In the contribution the stress of die materials of thermal fatigue is defi ned and material life is derived theoretically and compared with the measured values. The important properties of the die materials as thermal conductivity, coeffi cient of thermal expansivity, modulus of elasticity and mechanical properties are described. Binding to it single die materials as carbon steels and chrome-tungsten steels are analyzed. As the perspective die material for pressure die casting of ferrous metals appears molybdenum with regard to advantageous properties

E. Ragan

J. Dobránsky

P. Baron

T. Olejár

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117 METALURGIJA 51 (2012) 1, 117-120E. RAGAN, J. DOBRÁNSKY, P. BARON, T. OLEJÁR MATERIALS ON DIES FOR PRESSURE DIE CASTINGReceived – Prispjelo: 2010-12-27Accepted – Prihvaćeno: 2011-02-03Review Paper – Pregledni radISSN 0543-5846METABK 51(1) 117-120 (2012)UDC – UDK 669.14-418:539.37:620.17=111In the contribution the stress of die materials of thermal fatigue is defi  ned and material life is derived theoretically and compared with the measured values. The important properties of the die materials as thermal conductivity, coeffi   cient of thermal expansivity, modulus of elasticity and mechanical properties are described. Binding to it sin-gle die materials as carbon steels and chrome-tungsten steels are analyzed. As the perspective die material for pres-sure die casting of ferrous metals appears molybdenum with regard to advantageous properties.Key words: stress of material, thermal fatigue, service life, material propertiesMaterijal matrica za tlačno lijevanje. U članku je defi  nirano naprezanje materijala pri toplinskom umoru i iz-veden teorijski životno (radno) vrijeme materijala te uspoređene sa izmjerenim vrijednostima. Opisana su važna svojstva materijala, kao što je toplinska vodljivost, koefi  cijent toplinske rastegljivosti, modul elastičnosti i mehanička svojstva. U nastavku su analizirani materijali kao što su ugljični čelici te krom-volframovi čelici. Kao perspektivni materijal za tlačno lijevanje željeznih slitina se pojavljuje molibdan sa prestižnim svojstvima.Ključne riječi: naprezanje materijala, toplinski umor, radni vijek, svojstva materijalaINTRODUCTIONAt operation of pressure die casting dies are cycli-cally stressed on thermal fatigue. After exhausting of material sliding properties the end of die service life comes. For the more profound explanation it is neces-sary further to analyze thermal conditions and single important properties of die material during the opera-tion of pressure die casting. STRESS OF DIE AT PRESSURE DIE CASTINGAt entrance of liquid metal with high temperature into die a heat comes from the metal into the die surface according to Figure 1.The layer of die face can be considered as a whole. If this layer is free and we know its linear coefﬁ  cient of thermal expansivity α then at its temperature T1 and its temperature before entrance of liquid metal T0 the ex-tension of the layer is [1,2]. δ =a(T1-T0)  (1)Because the layer cannot be expanded the compres-sive stress σ arises in it        δmE σ =    (2)        m–1where m – Poisson’s constant, E – Young’s modulus of die material. If we substitute competent values E = 2,2·105 MPa, α = 1,3·10-5, m = 3,3, To = 300 °C, T1 = 600 °C we obtain σ = 1 200 MPa that evokes a tensile stress E. Ragan, J. Dobránsky, P. Baron, T. Olejár, Faculty of Manufacturing Technologies, Prešov, Technical University in Košice, Slovakia in the layer under the surface layer. However after draw-ing the casting from the die the surface layer cools and a tensile stress arises in it. By repeating this process as a thermal fatigue at the tensile stress it is arising a danger of cracks. After ten till hundred thousand cycles the cracks arise really and the life of the die material ends.THE SERVICE LIFE OF DIE MATERIALIf the number of the pressing liquid metal into the die N is considered as the quasicontinous function with indirect proportional dependence on the pouring tem-perature T then every elementary increase of the pour-ing temperature dT means lowering indirect proportion-al cycles of the die material service life  .Then it is:               dN dT = – k    (3)                 Nwhere: k – constant /°C.Figure 1   The surface layer 1 and undersurface layer 2 of the die at fi  lling the die with the liquid metal 3118   METALURGIJA 51 (2012) 1, 117-120E. RAGAN et al.: MATERIALS ON DIES FOR PRESSURE DIE CASTINGAfter integration from T1 up to T and from N1 up to N we obtain: T–T1=–klnN + klnN1 (3,1)We adapt it and we change on decadical logarithm [3,4]:  logN = A – k1t (3,2)where: A – constant,   /°C-1.The relation (3.2) corresponds to the measured re-sults according to Figure 2 where the logarithm of cy-cles N is dependent on the pouring temperature T at pouring single alloys of zinc, aluminum, copper and ferrous alloys (cast iron, steel).It is possible to reach the increase of die service life by single or total use of following measures [5,6]: the use of die material with the smallest thermal expansion, the smallest modulus of elasticity and the largest con-ductivity.Ceramic materials accommodate to the ﬁ  rst claim but they do not accommodate to the second claim and as very brittle ones they did not prove at tests. Molybdenum that is shown as a perspective die ma-terial accommodates partly to the third claim. It is how-ever expensive at lower temperatures it has area of brit-tleness and at higher temperatures with regard on its sublimation it is necessary to secure a surface protec-tion. For increase of die service life at pressure die cast-ing brass it is possible to use the alloys Inconel 617 and Haynes together with higher temperature of die.PROPERTIES OF DIE MATERIALSDie materials must fulﬁ   l properties going out of from stress of dies in operation. They are mainly ther-mal expansion and modulus of elasticity, mechanical properties, resistance to thermal fatigue [6] and chemi-cal stress of die.Thermal expansion and modulus of elasticityAt smaller thermal expansion and modulus of elas-ticity stress at certain thermal impact at die operation is also smaller. It dependences on modulus of elasticity of elementary metal that is only little changed by alloying addition. Modulus of elasticity and thermal expansion coefﬁ  cient of die materials are on Table 1 [7,8].Thermal conductivityIt is an important property. It shows also at running property of metal. In [9] length of ﬂ  ow is compared in dependence of the die material alloy Cu-Be and steel that is 44 percent shorter that at steel.Mechanical propertiesIt is possible to change mechanical properties by al-loying and heat treatment so that deformations are in elastic area. It is however possible only at casting of zinc alloys at temperatures low. The yield point is low-ered at high temperatures and plastic deformations oc-cur. It manifests that it is necessary to temper die steels on lower strength. Plastic deformations are tired out slower.Chemical stress of die materialsAt chemical stress of die material in the ﬁ  rst row there are alloys dissolving iron especially aluminum al-loys. It is possible to ﬁ  nd out it from equilibrium dia-gram of iron and elementary metal of alloy then Fe-Zn, Fe-Al, and Fe-Cu. The higher is temperature of alloy the higher is danger of chemical dissolving die. At brass zinc penetrates into die cracks. Also at aluminum alloys it can come to very expressive effect. According to equilibrium diagram Fe-Al at 700 °C aluminum creates the brittle intermetallic phases FeAl2, Fe2Al3 and FeAl3 [10,11]. At slight thickness they can inﬂ  uence unfavora-bly surfaces of casting and die and they can demolish the die. This appearance is well-known as a sticking. Especially improperly situated cores and parts of die exposed to ﬂ  owing alloy at ﬁ  lling die cavity are endan-Table 1 Important properties of die materialsMaterial Addition     / %Modulus of elasticity/ MPaCoeffi   cient of thermal expansion / m0C-1Thermal conductivity       / kJm-1s-10C-1Hardness  / HBSResistance against meltingCopper - 105 000 16,5 0,393 60 excellentBeryllium bronze 0,5 Be 110 000 16,5 0,0836 400 excellentHeat treat soft steal 0,4 C 220 000 11,7 0,0627 8 excellentLow alloy heat treat steel 0,3 C, 1 Cr 215 000 10,5 - 13 0,0418 300 – 400 goodHigh alloy heat treat steel 0,3 C, Cr, W, Mo, V 210 000 10,5 - 13 0,0188 – 0,0334 350 – 500 goodAustenite steel 0,1 C,18 Cr 200 000 19,5 0,0209 130 very badMolybdenum - 350 000 4,9 0,146 170 excellentMolybdenum - titanium - 350 000 3,2 0,125 200 excellentFigure 2    Dependence of die material service life at pressure die casting on pouring temperature119 METALURGIJA 51 (2012) 1, 117-120E. RAGAN et al.: MATERIALS ON DIES FOR PRESSURE DIE CASTINGgered. We face to attacking by the large hardness of die face and protective layers.   MATERIAL FOR DIESFor dies of pressure die casting carbon steels, low-alloyed steels, high-alloyed steels, chrome steels, tung-sten steels and chrome-molybdenum steels are used. -   Carbon steels are used for die parts being not in contact with liquid metal as the clamping die box, the guide and supporting plate of ejectors and the hydraulic drawer of cores.-   Low-alloyed steels are used especially for zinc al-loys. They are steels with the content 0,30 up to 0,45 % C alloyed with Cr respectively V and Mo. Further they are carbon constructional steels with high strength after heat treatment 1 000 up to 1 200 MPa. It is possible to reach martensitic structure into larger depth with carbide-created elements. At tempering they shift the decrease of martensit de-composition on pearlit into higher temperatures. It limits their use on lower temperatures of pour when the decrease of hardness is not large.-  High-alloyed steels contain so many carbide-created elements that by tempering at temperature higher than 400 °C it comes to increase of hardness meas-ured at room temperature marked as the secondary hardness. It is higher at larger number of the car-bide-created elements connecting with the origin of complex carbides and shifting into higher tempera-tures. Therefore the steels for dies shifted from the chrome and tungsten steels to the combinations W-Cr, W-Cr-Co, Cr-Mo, Cr-Mo-V, Cr-Mo-V-W. At hardening under temperature of the martensite crea-tion beginning the steel contains the martensite and the residual austenite that at tempering decomposes on the martensite or bainite and with it possible to contribute to the secondary hardness [12]. -   Chrome steels are martensitic with so large content of chrome that they are anticorrosive and heat-re-sisting and with so large content of carbon that they are hardenable. By additions of the carbide-created elements the ﬁ  ner grain at higher temperature of austenitization and shifting zone of secondary hardness into higher temperatures is reached.-   Tungsten steels are the steels with the content 0,30 up to 0,35 % C, 2 up to 2,5 % Cr and 4,5 up to 11 % W. In USA they are without further additions in Slo-vakia still with addition V and Ni and in Germany also Co. The tungsten-chrome steels have larger hardness into higher temperatures of tempering. They match on dies for resistance to wear and stick-ing e.g. in region of gate [13,14]. The thermal con-ductivity of the tungsten steels is not so good as at the steels without tungsten. Tungsten and cobalt im-prove however the resistance to wear [15]. -   Chrome-molybdenum steels have the content ap-prox. 0,30 % C, 2,8 up to 5 % Cr, 0,5 up to 2,5 % Mo most often also the addition of V sometimes also W and Co. the cobalt lowers the inclination to brittle fracture at higher temperatures and increases plastic properties in heat. With lowering content of chrome and further additions it was reached the considerable improvement of service life. The tests at die casting brass showed that the better resist-ance to cracks is at tempering on smaller strength 1  050 up to 1 300 MPa than on 1 600 MPa. At die casting aluminum alloys it would be necessary the increase effect of protective layers obtained by ni-triding or sulfonitriding recommended at these steels [16,17].   MATERIALS ON DIES DEVELOPMENT DIRECTIONSThe important factor is the steel quality not only ac-cording to composition but also according to cleanli-ness. It goes about low content of sulfur and phosphorus but also the smallest content of inclusions their ﬁ  neness and regular distribution. The inclusions always lower resistance to origin of cracks network. Remelting steel in vacuum is also important. It is possible to obtain the improvement of material service life by further change of the chrome, molybdenum, tungsten and vanad so that the suitable secondary hardness is reached at the small-est content of these elements. We obtain with it also si-multaneously the largest possible thermal conductivity. It is possible to reach the further improvement by low-ering hardness after tempering. [18,19]At large series of die casting in production of auto-mobiles the choice of materials on dies for zinc alloys aims at chrome-molybdenum steels used for die casting aluminum alloys. The analogous tending can be oc-curred for other alloy die cast.Molybdenum is appeared as the perspective material on dies for die cast. It has the relative high thermal con-ductivity the low coefﬁ  cient of thermal expansivity so that also at large modulus of elasticity the high stress does not rise. It is brittle in the interval of low tempera-tures. It can come to cracking. As to it has not the pro-tective layer on the die face e.g. nitriding one it comes to sublimation at die cast. [20]Further the composite materials have a perspective. According to [21,22] the ﬁ  lling chamber was from the composite materials reinforced by particles of titan (TiB and TiC) for die cast of aluminium alloys in USA.CONCLUSIONIn the contribution the cyclical stresses on thermal fatigue of die materials are described and analytically derived. Further exhausting of the material sliding prop-erties are derived with the end of die service life. The single properties of the die materials are also described. The development was very important at transition from zinc to aluminum and copper alloys at die cast and it can 120   METALURGIJA 51 (2012) 1, 117-120E. RAGAN et al.: MATERIALS ON DIES FOR PRESSURE DIE CASTINGdo service at transition from research to working of the ferrous alloys die cast. REFERENCES[1]  W. Leis, Tlakové lití, materiály, Slévarenství, 7(2007), 327-333.[2]  E.J. Vinarcik, High Integrity Die Casting Processes, John Wiley and Sons, New York, 2003.[3]  J. Nová, I. Nováková, J. Bradáč, Technologie I, TU Libe-rec, 2006.[4]  D. Bolibruchová, E. Tillová, Zlievá-renské zliatiny Al-Si, ŽU Žilina, 2005.[5]  E. Ragan a kol., Liatie kovov pod tlakom, FVT TU Košice so sídlom v Prešove, Prešov, 2007.[6]  J. Malík, Zlievárenské stroje a zariadenia, HF TU, Košice, 2006.[7]  E. Ragan, Proces liatia pod tlakom, FVT TU Košice so sídlom v Prešove, Prešov, 1997.[8]  M. Mihaliková, J. 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Neslušan, B. Barišič, Strojarstvo, 50 (2008) 6, 337 – 345.Acknowledgement:This work has been supported by Cultural and Educa-tional Grant Agency of Ministry of Education of Slovak Republic project KEGA 3/7167/09.Note:   The responsible for English language is the lecturer from TU Košice

Materials on dies for pressure die casting

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