Yaşlandırma ile sertleştirilebilen magnezyum-alüminyum alaşımlarının özelliklerinin geliştirilmesi

Abstract

Mukavemet/yoğunluk oranı yüksek olan magnezyum alüminyum alaşımlarının katılaşma ve yaşlanma özelliklerinin incelenmesine esas teşkil etmek üzere magnezyum alaşımlarının ergitilmesi ve dökümü, mekanik özellikleri, kullanım alanlarına değinilmiştir. Çökelme sertleşmesi mekanizmaları açıklanmıştır. % 6'dan % 12'ye kadar alüminyum içeren magnezyum alaşımlarının döküm yapısının dendritik olduğu görülmüştür. Bu alaşımlar 420 °C'ta 22 saat homojenleştirme işleminden sonra 150-300°C sıcaklıkları arasında değişik sürelerde yaşlandırma işlemlerine tabi tutulmuşlardır. İşlemler sonucunda, magnezyum alaşımlarının hem sürekli hem süreksiz çökelme gösterdiği görülmüştür. Alüminyum içeriği ve yaşlanma süresinin artmasıyla süreksiz çökelme oranının arttığı gözlenmiştir. Yaşlandırma işlemleri sonucunda yapılan X-ışınları çalışmalarında sertliğin artmasını sağlayan çökelti fazının Mg17Al 12, olduğu tesbit edilmiştir. 150-300°C sıcaklıkları arasında yapılan yaşlandırma işlemleri sonunda en iyi sertlik değerleri 200°C'taki yaşlandırma işleminde elde edilmiştir. 'Ayrıca magnezyum alaşımlarında alüminyum içeriğinin artmasıyla sertlikte bir artış gözlenmiştir.

Magnesium-aluminium allays are characterised by goad castability, excellent machinability and good hat formability. In addition of this properties the high strength ta weight ratios of casting and stifness of weight ratios of the wrought alloys make these alloys an attractive choice for many engineering applications. Magnesium-aluminium alloys are attracted attention on the basis far age hardening properties, the literature contains many works on age hardening phenomena of the alloys.. First investigation on this phenomena have been carried out by Meissner, Archer, Gann, and Schmid and Siebel. The correlation between precipitation and hardening has been shown by Talbot and Norton, who studied an alloy with 9.6 per cent Al. Metallographic investi gation of magnesium-aluminium allays aged at various temperature for a variety of period have been carried out by Fax and Lardner, and Frebel, Behler and Predel. The precipitation process and age hardening mechanism in magnesium-aluminium alloys have been studied by Clark. The mechanical properties af magnesium-aluminium alloys are improved by age hardening phenomena; having hold at ^20 C has the effect of taking most of aluminium into solid solution if the alloy is then heated at tem peratures in the range 150 to 250 C for several hours, precipitation occurs, and the mechanical properties are improved, as a result of the precipitation, the hardness increases. It can be clearly seen from the farm af the solubility curve far aluminium in magnesium, all allays containing more than 2 per cent of aluminium would exhibit precipitation hardening, but in practice, the small increase in hardness were observed in magnesium- VI aluminium allays having aluminium in the range of k per cent to 6 per cent Al. Far this reason, practial allays rarely contain less than 6 per cent of aluminium. In magnesium-alumi nuous) and cellular (di at various age hardenin precipitation shows a Id cally oriented structur discontinuous precipita starts at grain baundar alloys cellular precipi process as in other sol simultaneously with the wide temperature range. nium allays, scantinuaus) g temperature idmanstatten e in its over tion shows a ies. In magn tation is not id solution s general prec both general (conti- precipitation occur s. The continuous or crystallographi- aged conditions, the structure which esium-aluminium a low temperature ystems, but occurs ipitation over a It has been reported by most of the investigator that during ageing phenomena in magnesium-aluminium alloys, the only precipitate is nan coherent equilibrium Mg.-Al.p phase. IMa zones or intermediate precipitates have been reported. The tensile strength of magnesium-al were investigated by Fax.. It has been r function of aluminium, in the salutiontre ultimate tensile strength values up to 2k an elongation of 1G %, can be obtained fo taining 10 % wt aluminium, but the 0.2 % is rather law, 7 kg/mm2. In the fully he tian the ultimate tensile strength values to be unchanged, an the other hand 0.2 % increased to 10 kg/mm2, and the ductility reduced. Uminium alloys eported as a ated condition kg/mm2 and r an alloy con- praff stress at-treted condi- are observed proof stress is is dramatically Orientation relationships between the precipitate types and matrix in magnesium-aluminium alloys have been determined by Crawley and Milliken. They have found that, in continuous precipitation the orientation rela tionship is of the type [ill]. // [2ÎÎG].. and [Oil] // [000l]m x. InppSiscontinuous precipita tion, "precipitates nave the same type of orientation relationship with the matrix as the continuous precipi tate. Vll In the present investigation magnesium-aluminium alloys were manufactured in centrifugal investment casting machine by using lost wax technique. Firstly a wax pattern of casting part is obtained in a wax injection machine. The pattern is cleaned and polished by torch, and the pattern assembled. The pattern assemble then is placed into the stainless steel flask Second step is to farm investment mould. In order, to obtain investment mould, a special mixture of gypsum is added to water then mixed 30 second manually < afterwards 3 minutes by mixer. Before the gypsum is set, it is put into vacuum chamber for a period of 150 seconds in order to eliminate air bubbles and poured into the flask which contains pattern assembls and the flask containing pattern assemble toghether with plaster mould evacuated for one minute. Prepared plaster mould then is subjected to dewaxing treatment for a period of two hours at 150 C temperature and firing treatment for a period of three hours at 730 C temperature, thus mould cavity is obtained Meantime, centrifugal casting machine is prepared for casting, by starting to heat the empty crucible in which the alloy is going to melt. LJhen the temperature of crucible is reached to 500 C, small pieces of magnesium and aluminium is charged into the crucible. Then the system is heated up all the charge is melted. During melting argon gases introduces into the system because of oxidation tendency of magnesium alloys. When melting is completed the flask is taken from the dewaxing furnace and is placed into centrifugal casting machine. Then machine is started to turn and melted metal is forced to fill into mould cavity by means of applied centrifugal force. Finally the plaster mould is taken out from the system and it is broken in order to get the casting part out of plaster mould then the gating system is cut of. Thus casting part is obtained which has dimension of 100x90x4 mm. In present investigation the magnesium alloys with aluminium content from 6% wt to 1 2 % wt. was selected for study because these alloys is fairly easy to homo genize after centrifugal casting and yet offers a wide temperature range for precipitation. Vlll The precipitation behaviour of magnesium-aluminium alloys containing 6 tut % Al to 1 2 wt % Al was investiga ted by metallo graphic and X ray method, and hardness measurements. Specimens were solution annealed for 22 hours at o 420 C temperature in a resistance furnace and quenched in water. Age hardening treatments at 150,200,250 and 300 C were conducted in resistance furnaces for a period of 1 hour to 100 hour and terminated by air cooling Both solution annealing and age hardening treatments, specimens put into graphite crucible containing fine graphite powder to prevent oxidation. Hardness was measured as a function of; aging time on vickers hardness tester by using a load of 10 kg. Five readings were taken on samples for each aging condition. The microstructure of the aged specimens were examined in an optical microscope after mounting, polishing and etching by using nital reagent. The precipitation phase was determined in a Philips X-Ray dif f actometer operating at kQ k\J. JE0L 3304 model Scanning electron microscope was used in order to determine the variation of aluminium content in the structure. The microstructure of casting magnesium aluminium alloys containing 6 wt% to 12 wt % Al was shown in fig. 8.1 and fig. 8. 2 the microstructure of as cast condition had dendtric structure. It was also seen that the dendtric microsture was decreased as the temperature of the mould decreased. The microstructure examination showed that continuous and discontinuous precipitation were both observed in age hardened magnesium-aluminium alloys. The microstructure of continuous and discontinuous precipitation was shown in fig. 8.4. It was observed that the longer aging times and the higher aluminium c ontents lead to the higher the amount of discon tinuous precipitation. And a mosaic-like type of precipitate occured at the higher aluminium content and the longer aging times. This type of microstruc- tures were shown in fig. 8.7 and fig 8.8. IX The precipitation processes in magnesium aluminium alloys have been observed by the formation of Mg1 7A1. " plates of equilibrium phase without the intervention of transition phase of GP zones by Xray investigation. The amount of precipitate phase increased as a function of aluminium content and annealing time. No intermediate phase uas observed in this investigation. The age hardening curves for Mg-6wt % Al to Mg-11.5 wt% Al allays at 150°C were shown in fig 8.11 An appreciable increase of hardness were obtained alloys containing above 8 w t % Al. as shown in fig. 8.11. At aging treatment at 200 C, hardness of alloy was observed to increase appreciable more than aging at 150 C. These could be clearly seen in fig 8.12 No overaging effect were observed at 1 50aC and 200°C aging temperature within the test periods. Hardness curves for 250 C aging temperature showed that hardness results for 250 C aging treatment was slightly lower than 200 C aging as shown in fig 8.13. At this aging tmeperature 11.5 wt % Al alloy was found to incease in hardness to a peak value after 3 hr, and then to decrease in hardness up to 100 hour. Thus overaging effect was observed at this alloy. The age hardening curves far 300 C aging temperature showed that there were an appreciable increase of hardness below 9 wt% Al as shown in fig. 8.14. The initial rate of hardening was higher at this aging temperature. The general conclusions which may be drawn from the present investigation are that the rate of hardening on ageing at a given temperature increases as the aluminium content increases, the highest hardness values are determined at 200 C aging temperature. General analysis and chemical point analysis on a straight line was determined by. scanning electron microscope. The difference between maximum and mini mum aluminium solute content of magnesium-aluminium alloys containing from 6 wt % Al up to 1 2 wt % Al was shown to have two minimum values as seen in fig. 8.15.