated by gradual softeningof die surface material during operation of the die 9,11. This is caused by tempering due to high tempera-ture of aluminium alloy. Figure 9 shows micro-stru-ctures of die material near the die surface (Figure 9a)and three millimeters below the die surface (Figure 9b).The microstructure near the die surface (Figure 9a)has been changed due to high temperature that leads totempering of the martensite. The tempering of themartensite caused a decrease in hardness by about 130HV (Figure 10) on the die surface compared to hardnessof the die material deeper in the core. Decrease in diematerial hardness consequently led to decreasing mate-rial resistance against thermal fatigue cracking.CONCLUSIONSThermal fatigue cracking is a dominating surface de-fect of die-casting dies made of aluminium alloy. Firstcracks occur before 2000 cycles and propagate progres-sively in subsequent cycles. They occur closer to the diegate, where the temperature gradients are higher due tohigher melt temperature. Cracks also occur early on lo-cations with higher stress concentration, i.e. edges andcorners with small radiuses. Cracks which occur soonerusually become deeper and longer during subsequentcycles. The observed crack depth ranged from a fewtenths of a millimeter up to three millimeters and itslength varied from a few millimeters to 75 millimeters.The die material microstructure near the surface isgradually softening due to intense heating by the moltenaluminium alloy. This causes tempering of the marten-site and consequently a decrease in hardness. Surfacesoftening increases the initiation of cracks and their fur-ther propagation.REFERENCES1 L.J.D. Sully, Metals Handbook, vol. 15, 9th ed., ASM Inter-national, Metals Park, (OH), 1988.2 J.R. Davis (Ed.), ASM Speciality Handbook, Tool Mate-rials, ASM International, Materials Park, OH, 1995.3 D.F. Allsop, D. Kennedy, Pressure Diecasting, Part 2: TheTechnology of the Casting and the Die, Pergamon Press,Oxford, 1983.4 W. Young, Why die-casting dies fail, Paper no. G-T79-092,Proceedings of the 10th SDCE International Die CastingExposition and Congress, North American Die CastingAssociation, St. Louis, MO, 1979, 1-7.5 F. Kosel, L. Kosec, Mechanical Engineering Journal, 29(1983), E1-E8.6 J.F. Wallace, D. Schwam, Die Casting Engineering, (2000),50-58.7 B. Kosec, L. Kosec, J. Kopa~, Engineering Failure Anal-ysis, 8 (2001), 355-359.8 A. Persson, S. Hogmark, J. Bergstrom, International Journalof Fatigue, 26 (2004), 1095-1107.9 A. Persson, S. Hogmark, J. Bergstrom, Surface & CoatingsTechnology, 191 (2005), 216-227.10 A. Persson, J. Bergstrom, S. Hogmark, Journal of MaterialsProcessing Technology, 148 (2004), 108-118.11 D. Klob~ar, J. Tu{ek, B. Taljat, Materials Science and Engi-neering: A, 472 (2008), 198-207.Note: The responsible for English language is Bernarda Kosel.12 METALURGIJA 49 (2010) 1, 9-12M. MUHI^ et al.: THERMAL FATIGUE CRACKING OF DIE-CASTING DIESFigure 8. Cross-section of thermal fatigue cracks.Figure 9. Microstructure of die material: a) near the sur-face and b) 3 mm below the surface.Figure 10. Die material hardness versus depth fromsurface.

Pukotine na kalupima (alatima) za tlačno lijevanje uslijed toplinskog umora

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