Alaşım elementlerinin çinko-alüminyum esaslı ZA8 alaşımının sertlik ve darbe direncine etkisi

Abstract

Son yıllarda Çinko endüstrisi, günümüzde birçok alanda yaygın olarak kullanılan kovansiyonel basınçlı döküm Zamak alaşımlarına ilave olarak, daha yüksek alüminyum oranlan içeren yeni bir seri Çinko-Alüminyum esaslı döküm alaşımları geliştirmiştir. Bu alaşımlar günümüzde ZA-8, ZA-12 ve ZA27 alaşımlan olarak tanınmakta ve sahip oldukları üstün mekanik ve döküm özellikleri ile endüstriyel alanlarda kullanımları her geçen gün hızla artmakta ve birçok uygulamalarda bazı alüminyum, dökme demir, pirinç ve bronz gibi konvansiyonel döküm alaşımlarının yerini almaktadırlar. Bu alaşımlar basınçlı dökümde dahil olmak üzere hemen hemen bilinen bütün döküm yöntemleri ile üretilebilmekte, ZA8 alaşımı düşük alüminyum içeriği ve düşük ergime sıcaklığı dolayısıyla sıcak kamaralı basınçlı döküm yöntemi ilede başarıyla dökebilmektedirler. Bu özelliği ve diğer ZA alaşımlarına oranla daha üstün sürünme direncine sahip olması, bu alaşımın ZA alaşım ailesi içinde ticari olarak en ilgi çekeni yapmaktadır. Bu sebeple bu alaşım en çok ilgi çeken ve özelliklerinin optimum değerlere ulaştırılması için yoğun geliştirme çalışmalarının yapıldığı bir alaşımdır. Bu çalışmada ZA-8 alaşımına ilave edilen çeşitli miktarlardaki Al, Cu, Mg, Mn, Ti, Li, Si ve Cr gibi alaşım elementierinin sertlik ve darbe direncine olan etkileri incelenmiştir. Elde edilen sonuçlardan, ilave edilen Al, Cu ve Mg'un alaşımın sertliğini büyük ölçüde arttırdığı, fakat Mn, Ti, Li, Si ve Ctfun ise sertliği etkilemediği görülmüştür. Ayrıca % 7 Al içeriğine kadar alaşımın darbe direncinde bir artış olmuş, bu oranın üstündeki ilaveler ise darbe direncini düşürmüştür. Az oranlarda Cu, Mg, Mn, Tİ, Li ve Cr darbe direncini arttırmada yararlı olmuştur. Fakat, belirli miktarların üzerindeki ilaveler de ise darbe direncinde büyük düşüşler olmuştur. Az miktarlarda Si ilavesi ise darbe direncinde büyük düşüşler göstermiştir

Zinc-containing materials have existed for several thousands of years. However, it is only since about the middle of the current century that zinc-base cast and wrought products have achieved significant metallurgical acceptance. This was in part due to emphasis that was placed on obtaining a greater understanding of the ways in which the material's properties could be influenced by refinements in composition. The oldest-known, currently surviving, zinc alloy component dates from the pre historic Dacian settlement at Dordosch, Transylvania. This was an idiol which contained 87.52% zinc. Zinc-rich bracelets and other ornaments have been found from as early as 500 b.c. The first writer to give the name "zinck" to the metallic nature and indicated something of the physical qualities of the metal, including its fusibility and limitations in malleability. Zinc products appear to have been known in India as early as 1000 A.D. However, zinc was probably not smelted commercially in that country until about the 14th centry A.D. From evidence of the size of the zinc-bearing residues, which have been estimated at between 130000 and 170000 tons, and the numerous clay retorts which still remain today, it would appear that the Indian zinc-smelting industry was of an impressive magnitude. Knowledge of zinc smelting was brought to England from China about 1730, and in 1739 an English patent was issued for the process of distilliation of zinc following reduction of zinc-bearing sulphide ore with charcoal. Wrought zinc products date from 1805, when it was discovered in Sheffield that the ordinary brittle zinc could be warm-worked, i.e. rolled into sheet form, at temperatures in the range 100 to 150°C. The first commercial sheet zinc rolling mill was built in Liege, Belgium, during 1812. Today, all commercial cast and wrought zinc-rich materials are alloyed from high purity zinc, which has been refined by electrolysis in sulphate solutions. The majority of the commercial foundry alloys are based on the Zn-Al alloy system. Casting alloys are usually designated by an identification number which may, or may not, correspond to the nominal aluminium percentage. The largest usage of zinc-based foundry alloys is in pressure die casting with alloys 3 and 5. In this process molten metal is injected under pressure into stell dies into wich an impession has been formed. Great precision is possible, and products ranging from sewing machine parts to outomotive engine blocks can be cast Zinc pressure die casting alloys based on the eutectic composition in the Zn-Al system have found considerable indusrial applications since their first discovery in the 1920's. Their successis due to their good castability and fluidity, their ability to be cast in hot chamber pressure die casting machines allowing high production rates, and their good mechanical properties. During the last decades developments in the thin wall die casting tecnology have also permitted their imlementation into comlex shaped parts with reduced weights. The zinc industry responded with some significant countermeasures. In the early 1970s, the International Lead Zinc Research Organization, ILZRO, began to sponsor three different programs of research into zinc alloy die casting. These programs went straight to the engineering fundamentals underlying the die casting cycle; that is thermodynamics, hydraulics and mechanics. One, at the Battelle Columbus Laboratories in Columbus, Ohio, lead to the thinwall die casting system. Another, at the Commonwealth Scientific and Industrial ResearchOrganization in Melbourne, Australia, resulted in the Ausralian system. The third, at the BNF Metals Technology Centre in Wantage, England resulted in the British system. Each system has eliminated the cut -and -try approch to die casting, and all three enable the die caster to produce sound, high-guality castings with very thin walls. Equally important, the product designer may be less concerned with process restrictions and concentrate more on pruduct performance. This improved die casting capability equipped the zinc industry to retain some which would otherwise have been lost Most of the applications lost to plastics have not been regained, however, and probably will not be in the forseeable future. The plastics industry has been consistently refining and improving their materials and processes, as evidenced by the fact that they have taken over a number of decorative application in other industries, particularly home appliances. These non-outomotive gains are significant because they are based almost entirely on reduced cost; reduced product weight is not usually a factor. The components and their finish are often inferior to zinc die castings, but they are accepted because mey are adequate for the application at band. While the plasucs-versus-zinc struggle was taking place, the zinc industry was also developing a series of gravity cast alloys which could be used in sand and permanent mould foundries to compete with copper, aluminum and ferrous castings. This work has a produced a family of three alloys, designated as alloys 8, 12 and 27, the number in each case indicating the approximate aluminum content Alloy 27 is also being successfully die cast by the cold chamber process to produce components with high strenght and axcellent resistance to wear. xiii These two zinc industry thrust -alloy development and die casting improvement- have formed the basis for a totally new zinc alloy technology. Zinc alloys are used extensively in both gravity and die casting. When used as general casting alloys. Zinc alloys can be cast using such processes as high pressure die casting, low pressure die casting (iron, graphite, or plaster moulds), spin casting (silicone rubber moulds), investment (lost -wax) casting, continuous or semicontinuous casting, and centrifugal casting. A newer process involves semi solid casting, of which several techniques can be amployed. The ZA alloys are increasingly being specified for bearing and bushings in low speed high load applications. In plane bearing, the tradionaly material is bronze. In low to moderate speed, moderate temperature, lubricated aplications, the zinc alloys are becoming well established because of mir low coefficient of friction and excellent hardness. Zinc alloy bearings frequently equal to suprass bearing bronzes in performance and typically generate cast sawing of 15-40 %. The use of zmc-aluminum alloys as effective bearing materials is not new. developed in Europe over 40 years ago, high aluminum-zinc alloys similar in composition to the ZA27 alloy have been applied there for decades to bearings and many other parts which are customarly made of bronze in North America. Today, ZA12 and ZA27 are rinding increasing use as replacement to bronze in lower speed, low to medium temperature bearing and bushing applications. The market penetration of ZA alloys has been aided by the fact that traditionary high volume foundry metals have significant short commings mat detract from their inherent advantages: - cast iron has high energy and machining costs, protective finishes are nearly always required and there are industry enviromental problems. - bronze has high material and energy costs and the enviromental proplem of lead for many important alloys. - aluminium has limitations in strength, bearing properties and finishing along with moderately high energy casts. Of course, each of these classic materials does have distinct advantages in given applications. İn contrast, the zinc casting alloys have advantages that are higly attractive to foundries: xrv - excellent casting properties; - low energy consumption; - pollution free melting and casting; - excellent machinability; - lower material cost and density than bronze. The ZA alloys offer properties that are competitive or better than those of cast iron (Class 30), aluminium (AA356) and bronze (SAE 660). As a result, they compete in cast/performance with tradionaly high volume foundry metals in wide variety of applications. The ZA8 and ZA12 alloys provide both functional and cosmetic benefits; ZA27 is used when high strength is required. The ZA8 alloy was developed principally for permanent metal casting. It is curently cast by the graphite permanent mould and cold chamber die casting process and is also being evaluated for hot chamber die casting. ZA12 and ZA27 have made succesful transitions from sand casting applications to other processs. Both ZA12 and ZA27 perform well in cold chamber die casting. ZA12 is also cast by the graphite permanent mould and centrifugal mould process. Because of their high fludity, zinc alloys can be cast in much thinner walls than other die casting alloys, and mey can be die cast to tighter dimensional tolerances. Zinc alloys allow the use of very low draft angels; in some cases,a zero draft angle is possible. Zinc casting alloys have dendritic/eutectic microstructures. The hypoeutectic alloys solidfy with zinc rich rj dendrites, where as the hypereutectic alloys solidfy with aluminium rich dendrites. The ZA8 and ZA12 alloys solidfy with cored p dendrites, wheras ZA27 solidifies with a dendrites. It is critically importent that all zinc-aluminium casting alloys be carefully handled to prevent excessive pickup harmful impurity elements such as lead, cadmium, tin, and iron, among others. Cross contamination caused by melting the alloys in furnace used for casting copper and aluminium alloys or iron is particularly troublesome because these alloys contain elements harmful to zinc alloys. The aim of this study was to investigate and improve the impact strength of this alloy by modifying its coposition with small amount of alloying elements Al, Cu, Mg, Mn, Ti, Li, Si, and Cr. Aluminium content of the zinc-aluminium based alloy ZA8 ( Zn- 8% Al- 1% Cu- 0.03% Mg) has been varied from 5% Al (eutectic composition) to 11% Al (composition of alloy ZA12) and effect of this element on hardness and impact resistance has been determined. ît has been found that while the hardness of the alloys gradually increased with increasing aluminium, impact resistance increased with increasing aluminium up to 7% Al inclusive and then decreased gradually with further increase in aluminium content xv In addition, the effect of alloying elements of copper, magnesium, manganese, titanium, silicon, lithium, and chromium on these properties studied of the standart ZA8 alloy has also been investigated in the ranges from 0 to 3 % for copper, 0 to 1.0% for magnesium, 0.01 to 1.0% for manganese, 0.01 to 0.2% for titanium, 0.2 to 1.1% for silicon, 0.01 to 0.05% for lithium, and 0.005 to 0.05 % for chromium and the results showed that the hardness of the alloy considerably increased with increasing copper and magnesium, but, manganese, titanium, silicon, lithium, and chromium had not muck a significant effect on the hardness. Besides, small amount of additions of copper, magnesium, manganese, titanium, lithium, and chromium was benefical in increasing the impact resistance of the standart ZA8 alloy, but, additions above a certain amount caused the impact resistance to decrease at considerably high rates and its appear that, the presence of trace amounts of silicon caused a subsequent decrease in impact resistance of ZA8 alloy.