Hassas döküm Al-7Si-Mg alaşımının karekterizasyonu

Abstract

Çalışmada mekanik özellikleri, kimyasal bileşim ve döküm yönteminden etkilenen Al-7Si-Mg alaşımının, alçı esaslı kalıplara dereceli hassas döküm özellikleri araştırılmıştır. Öncelikle; borla birlikte dentritik yapıyı incelten titanyumun (% 0,02 - % 0,14 aralığında), ötektik yapıdaki silisyumu incelten stronsiyumun (% 0,010 - % 0,083 aralığında) ve ısıl işlem yapılabilme özelliği kazandıran magnezyumun (% 0,12 - 0,81 aralığında) mekanik özelliklere etkileri araştırılmıştır. Metallografik ve mekanik özelliklerin incelenmesi sonucunda; stronsiyum ile modifîye edilen Al-7Si-Mg alaşımlarında en yüksek kopma mukavemetinin % 0,021 Sr da sağlandığı, tane inceltmenin bor varlığında titanyum ilavesiyle alüminyum-titanyum peritektik noktasının altındaki titanyum değerlerinde elde edildiği ve en yüksek kopma mukavemetinin % 0,095 titanyum değerinde sağlandığı, magnezyum etkisinin incelenmesinde en yüksek mekanik özelliklerin % 0,45 Mg'la elde edilmesine rağmen, optimum özelliklerin % 0,35 Mg da elde edildiği tesbit edilmiştir. Döküm şartlarının etkilerinin, savurmalı döküm ve vakum ortamındaki kalıplara döküm şeklinde iki farklı teknikle incelenmesinde; optimum alaşım özelliklerini veren, %0.02 Sr ve % 0,1 Ti içeren Al-7Si-0,3Mg alaşımı kullanılmıştır. Al-7Si-Mg alaşımına 25°C ile 300°C arasındaki kalıp sıcaklıkları için döküm şartlarının etkileri; savurmalı döküm deneylerinde sıvı metale gaz verilme süresi, vakum ortamındaki kalıplara yapılan dökümlerde ise 80 Hz frekanslı 25 u ve 50 jj. genlikli titreşim uygulamaları ile incelenmiştir. Numunelerin döküm ve Tfi ısıl işlemi durumlarında mekanik özellikleri araştırılmış, optik ve SEM mikroskoplarında metalografik yapıları ve kopma yüzeyleri incelenmiş, DAS değerleri tesbit edilmiştir. Stronsiyum ilavesiyle büyümesi engellenerek kaba levhacıklar yerine küt küçük çubuklar şeklinde katılaşan ötektik yapıdaki silisyum, ancak Tfi ısıl işlemi sonucu küresele yakın forma dönüşmüş ve kısmende kabalaşmıştır. Kopma yüzeyi incelemelerinde; kopmanın genel olarak dentritler arası ötektik alanda geliştiği ve ilk olarak kırılgan silisyum fazında başlayıp-sünek yumuşak a fazında devam ettiği ve yüzey görünümünün metalografik yapıya bağlı olarak değiştiği, belirlenmiştir. Savurmalı döküm alaşımlarda, gaz verme süresi ve kalıp sıcaklığına bağlı olarak kopma mukavemetinin 214-246 MPa, sertlik değerinin96-110 VSD, DAS değerinin 30-35 um arasında değiştiği tesbit edilmiştir. Optimum gaz verme süresinin iki dakika olduğu görülmüştür. Gaz porozitesi oluşumunun engellenmesine karşın katılaşma porozitesi arttığından, vakum ortamında tutulan kalıplara yapılan dökümlerde kopma mukavemeti 210-217 MPa, sertlik 96-103 VSD ve DAS değeri 32-35 um arasında değişmiş ve tüm kalıp sıcaklıklarında savurmalı dökülmüş alaşımlardan düşük mekanik özellikler elde edilmiştir. Vakuma ilaveten 80 Hz, 25 um ve 50 um genlikte titreşimlerin uygulanması; 25°C ve 110°C kalıp sıcaklıklarında çekilme boşluklarını azaltarak ve yapıyı inceltip DAS değerini küçülterek, 222-240 MPa arasındaki kopma mukavemeti, 94-118 VSD arasındaki sertlik, 27-22um arasındaki DAS değeriyle mekanik özellikleri geliştirmiştir.Titreşim uygulaması, 300°C kalıp sıcaklığında DAS değerini küçülterek (31-28 um) sertliği arttırmasına (98-106 VSD) rağmen katılaşma porozitesini arttırarak kopma mukavemetini 25 um genlikte 193 MPa'a, 50 (im genlikte 187 MPa'a düşürmüştür. Al-7Si-Mg alaşımının vakum ortamındaki kalıplara dökümünde en iyi mekanik özellikler; 1 10°C kalıp sıcaklığında dökülmüş ve 50 um genlikle titreştirilmiş alaşımda 231 MPa kopma mukavemeti ve 118 VSD sertlik ve 24 jxm DAS değeriyle savurmalı döküm alaşımına yakın mukavemet ve en yüksek sertlik değerinde elde edilmiştir. Al-7Si-Mg alaşımında en yüksek mukavemet ve yapı özellikleri savurmalı döküm tekniğinde iki dakika gaz verilerek 1 10°C sıcaklıktaki kalıba dökülen alaşımlarda, 246 MPa kopma mukavemeti, 105 VSD sertlik ve 32 um DAS değeri ile bulunmuştur.

It has been found necessary to study the influence of processing parameters on the microstructure and mechanical properties of Al-7Si-Mg aluminum alloy which cast into plaster based mold by solid investment process. The properties of Al-7Si-Mg alloys were strongly dependent on the casting process used and chemical composition variations. First Si, Ti and Mg alloying additions were investigated and the optimum coposition was determined. The alloy with the optimum composition was to investigate casting conditions into plaster based mold by centrifugal casting and casting vacum-assisted molds, with the effect of vibrations 25um amplitude and 50um amplitude. Investment casting process uses an expandable pattern to produce a precision component that may be intricately shaped. As compared with other manufacturing processes, investment casting benefits those alloys that develop their best qualities in cast form, and can produce accurate items in material which can only be cast. However, specialized versions of the process can be highly affective in reducing costs on particular types of parts. The principles of investment casting were the same for both the solid mold and shell processes. The most widely used heat-treatable age-hardening aluminum-silicon- magnesium alloy was the class A 356 which has excellent casting characteristics and resistance to corrosion. This justifies its use in large quantities for sand and permanent mold castings. Several heat treatments were used and provide the various combinations of tensile and physical properties that make it attractive for many applications. Alloys used for automotive, marine, and aircraft industries have excellent castability and good weldability, pressure tightnness and good resistance to corosion. The process begins with producing a one-piece little tensile strength sample wax pattern by injecting wax into die. Wax patterns joined together to form a cluster. The refractories used in investment flask (solid mold) process were a plaster and silica mixture and serves as both binder and refractory. After suitable additions of water have been made investment mixtures mechanically stirred and vacuum- assisteded. The cluster of wax patterns and pouring cup were placed on open-end metal flask. The slurry was poured into the flask surrounding the cluster completely. Flask was vibrated and subjected to vacuum-assisted to remove air bubbles formed during the mixing of slurry. Slurry was then permitted to solidify by chemical action, thus providing a good solid mold arround the patterns. VI Once the investment was set, it hardens to comprise a green mold. The flask and its contents were placed into furnace at 150°C which can open on the button. The whole cluster-wax patterns, runners and sprue melt and run out from the furnace through the pouring cup. This leaves the foundry with a monolitic mold containing cavities of tensile samples with passages leading to them. Solid molds must be fired to burn out last traces of pattern material and attain a certain degree of permeability before they can be filled with metal. The molds were poured with the molten metal with centrifugal force in centrifugal casting machine or with the assistance of vacuum-assisted. These two methods enable to reproduce Al-7Si-Mg alloy tensile test samples from wax pattern. All technical equipment used in the experimental study were manufactured in the laboratory except the wax injection equipment and centrifugal casting machine. The properties of Al-7Si-Mg alloys were strongly dependent on the casting process used. Slow cooling of Al-7Si-Mg alloys due to the highly insulating nature of plaster based molds in flask investment casting tend to magnify solidification related problems like some internal porosity and low mechanical properties. The solidification must be controlled carefully to obtain good mechanical properties. Some elements were added was carried out to control grain structure and eutectic structure. Molten metal treatment to reduce hydrogen gas content and to remove inclusions. Vacuum-assisted pouring was used for evacuating the air from the mold. Centrifuging was increased by the force of pouring molten metal into the mold by spinning the mold assembly. Before investigating the optimum casting conditions in centrifugal casting and casting vacuum-assisted molds, the ideal amounts of Sr, Ti and Mg additions were examined. Stronsium modifies the eutectic structure. Titanium with small amounts of boron refines the grain structure. Magnesium was the basis for strength and hardness development in heat treatable Al-Si alloys. The variation of the silicon content 6-7.5 within the limits of Al-7Si-Mg alloy does not appreciably change the mechanical properties after heat treatment. However, the change of morphplogy of eutectic silicon could have some effect on mechanical properties. Under normal cooling conditions, silicon particles were present as coarse flakes. These act as crack initiators and appreciably decrease mechanical properties. Small additions of stronsium (from 0.010% to 0.021%) refine the coarse silicon eutectic and transform the flakes into fibrous form which results in improved mechanical properties, i.e. an increase in tensile strength from 165 MPa to 185 MPa (as cast) and from 200 MPa to 237 MPa (after the heat treatment). However further additions of Sr (0.021% to 0.083%) causes on undesirable intermetalic Sr compound to appear in the structure and increases porosity, resulting in a drop in tensile strength to 1 90 MPa (after heat treatment). The mechanical properties Al-7Si-Mg alloys were observed to be affected by the reduction in grain size and the accompanying increase in grain-boundry area. Reduction in grainsize was brought about by the efficent heterogenous nucleation of the a-aluminum phase. This can be achieved through crystal multiplication using mechanical or additives which were used to provide the necessary nuclei. Despite vu much study over several decades; the mechanism of grain refinement was not copletely understood. Al-Ti-B master alloys, containing 5% Ti and 1% B were added to A1-7Sİ- Mgalloy for refining the grains within the range of 0.019% - 0.14% Ti. It was found that grain refinement takes place in the existence of TİB2 in the molten aluminum and under the peritectic point aluminum-TiAİ3The strength continued to increase as titanium increase to 0.095% and the best tensile strength was observed at 0.095% Ti as 240 MPa. The effect of magnesium content from 0.12% to 0.81% in A1-7Sİ alloys has been investigated. The development of Mg2Si phase in this alloy was the basis for strength and hardness. It was probably due to the finer precipitation of Mg2Si, resulting from super saturation of the solute increasing with higher magnesium content. Tensile strength of this alloy increased with increasing Mg content and reached a maximum of 230 MPa at 0.45%. The maximum Mg level in Al-7Si-Mg alloys was 0.45%. It was very hard to keep the Mg tolerances within the strict limits, since Mg has a very high oxidation tendency. Some mechanical properties of this alloy started to worsen when the Mg content exceeds 0.45%. As a result, the optimum Mg content of this alloy was chosen to be 0.30% - 0.35%. Tensile strength value of 214 MPa was obtained at 0.35%) Mg level. Al-7Si-Mg alloy which was grain refined with 0.095% Ti and its eutectic structures was modified with 0.02% Sr to exhibit the best mechanical properties from the view point of chemical additives after the investigations in casting solid investment molds. Later, the effects of casting conditions on this alloy were investigated in the solid investment molds by two different pouring methods, centrifugal casting into the mold and casting into the vacuum-assisted mold. Cenrifugal casting was a pressure casting method in which the force of gravity for pouring molten metal into the mold was increased by spinning the mold assembly. The centrifugal equipment was the primary part of the system. In pouring vacuum- assisted mold, a vacuum-assisted pump was used to evacuate the air from the mold ahead of the stream of molten metal. Thus, there was less resistance to metal flow, and filling was more likely to be complete. The existence of gas in aluminum can be a major problem. Virtually all gas porosity observed in aluminum casting can be attributed to hydrogen and it was well observed the solubility limit of hyrogen was a function of temperature. As solidification progress and hyrogen was rejected at the solid-liquid interface, hydrogen gas bubbles will form. If these bubbles can not escape, porosity will be present in the solidified casting. The most common method for removing hydrogen from aluminum was inert gas flushing. Argon was one of the non-reactive purge gases. Hydrogen from the melt will then diffuse into an argon bubble and be carried away. The effects of argon degassing time and mold temperature on tensile properties of Al-7Si-Mg alloy in solid investment mold by centifuging casting wax examined. Degassing time was 1, 2 and 3 minutes with llt/min. flow rate and mold temperature changed as 25°C, 110°Cand300°C. V11I It was found that the best mechanical properties in 2 minutes degassing time and 110°C mold temperature as cast tensile strength 178 MPa and heat treatment tensile strength 246 MPa. Minimum mechanical properties also were found at 3 minutes degassing time and 300°C mold temperature as cast tensile strength 152 MPa and heat treatment tensile strength 198 MPa. The optimum properties were found as 2 minutes degassing time in all mold temperatures. For 2 minutes degassing time, the tensile strength was found to be 178 MPa (as-cast condition) and 246 MPa (heat-treated conditiion) at the casting mold tenperature of 110°C while 166 MPa (cast-condition) and 217 MPa (heat-treated condition) at the casting temperature of 300°C mold. The hardness of Al-7Si-Mg alloy by centrifugal casting were changed from 66 to 76 VSD as-cast conditions and from 96 to 110 VSD as-heat-treated conditions, depending on the mold temperature and degassing time. Dendirite arm spacing were observed to change from 30um to 35um when the mold temperature increased from 25°C to 300°C. The effects of the casting conditions pouring in the vacuum-assisted mold were investigated between 25°C to 300°C mold temperatures by only vacuum- assisted or vacuum-assisted with vibration. The molds were vibrated by 80 Hz and two different amplitude (25um and 50um). The best mechanical properties were found on the alloys exposed on 50um amplitude vibration with vacuum-assisted mold castind at 110°C mold temperature. The mechanical properties obtained were 171 MPa strength and 80 VSD hardness as-cast conditions, 240 MPa strength and 118 VSD hardness as-heat-treated conditions. DAS affected by the vibration in addition to the mold temperature. DAS decreased and microstructure became finer when vibration used and the amplitude increased from 25um to 50um. DAS was determined to vary from 32um - 35um in vacuum-assisted assisted mold casting, from 27 um - 31um when 25 um amplitude vibration was used from 22um - 28um when 50pm amplitude vibration was used when the mold temperature change from 25°C to 300°C. Casting in the vacuum-assisted molds were not affected very much by the change in mold temperature. However if the vibration was used the mold temperature becomes an important parameter. Tensile strength and hardness were 217 MPa - 103 VSD on the alloys cast in the only vacuum-assisted molds at the 1 10°C mold temperature which was the best mold temperature for mechanical properties. Tensile strength and hardness increased to 231 MPa - 1 1 1VSD by the use of vibration 25 um amplitude and to 240 MPa - 1 18VSD to the use of vibration 50 um amplitude as result of decreased DAS and reduce shrinkage porosities with vibration. On the other hand tensile strength and hardness changed from 210 MPa - 98 VSD with to use of vibration 25 um amplitude to 193 MPa - 100 VSD and with to use of vibration 50 um amplitude to 187 MPa - 106 VSD at the 300°C mold temperature. As a result of an increase in the volume of porosity, although DAS continues to decrease. IX The To treatment gives the optimum balance of strength and ductility and was most commonly employed with Al-7Si-Mg alloy casting. The heat treatment consists of solutionizing at certain temperatures close to the eutectic temperature, quenching and a combination of natural and artificial aging. The purpose of solution heat treatment of cast Al-7Si-Mg alloys was to obtain a maximum concentration of the age hardening constituent (Mg2Si) in solid solution, to homogenize the casting and to change the structural characteristics of the eutectic Si particles. The precipitation of very fine p (Mg2Si) phase during aging leads to pronounced improvements in strength properties. Both aging time and temperature determine the final properties. 0.20% amounts of Sr added to the melt to chemically modify the morphology of Si particles by neutralizes and promotes a fibrous silicon eutectic by retarding the growth rate of silicon. But after heat treatment spheroidized silicon structure was mainly obtained, also to coarsening of Si particles. Chemical and thermal modification were being used in conjuction to produce the desired properties of the casting. Tensile strength was increased from 178 MPa to 246 MPa and hardness was increased from 66 VSD to 105 VSD after heat treatment in optimum conditions of centrifugal casting. Tensile strength was increased from 171 MPa to 240 MPa and hardness was increased from 80 VSD to 118 VSD after heat treatment in optimum conditions of vacuum-assisted-assist casting. The fracture surface of tensile test specimen was investigated. The crack propagation occurs throught the interdendiritic regions. Scaning electron fractographs of samples indicated that the fracture surface consisted entirely of coalesced void or "dimles" and cellular region that was found to be characteristic of the fracture surface. The fracture observed under overload revealed that the crack initiation at the brittle Si particles, propagation of cracks in the interdendiritic regions and, rupture of matrix. The fracture mode changes from brittle to ductile deneding on the metallografic structure in castings. The best mechanical properties were obtained for two minutes degassing time and at 1 1 0°C mold temperature castings. Although during the casting into the vacuum-assisted-assisted molds the gas porosity eliminated, mechanical properties achieved were lower than the ones of centrifuging casting because of the existence of micro-porosity. The mechanical properties further improved to the levels of centrifuging casting by applying 80 Hz and 50 ujti amplitude vibtation to vacuum-assisted mold. The highest tensile strength obtained was 246 Mpa in the centrifuging casting, which is higher than sand casting and close to the metal casting values of Al-7Si-Mg alloys. The highest hardness value was 118 VSD with the vibration of 80 Hz and 50 |xm amplitude, casting into the vacuum-assisted mold.