SCI SINTERING 57 1 2025 06pdf
SCI SINTERING 57 1 2025 06pdf
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  1. Science of Sintering, 57 (2025) 69-85________________________________________________________________________ _____________________________ *) Corresponding author:gatlihan@pau.edu.tr https://doi.org/10.2298/SOS240605025AUDK: 539.375; 669.018; 546.62 An Investigation of the Effect of Cryo-Cooling on Natural Frequency and Wear Properties of Ageable Al Alloys Rıdvan Arslan1, Gökmen Atlihan2*), Sinan Aksöz11Department of Metallurgy and Materials Engineering, Technology Faculty, Pamukkale University, Kınıklı Campus, 20160 Denizli, Turkey. 2Department of Mechanical Engineering, Technology Faculty, Pamukkale University, Kınıklı Campus, 20160 Denizli, Turkey.Abstract: In this study, the influence of the cryogenic cooling on the natural frequency and wear behaviors of the three different aluminum alloys of AA2024, AA6061 and AA7075 were examined. In addition, hardness and microstructural observations were also performed to reveal the effect of cryogenic cooling on the wear performance of the Al alloys. Pin on disc test equipment was utilized to observe the wear performance of the Al alloys and field emission scanning electron microscope was used to go in depth about microstructural evaluations. For the determination of the natural frequency of the Al alloys designed withdifferent tapered angles from 0° to 1°, vibration test equipment was used. As a result of this work, it has been observed in experimental and analytical analysis that the natural frequency values decrease with the increase of the tapered angle of the fixed beam attached from theshort (b) side. The highest frequency values were obtained when the tapered angle was 0º in the experimental and analytical analysis of the fixed beam attached from the short (b) side. Again, the highest frequency values were obtained when the tapered angle of the fixed beam, which was attached from the short (b) side, was 1º in the experimental and analytical analysis. In addition, when the worn surfaces and wear data are examined in general, it is seen that the amount of oxide on the surface increases as a result of the applied cryo-cooling, and because of the increased oxide layer, metal-to-metal contact occurs during wear, and it acts as a solid lubricant by protecting the surface under the metal. In this way, it has been observed that the wear loss is reduced and the worn surface images are directly affected. The results of the analytical and experimental analyses of fixed-support beams indicate that the natural frequency values of the beam can be altered by modifying the tapered angle of thebeam and the location of the support. Lastly, no significant change was observed in natural frequency values with cryogenic effect. Keywords: T6 heat treatment; Ageable Al alloys; Cryo-cooling+Ageing; Analytical analysis; Wear properties. 1. Introduction In light of the advancements in technology, it has become imperative to transcend the conventional design and production services offered by numerous industrial sectors. This entails aligning their current offerings with consumer demands and necessities, which is a prerequisite for the advancement of technology [1]. In this context, numerous industrialsectors, including the aviation and space industry, transportation industry, defense industry,
  2. R. Arslan et al.,/Science of Sintering, 57(2025)69-85___________________________________________________________________________70and construction industry, continue to prioritize the development of designs and products that align with contemporary needs [2]. The prioritization of lightness, strength, cost, and high resistance to natural effects is evident in their designs [3,4]. In the field of materials, aluminum (Al) and its alloys, which are classified as light metals, have gained significant prominence in the industry since 1930 [5]. Al alloys are the most widely used material type after conventional steel and cast iron [6]. Their high specific strength makes aluminum and its alloys indispensable in today's and future technologies. In this regard, aluminum and its alloys have a broad range of applications, including in vehicle technology and space technology [7]. Aluminum and its alloys are classified into two categories: heat-treatable and non-heat-treatable. The dissolution of certain alloying elements (e.g., copper, zinc, and magnesium) in heat-treatable Al alloys during the heat treatment stages contributes to the improvement of the mechanical properties of the material. It is possible to enhance the mechanical properties of these alloys if desired. Consequently, the potential applications and the reasons for preferring Al and its alloys can be expanded. T6 heat treatment represents the pinnacle of these heat treatments, and it is evident that alternative and innovative technologies such as cryogenics have become pervasive in the present era. The process of cryogenic heat treatment is employed for the purpose of enhancing the mechanical properties of materials. Cryogenic heat treatments are applied at temperatures between -50°C and -190°C to improve the mechanical properties [8-10]. As industrial products undergo a process of lightening and shrinking, the resistance of the material in question to external natural effects becomes increasingly significant [11]. The mass change and natural frequency values are also affected by these effects. In the past, the effects of vibration on the system in designs with large masses were considered to be insignificant. However, in designs that are suitable for today's needs, mass changes make the effect of the material important despite the natural vibration effects to which it is exposed [12]. Materials are subjected to natural vibrations on a regular or irregular basis, depending on their intended use [13]. The natural vibration effects of aluminum alloys utilized in aircrafts or spacecrafts differ from those of aluminum alloys employed in machine or automobile fields. If the natural vibration value of the material coincides with the vibration value it is exposed to in the environment, it causes resonance frequencies. In this case, the material can vibrate at high amplitudes continuously, which can cause mechanical damage to the structure [14]. Another desirable feature of Al and its alloys is their resistance to wear. The wear resistance of aging Al alloys can be improved by heat treatments such as T6. Thanks to the applied aging and secondary processes such as cryogenics, their usage areas can be improved by improving their wear resistance [15-18]. A review of the literature in this field reveals that studies have primarily focused on improving the mechanical properties of aluminum and its alloys or examining the vibration properties. However, there has been a paucity of research investigating the vibration and cryogenic properties of aged aluminum alloys together. However, studies in which both features were examined simultaneously and improvements were made have not been observed [1,5,10,18]. The aviation structures on the planes are subjected to temperatures of approximately -75°C, which simultaneously affect their vibration and mechanical properties. Additionally, ship materials are subjected to approximately -75°C temperatures and prolonged periods in polar regions. In this study, the mechanical properties of the AA2024, AA606, and AA7075 Al alloys were enhanced through cryo-cooling (at -75°C for 168 hours) and T6 heat treatments. The effects of vibration at -75°C were also examined to gain insight into the cooling effects on the structure. Additionally, all samples were subjected to artificial aging. It was observed that these improvements did not directly affect the vibration properties. Nevertheless, the results of the analytical and experimental analyses of fixed-support beams indicate that the natural frequency values of the beam can be altered by modifying the tapered angle of the beam and the location of the support. In order to determine the effects of the applied heat treatment, an optical microscope, FESEM, EDS, X-ray, and microhardness data
  3. R. Arslan et al.,/Science of Sintering, 57(2025)69-85___________________________________________________________________________71were obtained. Upon examination of the test results, it was observed that cryo-cooling and aging heat treatments increased hardness and improved wear properties. However, no difference in vibration was observed. The experimental and analytical analyses of the built-in beams yielded results that were nearly identical. The discrepancy between the two results can be attributed to the analytical approach employed, which utilized the Euler-Bernoulli beam theory, and the omission of shear forces.2. Materials and Experimental Procedures 2.1 Experimental Hardness and Vibration Analysis Data In this study, AA2024, AA6061 and AA7075 alloys were used in the study, and given in Table I. All the alloys have aging applicable included. Tab. I Chemical compositon of the AA2024, AA6061, and AA7075 alloys. Content (wt.%) CuMnMgFeZnAA2024 3.8-4.9 0.3-0.9 1.2-1.8 0.5% - AA6061 0.6-1.1 0.2-0.8 0.8-1.2 0.5 - AA7075 1.2-2.0 0.3 2.1-2.9 0.5 5.1-6.1 In this study, AA2024, AA6061, and AA7075 Al plates with 2 mm wall thickness were produced after being designed in vibration test standards. The effects of the producedsamples, cryo-cooling (-75°), and aging heat treatment applied after the solution heat treatment on the natural vibration properties of the material and the effects of the change in the cross-sectional area that may occur in the material on the natural vibration values of the material were investigated. Cryo-cooling treatments (for -75ºC at 196 hours) were applied on the dry ice (dry ice can be dropped down to -75ºC) because they want to make the same condition on the aviation parts and surfaces. Standard sample dimensions were determined as a rectangle with L=200mm length, b=20mm width and 2mm thickness. In order to examine the effects of changes in the cross-sectional area of the material on the vibration value, depending on the initial dimensions, samples with different cross-sectional areas were designed with an increase of 0.1º in the range of h/L ratio θ between 0º and 1º. For each alloy, two different gauge series were created as a short side (b) and a long side (B). After dissolution at 510°C, all the samples obtained were subjected to cryo-cooling for 168 hours at -75°C in a dry ice environment. The obtained samples were then subjected to aging heat treatments at 160°C at 1-hour intervals (vibrations and hardness were taken at every hour interval), up to a total of 15 hours (Fig. 1. Graphical Abstract). The aging period was completed when the increase in hardness of the structure ceased and the decrease occurred (over-aging). For aging, the HV0.1 value was obtained in 45 seconds in the Hardway brand DV-1AT-4.3 Model hardness device after each hour of aging. For the optimization of thesamples, 7 hardness data were taken from each sample and the simple average values were given. After the samples were produced, the wear behavior of the pin-on wear analysis was examined with a ASTM G99 standard (Disk have 60HRC hardness) under 5 N and 15 N loads with 1m/s sliding speed. For the characterization of the samples; they Sanded with 400, 600, 800 and 1200 grit sandpapers, then polished with 3 and 6 microns felts. The polished samples were etched in Keller Reagent solution for 60 seconds. Optical microscope (OM) images of the samples were obtained a with NIKON brand ECLIPSE-LV150NL model device. Surface analysis and element distribution of the samples were examined with ZEISS Brand and SUPRA 40 VP Model Field Emission Scanning Electron Microscopy (FESEM) and Element Dispersion
  4. R. Arslan et al.,/Science of Sintering, 57(2025)69-85___________________________________________________________________________72Spectroscopy (EDS) device mounted on them. The samples were analyzed with X-ray diffraction via Bruker D8 advanced model device and used 0.004 scanning steps. In vibration analysis, the samples were fixed to the test setup in two different ways, as short sides and long sides, in order to examine the effect of changes in the cross-sectional area of the beam on the natural frequency. An accelerometer (PCB35C22) is attached to the samples on the support. In the direction of the applied force, the natural frequency value of the material was transferred to the data collection device (DEWE 43A) and then to the software program (DEWESoft X) and recorded. Working phases including sample preparation, cryo-cooling, heat treatment, natural vibration test, and microstructure analysis are given in the Graphical Abstracta in Fig. 1. Fig. 1. Graphical abstract. 2.2 Analytical Vibration Analysis of Beams In general, the normal stress relationship occurring in the beam can be expressed by Eq. 1. The bending moment of the beam is shown in Fig. 1 (Graphical Abstract – Bending Moment of Beam) [19]: = . (1) According to the Bernoulli-Euler hypothesis, the deformation at a certain distance from the neutral surface is given in Eq. 2.: =(+)−= (2) here r is the radius of curvature. The relationship between bending moment and normal stress is given in Eq. 3 [20].: ==22 (3) Load moment relationship for a loaded beam;
  5. R. Arslan et al.,/Science of Sintering, 57(2025)69-85___________________________________________________________________________7322== () (4) Eq. 4 can be written as: 22(22) = () (5) The d'Alembert principle is used for dynamic load. Mass and acceleration should beadded to equation 5. Thus, the following Eq. 6 is obtained: 22(2(,)2) = (, )− 2(,)2 (6) here, , A and I demonstrate the density of the material, cross sectional area of the beam and the moment of the inertia of the beam respectively. The natural frequency of the beam does not depend on the applied force, but on the material properties and geometry. Thus, (, )can be assumed to be zero and equation 7 is obtained: 22(2(,)2) + 2(,)2= 0 (7) This is a homogeneous equation. Eq. 8 can be written as the time dependent harmonic component of the coordinate and boundary conditions: (, )= ∑∞=1sincos (8) here An is the amplitude and n is the mode of the natural frequency. When Eq. 8 is solved by Eq. 7, the frequency (ωn) of the beam with variable cross section is found in Eq. 9. The natural frequency of the variable cross-section beam can be found by Eq. 10 [21]: =222√ (9) =2 (10) Since the beam has a variable section (Fig. 1- Graphical Abstract – Variable Section Beam), the variable section [22] is defined by the f(x) function. θ is the tapered angle, β is the variable section parameter. hx half thickness based on x, h1 is the maximum half thickness and h2 is the minimum thickness [23]. ℎ= ℎ1[1 −()] (11) = (1 −ℎ2ℎ1) (12) ()= (13) The moment of inertia (I) is calculated according to the selected function. 3. Results and Discussion 3.1. Microstructural Examinations
  6. R. Arslan et al.,/Science of Sintering, 57(2025)69-85___________________________________________________________________________74 Fig. 2 shows the OM images of the AA2024, AA6061 and AA7075 alloys in the raw state, with the resonance frequency values after aging, and of the sample with the highest hardness. When Fig. 2 is examined, no significant difference was observed in the microstructure of the cryo-cooling process applied to the samples after dissolution and the aging heat treatment after cryogenic. For more detailed examinations, FESEM and EDS images of the samples were given in Fig. 3. Fig. 2. Optical Microscope Images of AA2024, AA6061 and AA7075 Al Alloys. When Fig. 3 is examined, in the general EDS analyzes; It is seen that Al, Cu, Mg, Mn, Si and Zn elements for AA2014 alloy. Also, Al, Mg, Si, Cu, Zn, Ti and Mn elements was detected for AA6061 alloy and Al, Zn, Mg, Cr, Ti, Si and Mn elements contents for AA7075 alloy. The base material of all alloys is Al and all the materials could be aged. These contents were found to be in agreement with the factory data obtained. As can be understood from the material contents, it is understood that the O content is not included in the structure and is generally similar to the catalog information given [24-26]. Hard precipitates homogeneously dispersed in the microstructure improve the hardness and strength values in the main phase depending on the precipitation mechanisms in the structure [27]. When Fig. 4 is examined, hardness data of AA2024, AA6061 and AA7075 alloys are included. When Fig. 4 is examined, it is seen that there is an increase all the samples with the ageing time at the high hardness value. The increase in hardness can be said to be due to the effect of possible secondary hard phases in the structure [27,28]. It is known that hardness can be increased in the microstructure of the material thanks to the θ phases formed by the hardness, dissolution, cryogenic and subsequent aging heat treatment [29,30]. When Fig. 4 is examined, it is seen that there is a significant increase in hardness after each hour of aging heat treatment after dissolution. The highest hardness was obtained for AA7075 alloy as 142 HV after 12 hours of aging. While the highest hardness value in AA2024 alloy was obtained as 134 HV in 6 hours, 92 HV hardness value was obtained in AA6061 sample after 12 hours of aging. It is known that these hardness values are due to the thin or clustered phases formed during the aging heat treatment of precipitates such as Al2Cu, Mg2Si,
  7. R. Arslan et al.,/Science of Sintering, 57(2025)69-85___________________________________________________________________________75Al2CuMg, which may occur in the structure depending on the alloy content [29,31,32]. The increases in hardness were found to be compatible with studies in the literature. [33,43]. Fig. 3. FESEM and EDS analysis of Maximum hardness values for the a) AA2024, b) AA6061 and, c) AA7075 Alloys. Fig. 4. Hardness Values of AA2024, AA6061 and AA7075 Alloys After Dissoluted at 510°C for 2h+ Cryo-cooled at -75ºC for 168 hours + Ageing at 160ºC for 16h (hardness was for per hour).
  8. R. Arslan et al.,/Science of Sintering, 57(2025)69-85___________________________________________________________________________76 It is seen that the secondary hard phases formed by the aging heat treatment applied to ageable Al alloys positively affect the mechanical properties of the material. It is understood that in addition to the aging heat treatment, the cryogenic cooling process also plays an important role in the ageable Al alloy series. In addition, by reducing or eliminating residual stresses in the internal structure resulting from production, processing, and heat treatment, the surface properties, wear resistance and mechanical properties of Al alloys can be improved simultaneously. [10,33,42-45]. 3.2. Pin-On Disc Wear Results In this study, dry wear tests were applied to AA2024, AA6061 and AA7075 Al alloys, which were tested in different processing steps, with Pin-on Disk method under 5N and 15N loads. Wear tests; Untreated, Dissolution + Cryo-cooling (168 hours at -75ºC) and Dissolution + Cryo-cooling (168 retentions at -75ºC) were performed on samples aged for 12 hours at +160ºC. With the cryogenic application in this study, the hardness increased and the wear properties improved with the formation of the Al2Cu, Mg2Si, and Al2CuMg secondarily phases. In the wear test, the weight loss in the structure under 5 N and 15 N loads was converted to volume loss with the equation given in Eq. (14). Volume losses according to alloy type and applied process steps are given in Figure 5. As a result of the wear test applied under different processes and different forces, it was observed that there was an increase in the volumetric loss value due to the increase in load. It is seen in Figure 5.a that the volumetric loss rate caused by the wear effect applied under 5 N load has balanced values close to each other in the 3 sample series. The reason for this is that there are differences between the sample loss rates due to the increase of the applied load and the increase of the load, and it is thought that the wear is effective by breaking the surface wear effect on the surface due to the increase in the load. Wear traces can be seen in the FESEM analysis of the worn surfaces under 15 N in Fig. 8: Volume loss (mm3)=Mass loss (g)Density (gcm3)1000 (14) Fig. 5. Volume loss as a result of wear analysis of AA2024, AA6061 and AA7075 Al alloys according to the applied treatment types. As a result of the wear test applied under different processes and different forces, it was observed that there was an increase in the volumetric loss value due to the increase in load. It is seen in Fig. 5a that the volumetric loss rate caused by the wear effect applied under 5 N load has balanced values close to each other in the 3 sample series. The reason for this is that there are differences between the sample loss rates due to the increase of the applied load and
  9. R. Arslan et al.,/Science of Sintering, 57(2025)69-85___________________________________________________________________________77the increase of the load, and it is thought that the wear is effective by breaking the surface wear effect on the surface due to the increase in the load. In the FESEM analysis of the eroded surfaces below 15 N in Fig. 4, fracture traces are observed. As seen in Fig. 5, when the volume losses of the applied processes and materials areexamined, it is seen that the volume loss of the sample that underwent cryogenic treatment after dissolution treatment is less than the standard sample. It is understood that the effect of this volume loss is realized as a result of the increased hardness [33] thanks to cryo-cooling, and the improvement of wear properties [10]. It is seen that the volume loss of the aged sample after cryogenic treatment is much less compared to the other 2 samples. On the other hand, it was determined that the decrease in volume loss increased the material hardness of the precipitated phases due to the applied aging process [34] and the volume loss was less as a result of wear. The friction coefficient values obtained as a result of the adhesive wear tests performed with the contact of two metal surfaces under the force applied by the pin-on disc method are given in Fig. 6. When the analyzes are examined, the friction coefficients of different samples applied the same treatment are shown in Fig. 6.a, b and c. It has been determined that AA6061 alloy has the lowest friction coefficient in the post-solution cryogenic (b) and post-cryogenic aged samples (c) series compared to the other samples, and the post-cryogenic aging process reduces the friction coefficient in the AA6061 alloy. In Fig. 6, f and g, the friction coefficients of the same samples in different processes are examined. As a result of the examination, it was determined that the friction coefficients of the aged samples (post cryogenic aging samples) were the lowest for each Al series, while the raw samples were the highest. Fig. 6. Sliding distance vs coefficient of friction graphs for AA2024, AA6061 ve AA7075 Al alloys subjected to different processes. When Fig. 7 is examined, the effects of the treatments applied to AA2024, AA6061 andAA7075 Al alloys on the wear rate are shown. The wear rates of the samples were calculatedby Eq. 5: Wear Rate (mm3/Nm) =Volume loss (3)Normal load (N). Sliding distance(m)(15) The wear rate is a more accurate description of the wear properties for materials, especially metals, alloys, and composites. It is used as an indicator of the wear characteristics of sliding surfaces under the applied load, test speed, and sliding distance or time [35]. When the wear
  10. R. Arslan et al.,/Science of Sintering, 57(2025)69-85___________________________________________________________________________78rates of AA2024, AA6061 and AA7075 Al alloys are compared, it is thought that the wear rate in the aged sample series after cryogenics decreases depending on the type of process applied, and this decrease is due to the increase in the material strength properties after the aging process applied to the material. It was determined that the wear rate of the cryogenically applied sample series decreased less than the base sample. Fig. 7. Wear rate datas of AA2024, AA6061 and AA7075 Alloys. The wear surface FESEM images obtained as a result of the wear of AA2024, AA6061 andAA7075 alloys on the metal surface separately under 5 and 15 N loads for 60 minutes are given in Fig. 8. When Fig. 8 is examined, it can be seen that there are changes in the wear surfaces depending on the applied heat treatment processes. Fig. 8. AA2024 (a, d, g), AA6061 (b, e, h) ve AA7075 (c, f, i) wear surface images of Al alloys (a, b, c: Base Materials, d, e, f: Dissolutioned + Cryo Heat Treated, g, h, e: Dissolutined + Cryo Heat Treated + Aged). Fig. 8a,b,c shows the FESEM images of the samples belonging to AA2024, AA6061 and AA7075 standard materials after wear under 15N load. Fig. 8d,e,f shows FESEM images of
  11. R. Arslan et al.,/Science of Sintering, 57(2025)69-85___________________________________________________________________________79the abraded surfaces under 15N load after Dissoluted + -75ºC for 168 hours Cryo-cooling. Fig. 8g,h,i shows the FESEM images of the abraded surfaces under 15N load, after 168 hours of Cryo-cooling at -75ºC. When the FESEM images were examined, it was observed that there were significant effects on the surfaces after wear in general, and that the surface defects and smearing in the standard samples were intense. As a result of the applied cryo-cooling, it is observed that the amount of oxide on the surface increases, while these oxide surfaces act as a solid lubricant by protecting the surface under the metal when metal-to-metal contact occurs, thanks to the sliding mechanism created by these oxide surfaces during wear [36]. Especially the rapid oxide formation of Al alloys and the lubricating effect provided by the formed oxide layer are important [37] as in this study, it played an important role in the improvement of the surface properties and wear rates with the oxide layer [10]. When Table II is examined, it is seen that the natural frequency of the Al beam does not change when the tapered angle is 0º, however, there is a change in the natural frequency values with the change of the tapered angle (between 0º and 1º) Tab. II AA2024 (Upper), AA 6061, and AA7075 (Bottom) Alloys Vibration Data after cryo-cooling.
  12. R. Arslan et al.,/Science of Sintering, 57(2025)69-85___________________________________________________________________________80Fig. 9a shows the natural frequency values of the AA2024, AA6065 and AA7071 recessed beams attached from the short (b) side according to the tapered angle. It was observed that the frequency values of the beam decreased with the increase of the tapered angle both in the experimental analysis and in the analytical analysis. In the experimental and analytical analysis, the highest frequency values were obtained when the tapered angle was 0º. Again, when the tapered angle is 1º, the smallest frequency values were obtained experimentally and analytically. Fig. 9.(b) shows the natural frequency values according to the tapered angle of the recessed Al beam attached from the long (B) side. It has been observed that the frequency values of the beam increase with the increase of the tapered angle both in the experimental analysis and in the analytical analysis. In experimental and analytical analysis, the smallest frequency values were obtained experimentally and analytically when the tapered angle was 0º. Again, when the tapered angle is 1º, the maximum values were found at 1º experimentally and analytically. Experimental and analytical analyzes were close to each other. The reason for the difference between them is the use of the Euler-Bernoulli beam in the analytical analysis and the neglect of the shear forces. In addition, it is thought that the internal structure or production errors in the samples used in the experimental analysis are caused. Fig. 9. Natural frequency value of Alloy AA2024, AA6061 and AA7075. a) AA2024, AA6061 and AA7075 The natural frequency values according to the tapered angle (section angle) of the fixed beam attached from its short side (b), b) AA2024, AA6061 and AA7075 natural frequency values according to the tapered angle of the fixed beam attached from its long side (B).
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