Ötektik altı Al-Si alaşımlarında 5/1 tibor ön alaşımı ile tane inceltme işleminin sertlik ve mikroyapı üzerindeki etkisinin belirlenmesi

Abstract

Hafif metaller grubundan olan Al-Si döküm alaşım larının mekanik özellikleri mikroyapıya bağlı alarak de ğişim göstermektedir. Mikroyapının katılaşması sırasın da ve katılaşmadan sonra kontrol altında bulundurulması ve amaçlara göre yönlendirilmesi dökümde büyük önem ta şımaktadır. Ötektik altı Al-Si alaşımlarının yapısı genellikle dallantılı bir görünümde olan o< birincil fazı ile bu dallantılar arasında kümelenmiş ötektik karışımdan olu şur. Ana yapıyı oluşturan c<- Alüminyum yumuşak ve sü- nek bir fazdır. Herhangi bir çekme yada bükme durumun da kapmalar bu yumuşak ana yapı sınırlarında oluşmakta dır, dolayısıyla bu yapıya sahip olan malzemelerin çek me dayanımı ve % uzama değerleri kötüdür. Alüminyum dökümlerde en önemli döküm problemini oluşturan gaz ve kaba porozite tane boyutu ile doğrudan ilişkilidir. İnce taneli yapılarda porozite dağılımı daha ince ve daha az tehlikeli olduğundan malzemenin me kanik özellikleri, öncelikle çekme dayanıım ve uzama de ğerleri bu tür yapılarda oldukça iyidir.

Aluminum casting alloys are making rapid strides tomard more extensive engineering use. Certain engineering advantages are inherent in the use of aluminum allays for casting. Light weight (per unit volume) is the one most commonly cited,- Some of the numerous other desirable properties include: - A wide range of mechanical properties - Architectural and decorative value - Corrosion resistance - Nontoxicity - Electrical conductivity - Ease of machining - Casting properties - Lower casting shipping costs per piece, The aluminum-base alloys may in general be charac terized as eutectic systems, containing intermetallic compounds or elements as the excess phases. All the properties of interest 'are influenced by the effects of the various elements with which aluminum is allayed. The principal alloying elements in aluminum-base casting alloys are copper, silicon, magnesium, zinc, chromium, manganese, tin, and titanium. Silicon is present in all commercial aluminum casting alloys and used as an allaying element in amounts up to about 14 per cent Si. The solubility of Si in aluminum, the «K phase, is limited to 1,65 percent at 577 C and less than D.05 per cent at room temperature. Undisolved silicon is present VI as p, silicon particles containing an extremely small percentage of aluminum. The size of the silicon-rich S particles may be varied greatly. A rather coarse particle size occurs with normal melting and sand- casting practice. The general effect of increasing silicon content is the increase of the strength until the eutectic silicon percentage is reached. Some treatments are applied for control of metallüg- raphic structure of casting allays. The importance of structure in cast allays lies mainly in the structure- sensitive properties which can be utilised in engineering These properties are determined primarily by the influence of the microsturcture on the behavior of dislocations in the lattices of the individual crystals. Unlike wrouth materials, in which further opportunities exist for changingboth structure and dislucation density, the initial microstructure is frequently the main vehicle for the control of properties, althoug subsequent heat treatment plays this role in some cast allays. The principal factors governing the final metallographic structure of a casting may be listed as follows: - Constitution and thermal properties of the alloy - Casting design and dimensions - Thermal properties of the mould - Superheat and final casting temperature - Conditions for heterogneous nucleation - Subsequent heat treatment Three factors has been shown to be significant in determining the metallographic structure of castings, viz. nucleation, growth behavior and crystal multipli cation,, The most common aim in structure control is the pursuit of refinement through one or some of these possibilities. Practical meausures for size control and modification can be grouped as follows: - Variations of cooling rate - Chemical treatment of the liquid metal - Manipulation of melt superheat - Agitation during freezing Vll The association of rapid cooling with fine grain size arises from the influence of undercooling on the comparative rates of nucleation and growth. The under cooling of a melt to a lower temperature increases the number of effective nuclei relative to the growth rate, the latter being restricted by the rate at which the latent heat of crystallisation can be dissipated. Slow cooling, conversely, favours growth from few nuclei and produces coarse grain structures. Research over a wide range of cooling rates has indicated that the dentrite arm spacing conforms to the relation dra=C, where d is the spacing, r the cooling rate and a and c are constants. In the case of eutectics the lamellar spacing is associated with the local rate of freezing in accurdance with the relationship/''* r =constant, where ^ is spacing, r is the rate of freezing and n = 0.5. The cooling rate in castings is to a large extent governed by the design and thermal properties of the casting itself. Cooling rate is, however, affected by variation in the mould material, the highest heat diffusivity being obtained in practice with the metal moulds used in gravity die casting. Amoung normal casting variables the most useful influence upon cooling rate is that of pouring temperature, an increase in which diminishes the freezing rate by preheating the mould, so reducing the rate of heat transfer during crystallization. Maximum undercooling for grain refinement thus requires a low casting temperature. Highly effective grain refinement can be accomplished by inuculation - the addition to the melt of small amounts of substances designed to promote nucleation- although in certain cases the function may be to modify the growth rather than the nucleation process. The mechanism for grain refining has been studied extensively in the literature. Various explanations are advanced for the refining function of alloy additions. The most direct of these is the formation of stable particles as nuclei in the melt, for which small additions are usually sufficient. A further important effect is that of growth restriction due to solute concentration gradients and constitutional undercooling. In the liquid adjoining the crystal. This effect is always present to some extent from the normal alloy content. In the peritectic hypothesis, on the other hand, refinement by most inoculants in aluminum is attributed to peritectic reac tions common to their respective systems with the parent metal. There is much evidence to suggest that both mechanisms are operative in certain circumstances. Properties improved by grain refining include reduced ingot cracking, improved feeding, reduced macro-and micro-segregation, and porosity redistribution. Often, vixi dynamic properties and mechanical properties such as elongation and tensile strength are also improved. Effective grain refinement in aluminum allays is obtained from small additions of titanium and titanium -boran alloys. A coarse equiaxed grain structure, a coarse columnar structure, or even a twin columnar grain structure (TCG) can be eliminated by the addition of these grain refiners into the melt. Past studies have shown that there are several variables that deter mine the response of grain refiner additions to hypoeutectic silicon alloys. The titanium residual level and wheather it exceeds the peritectic value, the casting conditions (including local solidification rate), the hypoeutectic silicon alloy, the addition rate, the agitation and the grain refiner addition are all influential. A further influence upon nucleation may be exercised by superheating the metal to a ligher temperature than that used for pouring. The influence of superheat is held to be associated with the behavior of heteroge neous nuclei during thermal cycling. The most usual effect is to produce coarsening of the grain structure. This has been attributed to the elimination, by solution, either of foreign nuclei themselves or of small amount of solid parent metal which remain stable above the liquidus temperature in the surface pores of foreign particles or the walls of the container. In certain cases, however, super heat produces the pposite effect of grain refinement, a notable example being seen in the treatment applied to magnesium-aluminum alloys. The mechanism of refining by superheat has been explained as the solution of relatively large included particles of nucleating compounds and their precipitation in more finely divided and crystallographically favourable forms on coaling towards the liquidus temperature. It has long been realised that nucleation could be brought about by physical disturbance of undercooled liquid, for example by stirring or gas evolution. This may be attributed to the Widespread distribution of nuclei originally produced at the surface, or to the fragmentation of already growing Crystals. In this study, grain refining behavior of two hypoeutectic Al-Si alloys containing approximately 6-6.30 % and 8-9 % wt silicon was investigated by using 5/1 TİBDR as a grain refiner. The aim of the study was to optimize refining conditions to improve grain IX refiner performance and microstructure. For this purpose, titanium content of the allays, casting/ inoculation temperature, holding time after addition of the grain refiner and coaling rate in the moulds were selected as experiment parameters. The desired titanium levels was achieved by adding 5/1 TİBDR tablets to the melt in the calculated stoichiometric ratios to give 0.05, 0.1 and 0.2 % wt titanium in the both alloys studied. Grain refining and casting of the melts was made at two different temperatures, 700 C and 75G C after adding the grain refiner and holding for 15.30, k5 and 60 minutes at these temperature levels. the melt was poured into graphite and permanent moulds which give different coaling rates. The micro- structures developed without using grain refiner, and under different refining conditions were examined both by optical and scaning electron microscope (SEM). Grain refining performance for the different refining conditions was determined by comparing the microstruc- tures with each other and with the nanrefinied initial microstructures. The hardness of the samples was measured by using Brinell hardness test method. Experimental investigations have revealed that for anallay having 6-6.30 % wt Si at 0.05 % wt Ti level and holding 15-30 minutes at 700-75QaC inacculatian/casting temperature an effective grain refining can be achieved, However no further refining could be obtained for 0.1-0.2% wt. Ti levels. It has been determined that tendency for grain refinement has also increased with increasing Ti level from 0.05 % to 0.1 % but has not increased for Ti level higher than 0.1 % wt.