Yüksek Mukavemetli (2024,6061,7075) Alüminyum Alaşimlarinin Jomminy Yöntemi İle Suverme Duyarliliği

Abstract

Bu çalışmada uçak sanayii ve savunma sanayiinde yaygın bir şekilde kullanılan yüksek mukavemetli 2024, 6061 ve 7075 Al-alaşımlarının suverme duyarlılığı araştırılmıştır. Bu amaçla standartlara uygun Jomminy numuneleri her üç Al-alaşımından da hazırlanarak uygulamada kullanılan sıcaklıklarda çözeltiye alma işlemine tabi tutulduktan sonra Jomminy testi uygulanmış ve takiben uygun sıcaklık (2024 490°C- 0.5 saat, 6061 565°C-0.5 saat ve 7075 465°C-0.5 saat) ve sürelerde (2024 doğal yaşlandırma: oda sıcaklığında 5 gün, 6061 175°C-8 saat ve 7075 135°C-13 saat) yaşlandırılmıştır. Çözeltiye alınmış ve yaşlandırılmış şartlardaki Jonıminy numunelerinde suverme ucundan itibaren 7mm aralıklarda sertlik (HV) ölçümleri yapılmıştır. Kritik noktalan standartlardaki sertlik değerlerinden tesbit edilmiştir. Jomminy numunelerinin değişik noktalarından alman numunelerin mikroyapılan optik ve taramalı elektron mikroskobu ile incelenmiştir. Yukarıda belirtilen her bir Al-alaşımından çekme numuneleri hazırlanarak çözeltiye alma, değişik ortamlarda soğutma (suverme) ve yaşlandırma sonrası test edilerek çekme özellikleri incelenmiştir. Deneysel çalışmalar neticesinde elde edilen sonuçlara göre 7075 Al-alaşımının maksimum, 2024 Al-alaşımının orta seviyede ve 6061 Al-alaşımının ise bu üç alaşım arasında en az suverme duyarlılığına sahip olduğu gözlenmiştir. Mikroyapısal gözlemler ile Jomminy test sonuçlarının da uyum içerisinde olduğu tesbit edilmiştir.

Although composite materials have opened a new dimension in the aerospace structure industry, aluminium alloys are still considerable potential materials being used in the airframe of aerospace structures. Most of the duminium alloys used in the aerospace industry are strengthen by heat treatment. These alloys are well known as heat treatable alloys and show high strength. Strengthening of these alloys using heat treatment mainly based on ageing of the alloy after solution treatment and a rapid quenching which introduce super saturated of solid elements and high vacancies, that gives increased strength by lattice distortion and helps ageing process. Response of strengthening by heat treatment of Al-alloys well studied by different investigaters and published in many literature [1"19]. However, applications of Jomminy test is very limited191 and not well studied in Al-alloys as in steel. In this study, hardness (quench sensitive) of three high strength Al-alloys (2024-6061 and 7075) has been investigated using mainly Jomminy test. Hardness and tensile data were also collected. Also, in order to compare the results with standard quenching techniques, quenching of many separate specimens in different media (water, air, fan-air and furnace) have been done. Alloys with composition given in Table's 1 were sampled in accordance with Jomminy specimen rules and the parts shown in Figure's 1 have been heat treated according to the conditions stated in Table's 2. Table's 1. Alloys composition used in this study. C=70mm v,^=25111111 a=125m r=25mm 20r (a) 20mm (b) VI K. h =20mm ^ do=10mm -J. Lv=50mm Lt= 110mm Lo=60mm dfl2mm (c) Figure's 1. The samples and sizes used in the analysis. a) Jomminy sample b) Part specimen c) Tensile specimen. Table's 2. Heat treatment data of the alloys. Examinations of the samples have been carried out on the as quenched specimen using hardness test (conducted on the Jomminy samples starting from the quenched end with 7mm intervals), tensile test and metallographical studies (by optic and scanning electron microscopy). Similar investigation continued on the aged samples and the results are given below. Hardness values obtained on the Jomminy samples as in the quenched alloys (2024, 6061 and 7075) were plotted in the combined Figure's 2. Vll o CO oo CM § CO CM X(mm) Figure's 2. Hardness changes with cooling rate on Jomminy sample. As may seen from the figure, alloys have shown increased hardness at the quenched end in comparison to the air cooled end. Amount of this increase is higher in the alloys 2024 and 7075 than 6061. Hardness of the as quenched alloys can be attributed to two reasons. One of these is the introduction of more solute element in the matrix by solution treatment and quenching of the sample. This treatment temperature produced highly saturated solid solution depending on the cooling rate. Other effect is the increasing of vacancy concentrations in the sample by solution heat treatment (Figure's 3). Hardness results obtained from the aged jomminy samples (2024 natural aged, 6061 and 7075 artificially aged) have been drawn depending on cooling rate in Figure 4. It shows that all alloys indicated increase in hardness. The increase upon ageing is higher in 7075 alloy and secondly, in 2024 alloy. As can be seen from the figure, cooling rate is more effective on the hardness and thus the water quenched Jomminy end shows the highest hardness. Increases in hardness of 6061 is greatly influenced by ageing but the value of hardness on the water quenched and air quenched sides are not so far. Vlll 700 600 o 500 400 & 300 E H 200 100 0 IO-ıo 10 -8 I0"6 I0"4 I0"2 Vacancy concentration 1200 1000 800 600 400 = 200 10° Figure's 3. Equilibrium concentration of vacancies in pure aluminum as a function of temperature. 140 120 r*-*^ £ 100 CD ^£. O. X Figure's 4. Hardness results obtained from aged Jomminy samples. 7075 alloys shows standard T6 hardness value at 7 mm from the water quenched end (7075-T6=157 Hv) in the aged conditions. Alloy 2024 indicates the standard value of hardness at 21 mm from the quenched end (2024-T4=126 Hv). However 6061 alloy (6061-T6=95 Hv) presents the standard value almost all over the specimen. IX From the above results this can easily said that alloy 7075 shows rapid hardness and high quench sensitivity. This was followed by 2024 alloy. Although alloy 6061 showed increased hardness after ageing, this cannot be considered as quench sensitive due to the value of the hardness being almost equal on all over the samples regardless of cooling rate. Increases in the value of hardness after ageing can be attributed to the ageing characteristic of alloys. The particles of MgZn2 are being the most rapid precipitaters (5-10 seconds) in the 7075 and was followed by Q12AI particles in 2024 alloy. Mg2Si another particles which precipitates in 30 seconds in the alloy of 6061. All ageing particles have hardening effect on the alloy depending on the precipitation rate, size and distribution. A separate study with parts specimen given in Figure 5 which was carried out in different cooling situations has confirmed above results and discussions. 2 1 CO CD co 10000 1000 100 Cooling rate Figure's 5. Hardness value with cooling rate of parts specimen. Mechanical properties of both quenched and aged specimens were as expected. That elongation was dropped with increasing of cooling rate (Figure's 6 a,b). However, the yield (Figure's 7) and tensile strength (Figure's 8) show increased value with increasing quenching rate and ageing. Microstructural studies of the alloys are shown in Figure's 9 and it indicates suitable structure as in the literature. 100 Cooling rate 10000 (a) 100 Cooling rate 10000 (b) Figure's 6. Elongation changes of the alloys with cooling rate, a) As quenched alloys b) As aged alloys. XI 100 1000 Cooling rate 10000 Figure's 7. Yield strength changes of the alloys with cooling rate. 10000 Figure's 8. Tensile strength changes of the alloys with cooling rate. XII (c) Figure's 9. SEM micrographs of 2024 (a), 6061 (b), 7075 (c) alloys.