7075 alüminyum alaşımında basamaklı yaşlandırma ile özelliklerini kontrol imkanları
Possibilities of controlling the properties of 7075 aluminium alloy with step aging process
- Tez No: 19302
- Danışmanlar: PROF.DR. AHMET ARAN
- Tez Türü: Yüksek Lisans
- Konular: Makine Mühendisliği, Mechanical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1991
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 90
Özet
ÖZET Yüksek dayanımı ve buna karşılık düşük yoğunluğu nedeniyle özellikle havacılık endüstrisinde yaygın olarak kullanılan 7075 alüminyum alaşımına dayanım ve gerilmeli korozyon direnci özellikleri bakımından bir optimizasyon sağlayacak ısıl işlemin bulunması araştırmacıların dikkatini çeken bir konu olmuştur. 7075 alüminyum alaşımına, maksimum dayanım özelliklerini sağlamak için T6 ısıl işlemi uygulanmaktadır. Ancak bu durumda malzemenin gerilmeli korozyon direnci düşmektedir. Gerilmeli korozyon diren cini artırmak için uygulanan T73 aşırı yaşlandırma ısıl işlemi ise mekanik dayanımda %15'e varan düşüşlere sebep olmaktadır. Belirtilen bu iki işlemin sağladığı avantajları tek bir işlemle aynı anda malzemeye kazandırmak için çeşitli, kademeli yaşlandırma işlemleri geliştirilmiştir. Bunlardan en yaygın kullanılanı retrograsyon ve yeniden yaşlandırma (RRA-Retrogression and Reaging) ısıl işlemidir. Ancak bu işlem esnasındaki karışık çökelme tepkimeleri ve bu tepkimeleri kontrol eden parametrelere henüz tam olarak hakim olunamadığından işlem koşulları tam belirgin değildir. Bu nedenle işlemin çeşitli bileşimlerdeki ve boyutlardaki ürünlere uygulanabilirliği düşüktür. Yapılan bu deneysel çalışmada çökelti yapısının korozyon ve mekanik dayanıma etkisi incelenmiş, ve tasarlanan deney metodu ile dayanım ve korozyon diren cinin optimizasyonu için değişik bir yöntem bulunması amaçlanmıştır. Bu doğrultuda biri çözündürme işleminden hemen sonra çeşitli sıcaklıklarda doğrudan, biri de bu işlemden sonra malzemeye su verilerek orjinal T6 koşullarına olmak üzere iki basamaklı bir yaşlandırma işlemi uygulanmış ve böylece çökelmenin kontrolüne çalışılmıştır. Ayrıca elde edilen verilerden, kullanılan malzemenin çökelme yapısı hakkında bir sonuca ulaşılmıştır. -v-
Özet (Çeviri)
POSSIBILITIES OF CONTROLLING THE PROPERTIES OF 7075 ALUMINIUM ALLOY WITH STEP AGING PROCESS SUMMARY Aluminium alloys have been used in increasing amounts in the construction of airframes of both civil and military aircraft since World War II. Considerable improvements have been achieved during these four decades both in terms of engineering properties and, possibly more importantly, in terms of understanding the limitations that restrict their use. Since aluminium alloys are predominantly used in virtually all aircraft structures, improvements in their performance are per haps of more benefit to the aircraft manufacturer than to the aluminium industry. The advantageous characteristics of aluminium are partly inherent in the material, and are partly the result of extensive research and development work in in dustry laboratories. One of it is chief advantages is lightness; others include resistance to corrosion, non toxicity, workability (it can be fabricated by more dif ferent methods than any other metal), high strength- weight ratio in alloys, attractive apperance, suscep tibility to an almost infinite range of finishes, high electrical conductivity, high conductivity for heat, ef ficient reflection of light and heat, nonmagnetisra and availabilty in a wide range of basic forms. In addition to having favorable properties, aluminium has ex perienced a remarkably stable price history from the time it was first made available at a price to make volume use possible. Since aluminium alloys are currently the predominant choice in the limited selection of aircraft structural materials avaliable, their use is hardly hindered by technical difficulties, and there is little incentive for performance improvement, especially if any extra risk or extra cost were to be introduced in the implementation of the development. It can be seen from Fig.l that the distribution of material in past military and civil airframes has favoured aluminium alloys, but that new designs of the 1990s could well include a sig nificant use of composite material. -VI-V) 100 80. 60 40 (a) S 20 h- < 2 100' 80- JfflHOM k/y/yy-r- ^te < er 3 (b) u E? 60 to 40 20 0 _^^a 1960 V- a: civil Figure 1. Approximate distributions of materials used in existing civil and military aircraft and those predicted to be selected for new aircraft in the 1990s. The potentia metallic systems, titanium alloys, further potetial fo the disadvantages risks involved, materials are cons those areas in whic sidered deficient, able in a complete applications, in will allow. With material costs are 1 competition f including supe has led to rene r improvement of of cost and the The advantage idered in order h aluminium al Aluminium all range of product a large size of conventional al low. rom new composite and rplastic forming of wed evaluation of the aluminium, despite increased technical s of the competing to be able to analyse loys could be con- oys need to be avail- forms and, for many product as equipment uminum alloys basic The technical materials has taken pla at improving the safety of the high costs invo and manufacturing techn been directed towards corrosion, or improving towards improvements could lead to a reducti development of aluminium base ce in small steps, usually aimed aspect of the materials, because lved in introducing new materials ologies. Thus, development has enhancing resistance to stress fracture toughness, rather than in strength or stiffness, which on in structural mass. -vii-Features that limit the structural efficiency of aluminium alloys occur with problems such as stress cor rosion cracking. It has long been understood that very high strengths may be obtained from conventional aluminium alloys, particularly in the 7000 series (AlZnMg), but that these high strength levels tend to be associated with low levels of resistance to stress cor rosion cracking. Developments in the heat treatment practices of the high strength alloys such as the double aging treat ments have resulted in a better balance between strength and resistance to stress corrosion and its as sociated problems but with a strength level reduced to no more than 500 MNm-2 (0.2 % proof stress). Recently, progress has been achieved in the development of rapid high-temperature aging treatments that impart high stress corrosion resistance with little loss in strength to the selected 7000 series alloys (especially 7075 alloy). The applicability of this tecnique of retrogressive aging to a range of alloys and products needs to be evaluated. But it does appear possible to produce 7000 series alloys capable of operating at very high strength levels with a minimal risk of stress cor rosion cracking. 7000 series aluminium alloys are strengthened by precipitation hardening. The major alloying elements of aluminium are cop per, magnesium, zinc and silicon. Copper increases the precipitation hardenability of aluminium alloys with ad dition of 1-10%. Because of the decreasing solubility of copper in aluminium matrix with decreasing temperature, precipita tion hardening conditions are provided. Therefore the precipitation hardening was first found in the AlCu al loys. Precipitation hardening is consist of three steps. The first is solution heat treating, the second is quenching and the third is ageing. -viii-The purpose of the solution heat treatment is to obtain in solid solution the maximum practical con centration of the hardening solutes such as magnesium or zinc. It is known that the solubilities of the these elements increase markedly with temperature, especially just below the eutectic melting temperature. Further more, the rate of solution increases with temperature, because of increased diffusion rate. Consequently, the most favorable temperature for effecting maximum solu tion is very near that which melting occurs. Actual melting must be avoided, however, since it produces in- tergranular networks of nonductile eutectic. If exten sive, there may decrease both strength and ductility. In addition quench cracking is sometimes encountered when the normal solution temperature is exceeded. Quenching is in many way the most critical step in the sequence of heat treating operations. The objective of quenching is to preserve as nearly intact as possible the solid solution formed at the solution heat treating temperature by rapidly cooling to some lower tem perature, usually near room temperature. From the preceding general discussion, it is evident that this statement applies not only to retaining solute atoms in solution but also to maintaining a certain minimum num ber of vacant lattice sites to assist in promoting the low-temperature diffusion required for zone formation. The solute atoms that diffuse to grain boundaries as well as the vacancies that migrate (with extreme rapidity) to disordered regions, are irretrievably lost for practical purposes and fail to contribute to the subsequent strengthening. During aging of Al-Zn-Mg-Cu alloys, the develop ment of spherical GP zones occur. These zones have their maximum strengthening effect when they are ex tremely small, and they lose their effectiveness rapidly as they increase in size above about 30 A0. Precipitate structures are highly significant, be cause they frequently indicate the metallurgical condi tion of the alloy, its mechanical characteristics, and corrosion behaviour. The precipitates are fine par ticles of phases that were in solution in aluminium at an elevated temperature and precipitated from solid solution at a lower temperature. Precipitates form at grain of fragment boundaries, dislocations and vacancies. Their size, shape, and location depend on the thermal conditions leading to their formation. -IX-During quenching after solution heat treatment, dislocations and precipitates develop. Dislocations generally appear as helices, loops or tangles generated as a result of quenching strains and the condensation of excess vacancies. Their number depends on solute con centration, which controls vacancy concentration to a large extent. Precipitate particles are not dedected in rapidly quenched material but are found in increasing numbers as the cooling rate decreases. They are first seen as ex tremely fine particles at grain boundaries. At slower cooling rates, the particles increase in size and num ber. In addition to the precipitate structures, solute -depleted regions adjacent to grain boundaries develop. Sequence of precipitation in 7000 series aluminium alloys is; Supersaturated Solid Solution > GP zone (coherent)-- - *V(semicoherent) » n. (MgZn2> > T The highest strength for 7000 series alloys is ob tained by the T6 temper, but the high corrosion suscep tibility in this condition can limit the applicability of the alloy in many cases. The corrosion resistance can be increased using overaged tempers such as T73, but these increases are mostly accompanied by strength decreases. In 1974, a treatment which is known as“retrogression and reaging -RRA”has been proposed by Cina to obtain the T6 strength together with the T73 SCC- resistance. This treatment is applied to the material in the T6 condition. Because of the unknown parameters and complex precipitation process, this (RRA) heat treatment can't be applied effectively. Therefore researches are con centrated on more practical and applicable step aging treatments. The purpose of this work is to research a more practical and applicable heat treatment which optimizes the strength and stress corrosion resistance of 7075 al loy by controlling the precipitation process and precipitate microstructure. This precipitation microstructure determine the properties of alloy. For this purpose a double aging heat treatment was used -x-which involves a direct quenching to the first aging temperature after the solution heat treatment and then quenching the material to the room temperature, after that second aging which was originally T6 (at 120°C3 24 hours aging) heat treatment. The conclusion arrived with these process is as follows. There is three types of nucleation and precipitation during direct aging; when the direct aging temperature is lower than 160°C the coherent GP zones formed and they serve as nuclei to precipitates. Be cause of this fine particles the hardness is increased highly. If the temperature higher than 200°C, non coherent and coarse (1)(MgZn2) phase occurs and it has not contribution to strength but the corrosion resis tance increases. If the temperature is between 160- 200°C, partly coherent (1') is the main costituent with finer distribution and these transform to noncoherent particles with increasing aging time. -xi-
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