The Influence of Heat Aging Treatments on the Cavitation Erosion Behavior of a Type 6082 Aluminum Alloy

It is known that a number of parts that operate in liquid media, such as the propellers of motorboats and pleasure river vessels, as well as the rotors of household pumps and the radiators and pumps in the cooling system of motor vehicles are made, as a rule, of aluminum-based alloy. Research during maintenance leads to the conclusion that, in certain operating conditions, due to the turbulent character of the flow, with pressure drops to below the vaporization level, it inevitably reaches the threshold of cavitation, which manifests itself through its effects, especially through erosion. To increase the lifetime, these alloys are currently subjected to techniques to improve the structure’s resistance to the cyclic stresses of cavitational microjets. Among these techniques are volumetric heat treatments, which lead to changes in the microstructure and mechanical property values, with an effect on the behavior and resistance to cavitation erosion. This paper studies the influence of heat aging treatments on the cavitation erosion behavior of an aluminum alloy type 6082, in the cast state. The heat treatments applied were 140 °C/1 h, 12 h, 24 h and 180 °C/1 h, 12 h, 24 h. The MDEmax and MDERs parameters were determined and a correlation could be made between the values of the mechanical-resilient characteristics and the resistance to cavitation erosion in the case of aluminum alloy 6082.


Introduction
The use of aluminum alloys in industrial fields is one of their widest applications, due to the low specific mass and the values of the mechanical properties that confer resistance to various mechanical and hydrodynamic stresses. In the automotive, agro-industrial, and naval fields, aluminum alloys are used for the manufacture of the most diverse equipment, from storage containers to irrigation systems, to cooling systems in the construction of hydro-pneumatic systems, to pumps of thermal engines of agricultural vehicles, to pump rotors from water wells, and to propulsion propellers of boats and pleasure vessels [1][2][3][4][5].
The lifetime of such parts, which work with liquids, is affected by the hydrodynamic flow regime, because through pressure variations, the flow becomes cavitational, when the local pressure drops below the value of the vaporization pressure [3,[5][6][7][8][9][10]. Known as cavitation erosion in such situations, after a certain duration of operation, the surfaces of the parts in the cavitational field are subject to this process. Cavitation erosion is one of the manifestation effects of cavitation corrosion, well-known and developed in the specialized literature. This effect is mechanical in nature and consists in the creation of caverns due to ruptures produced in the structure of the surface material due to cyclic stresses (local fatigue) On the experimental samples in the T651 state, heat treatments of artificial aging were performed on experimental samples with dimensions 10 mm × 10 mm × 50 mm as follows: aging at 140 • C, with three maintenance periods, 1 h, 12 h, and 24 h; artificial aging at 180 • C, with three holding periods, 1 h, 12 h, and 24 h. The heat treatments were carried out in a Nabertherm furnace, within the Laboratory of Metallic Materials Science and the Physical Metallurgy Section of the Polytechnic University of Bucharest. For each type of heat treatment, six tests were performed to determine the mechanical properties: breaking strength, yield strength, elongation, toughness, hardness, and microhardness. Grain size determination was performed according to ASTM E3, ASTM E 407, ASTM E112, using Barker electrolytic reagent at ×100 power. Structural investigations were performed on an OLYMPUS microscope. The determination of the mechanical characteristics of the test samples subjected to different heat aging treatments was carried out on a universal testing machine Walter + Bai AD Switzerland model LFV 300. The impact resistance was carried out by determining the KCV values on the Pendulum Charpy hammer type Walter Bai with 300 J. Macroscopic structural analysis was performed on an OLYMPUS SZX stereomicroscope, equipped with QuickMicroPhoto 2.2 software, and microscopic analysis on a REICHERT UnivaR type optical microscope equipped with Image Pro Plus software. The analysis of the samples by X-ray diffraction was carried out with the D8 Discover diffractometer, Bruker-Germany, Cu anode tube (λ = 1.540598 Å), 1D Lynx Eye detector. The diffractograms were recorded with an angular increment of 0.040, at a scanning speed of 1 s/step, angular range of 2θ = 10-100 • . Scanning electron microscopy analysis was performed on a Thermophischer Quattro S scanning electron microscope.
The experimental program, for cavitation testing of the structure's resistance, took place in the Cavitation Erosion Research Laboratory of the Politehnica University of Timis , oara, on the standard vibrating apparatus with piezoceramic crystals [6]. During the tests, the requirements described by the international standards ASTM G32-2016 [37], the laboratory custom [38,39], were followed, regarding the total duration of cavitation (165 min) and the 12 intermediate periods (each of 5 and 10 min and 10 of 15 min each), the one related to tracking the evolution of the destruction in the area of the cavity surface. The functional parameters of the vibrator, which determine the hydrodynamics of the vibratory cavitation, that is, the intensity of destruction, were rigorously controlled by a special software implemented in the computer connected to the vibrator. Their values, according to ASTM G32 standards, are: • Double vibration amplitude = 50 µm; • Vibration frequency = 20,000 ± 1% Hz; • The power of the electric ultrasound generator = 500 W; • Liquid medium = double distilled water; • Liquid temperature = 22 ± 1 • C. Clarification: The construction of the vibrating mechanical system is for the DIRECT TESTING METHOD (with the vibrating sample caught in the mechanical system), for metallic and non-metallic samples with a diameter of 15.8 mm and a length of 16 mm and with masses of 16-17.5 g. To generate erosion by vibrating cavitation, for light alloy samples, such as aluminum, due to the very low mass, even if the sample complies with the required geometric dimensions, the INDIRECT TESTING METHOD (stationary sample method) was used. In this sense, a special sample fixation system was built, as can be seen in Figure 1.
According to laboratory custom, in order to comply with the requirements of ASTM G32-2016, in order to reduce the influence of the surface quality on the results, before starting the test, the surface that was to be exposed to cavitation was polished to a roughness Ra = 0. with the aim of highlighting and understanding the way degradation by erosion, that is, the mechanical response process of the surface structure to the cyclic stresses of microjets and shock waves generated by the implosion of cavitation bubbles. the required geometric dimensions, the INDIRECT TESTING METHOD (stationary sample method) was used. In this sense, a special sample fixation system was built, as can be seen in Figure 1.  According to laboratory custom, in order to comply with the requirements of ASTM G32-2016, in order to reduce the influence of the surface quality on the results, befor starting the test, the surface that was to be exposed to cavitation was polished to a roughness Ra = 0.2...0.8 µm [6]. From each type of state (semi-finished and with hea treatment), as a certainty of the accuracy of the experiment, as required by customs, thre samples were tested. Before the start of the test and after each intermediate period o time, the eroded surface of each sample was photographed with the Canon Power Sho A480 and analyzed with the OPTIKA Vision Pro3 microscope from the Cavitation Ero sion Research Laboratory, with the aim of highlighting and understanding the way deg radation by erosion, that is, the mechanical response process of the surface structure to the cyclic stresses of microjets and shock waves generated by the implosion of cavitation bubbles.

The Mechanical Behavior of the Experimental Samples
For ease of presentation, description, analysis, and understanding of the research results, the symbols used to mark the samples, depending on the nature of th semi-finished alloy and the parameters of each heat treatment regime (temperature holding time, cooling mode), are as follows: U-state of the laminated semi-finished product (delivery state), without other hea treatments; UP-state obtained by artificial aging at 160 °C, with a holding time of one hour and cooling in the oven; UL-state obtained by artificial aging at 160 °C, with 12 h holding time and oven cooling; UI-state obtained by artificial aging at 160 °C, with 24 h holding time and cooling in the oven;

The Mechanical Behavior of the Experimental Samples
For ease of presentation, description, analysis, and understanding of the research results, the symbols used to mark the samples, depending on the nature of the semifinished alloy and the parameters of each heat treatment regime (temperature, holding time, cooling mode), are as follows: U-state of the laminated semi-finished product (delivery state), without other heat treatments; UP-state obtained by artificial aging at 160 • C, with a holding time of one hour and cooling in the oven; UL-state obtained by artificial aging at 160 • C, with 12 h holding time and oven cooling; UI-state obtained by artificial aging at 160 • C, with 24 h holding time and cooling in the oven; UOP-state obtained by artificial aging at 140 • C, with a holding time of one hour and cooling in the oven; UOL-state obtained by artificial aging at 140 • C, with 12 h holding time and cooling in the oven; UOI-state obtained by artificial aging at 140 • C, with 24 h holding time and cooling in the oven; The results regarding the mechanical behavior of the experimental samples are shown in Table 2, and suggestively represented in Figure 2. shown in Table 2, and suggestively represented in Figure 2.   A careful analysis of the experimental data allows the formulation of the following observations.
By applying heat aging treatments to aluminum alloy type 6082 products, in T651 condition, either at 140 • C or at 180 • C, with different holding times, there is an increase in mechanical strength, yield strength, and a decrease in cooking and resilience, a fact that allows a prediction of the increase in resistance to cavitational erosion. Thus, when applying an aging at 140 • C, the breaking strength can increase by 7-13% (differentiated according to the duration of holding at the temperature), the yield strength increases by 18-34%, the necking increases by 17-77%, and the resilience decreases by about 18-36%; when aging at 180 • C, the breaking resistance can increase by 18-94% (differentiated according to the duration of holding at the temperature), the yield strength increases by 39-186%, and the resilience decreases by about 4-43%. The explanation of the increase in mechanical characteristics is given by the precipitation of intermetallic compounds, in the metal matrix of solid solution α, precipitation that becomes abundant when the duration of maintenance at the aging temperature of 180 • C increases. This is highlighted by the structural analysis, both with the optical microscope and the X-ray diffraction analysis, presented in subchapter 3b.
In Figure 3b,d, we show the structure after applying aging at 140 • C for the longest holding time, 24 h, and in Figure 3e,f, the structure is highlighted after applying aging at 180 • C at the longest holding time, 24 h. It is thus noted that at a temperature of 180 • C and a duration of 24 h, there is the most abundant precipitation of intermetallic compounds in the solid solution matrix α, a fact that explains the almost doubling of the mechanical strength and yield strength, compared to the control sample, in T651 condition, not subject to aging.
The X-ray diffraction analysis in Figure 4 confirms the results of the metallographic analysis, in the sense that by applying the aging treatments, the lattice parameter increases considerably, a sign of the hardening of the solid solution α.

The Cavitational Erosion Behavior of the Experimental Specimens
The results of the cavitation test are expressed by the diagrams in -For the average erosion rate: where A is the scale parameter, established from the condition that the deviations of the experimental points from the curve are minimal; B is the shape parameter of the curve. In Figure 3b,d, we show the structure after applying aging at 140 °C for the longest holding time, 24 h, and in Figure 3e,f, the structure is highlighted after applying aging at 180 °C at the longest holding time, 24 h. It is thus noted that at a temperature of 180 °C and a duration of 24 h, there is the most abundant precipitation of intermetallic compounds in the solid solution matrix α, a fact that explains the almost doubling of the mechanical strength and yield strength, compared to the control sample, in T651 condition, not subject to aging. The X-ray diffraction analysis in Figure 4 confirms the results of the metallographic analysis, in the sense that by applying the aging treatments, the lattice parameter increases considerably, a sign of the hardening of the solid solution α. states: (a,b)-state T651 (control sample); (c,d) state T651 + aging at 140 °C/24 h (UOI sample); (e,f)-T651 state + aging at 180 °C/24 h (UI sample); (b,d,f) detail images of (a,c,e). The X-ray diffraction analysis in Figure 4 confirms the results of the metallographic analysis, in the sense that by applying the aging treatments, the lattice parameter increases considerably, a sign of the hardening of the solid solution α.

The Cavitational Erosion Behavior of the Experimental Specimens
The results of the cavitation test are expressed by the diagrams in                                It should be noted that, according to all the studies in this field, mentioned in the specialized literature [39], the dispersions of the experimental values, the evolution forms of the averaging curves, and the values of the indicated parameters are an effect of the nature of the semi-finished product, of the parameters of the heat treatment regimes (treatment type, temperatures, durations), and of the microstructure and mechanical properties (hardness, mechanical resistance to breaking, resilience, etc.).  It should be noted that, according to all the studies in this field, mentioned in the specialized literature [39], the dispersions of the experimental values, the evolution forms of the averaging curves, and the values of the indicated parameters are an effect of the nature of the semi-finished product, of the parameters of the heat treatment regimes (treatment type, temperatures, durations), and of the microstructure and mechanical properties (hardness, mechanical resistance to breaking, resilience, etc.). In order to certify the accuracy of the test on the three samples, in the diagrams showing the variation of the mass loss M(t) with the duration of exposure to cavitation, the upper and lower tolerance intervals, that is, the area of the dispersion band of the experimental points, calculated with the Mathcad program, were delimited. The statistical relationships used have the following forms [39]: the equation of the exponential regression curve (rel.1); -standard error of estimation: where -M(t) i is the mass of material lost through erosion defined by the averaging curve (rel. 1) at time t i ; -M i is the average experimental value of the values obtained by experiment at time ti, on the three samples.
As a result of the complexity of the hydrodynamics of cavitation and the dependence of the resistance of the required surface structure on a multitude of factors (microstructural defects, type of microstructure, chemical composition, physical-mechanical properties), the tolerance ranges accepted for this type of stress (≤10%) range from 99% to 90% [39], whose upper "S" and lower "I" limits are calculated as follows: for the 99% tolerance interval: for the 90% tolerance interval: where σ is the average standard deviation of the experimental values from the averaging curve M(t). Clarification: The diagrams in Figures 5-18, through the evolutions of the averaging curves, the dispersion of the experimental values, in different intervals, compared to the averaging curves, show the behavior and resistance of the surface structure to the cyclic stresses of microjets and shock waves generated by vibrating cavitation. The values of the parameters Mmax (the mass lost through erosion, defined by the curve M(t) at 165 min) and vs (the value of the speed from 165 min defined by the curve v(t), known in the specialized literature as the stabilization speed of erosion, or the value towards which the v(t) curve asymptotically decreases [6,9,[38][39][40]) are indicators of the structure's resistance to cavitation stress, and by comparison, they serve to identify the material (structure) with the best resistance and behavior to cavitation erosion generated in the device used [6, 9,39].
It should be noted that, according to all the studies in this field, mentioned in the specialized literature [39], the dispersions of the experimental values, the evolution forms of the averaging curves, and the values of the indicated parameters are an effect of the nature of the semi-finished product, of the parameters of the heat treatment regimes (treatment type, temperatures, durations), and of the microstructure and mechanical properties (hardness, mechanical resistance to breaking, resilience, etc.).
The metallographic stereomicroscope analysis of the cavitationally eroded surfaces allowed the fractographic evaluation, as well as the definition of the production mechanism of this phenomenon. The results of the stereomicrostructural analysis are shown qualitatively in Figures 19-25 and quantitatively in Table 3. In the control sample, Figure 18, an attack generated by aggressive cavitational erosion is noted, with macroscopically visible caverns, located on a large surface, about 85% of the entire surface exposed to erosion, and the most stressed surface has an extension of 74% (as can be seen from Table 3, which centralizes the quantitative results presented in Figure 18). The maximum depth of corrosion penetration reaches 490 µm (Figure 19d), a depth that is considered very high, placing this alloy in the framework of metallic materials with low resistance to cavitational erosion. When applying the aging treatment at 140 • C, regardless of the holding time, a slight improvement in the resistance to cavitational erosion is observed (Figures 20-22), in the sense of the decrease in the surface most affected by the corrosive attack, from 71% (at a 1 h hold) to 69% (at a 12 h hold), reaching 66% (at a 24 h hold). Also, the maximum penetration depth of the corrosive attack also registers a decrease compared to the control sample from 337 µm (after a 1 h hold, Figure 20d) to 314 µm (after a 12 h hold, Figure 21d), reaching 267 µm (after a 24 h hold, Figure 22d). It is noteworthy that, although the values obtained after the application of the aging heat treatment, both of the attacked surfaces and of the penetration depth, are high, they maintain the alloy in the assessment of a low resistance to cavitational erosion. When applying the aging treatment at 180 • C, regardless of the holding time, a considerable improvement in the resistance to cavitational erosion is noted (Figures 23-25), both compared to the control sample and compared to the samples aged at 140 • C. Thus, the surface area most affected by the corrosive attack decreases, to 60% (after 1 h and 12 h maintenance) and to 54% (after 24 h maintenance). Also, the maximum penetration depth of the corrosive attack also registers a decrease compared to the control sample, to 253 µm (after a 1 h hold, Figure 19d) and to 249 µm (after a 12 h hold, Figure 21d).  24 h hold). Also, the maximum penetration depth of the corrosive attack also registers a decrease compared to the control sample from 337 µm (after a 1 h hold, Figure 20d) to 314 µm (after a 12 h hold, Figure 21d), reaching 267 µm (after a 24 h hold, Figure 22d). It is noteworthy that, although the values obtained after the application of the aging heat treatment, both of the attacked surfaces and of the penetration depth, are high, they maintain the alloy in the assessment of a low resistance to cavitational erosion. When applying the aging treatment at 180 °C, regardless of the holding time, a considerable improvement in the resistance to cavitational erosion is noted (Figures 23-25), both compared to the control sample and compared to the samples aged at 140 °C. Thus, the surface area most affected by the corrosive attack decreases, to 60% (after 1 h and 12 h maintenance) and to 54% (after 24 h maintenance). Also, the maximum penetration depth of the corrosive attack also registers a decrease compared to the control sample, to 253 µm (after a 1 h hold, Figure 19d) and to 249 µm (after a 12 h hold, Figure 21d).

Scanning Electron Microscope Analysis of Cavitationally Eroded Samples
The results of the scanning electron microscope analysis are shown in Figure 26. It is noted that in all investigated samples, the corrosive attack is very intense, both in the control sample (Figure 26a

Scanning Electron Microscope Analysis of Cavitationally Eroded Samples
The results of the scanning electron microscope analysis are shown in Figure 26. It is noted that in all investigated samples, the corrosive attack is very intense, both in the control sample (Figure 26a,b) and in the samples subjected to the aging treatment, either at 140 °C 24 h (Figure 26c,d) or 180 °C/24 h (Figure 26e,f). The attack is intergranular, with a brittle appearance, faceted cleavage, and propagation of the intergranular fracture front. The similarity of the aspects, even if different values of the cavitational erosion penetration depth are recorded, show the same manifestation of the intergranular erosion phenomenon, specific to metallic materials with low resistance to this form of corrosion.

Discussion-Discussions and Interpretations of the Experimental Results Regarding the Behavior and Resistance to Cavitation
The diagrams characteristic of the erosion generated by the shock waves and the microjets resulting from the implosion of the cavitational bubbles formed in the process of vibrating cavitation are shown in Figures 5-7, 9, 11, 13, 15 and 17, which contain data that allowed the analysis of the behavior and strength of the structure of the types of material states after the variation of cumulative mass losses with the duration of exposure to cavitation. Figures 6, 8, 10, 12, 14, 16 and 18 contain data that allowed the analysis of the behavior and resistance of the structure of the types of material states after the variation of the erosion speed (the average speed of losses of mass during the intermediate duration of the cavitation attack) with the duration of exposure to cavitation.
The data from the diagrams presented in Figures 5-7, 9, 11, 13, 15 and 17 lead us to the following findings: - The values of the tolerance fields (between 97% and 99%), related to the dispersion bands, justify the accuracy of the test program, through the rigorous control of the parameters of the vibrator that dictate the intensity of the destruction by cavitation, regardless of the state (semi-finished or with heat treatment); the structures of the

Regarding the Behavior and Resistance to Cavitation
The diagrams characteristic of the erosion generated by the shock waves and the microjets resulting from the implosion of the cavitational bubbles formed in the process of vibrating cavitation are shown in Figures 5-7, 9, 11, 13, 15 and 17, which contain data that allowed the analysis of the behavior and strength of the structure of the types of material states after the variation of cumulative mass losses with the duration of exposure to cavitation. Figures 6, 8, 10, 12, 14, 16 and 18 contain data that allowed the analysis of the behavior and resistance of the structure of the types of material states after the variation of the erosion speed (the average speed of losses of mass during the intermediate duration of the cavitation attack) with the duration of exposure to cavitation.
The data from the diagrams presented in Figures 5-7, 9, 11, 13, 15 and 17 lead us to the following findings: - The values of the tolerance fields (between 97% and 99%), related to the dispersion bands, justify the accuracy of the test program, through the rigorous control of the parameters of the vibrator that dictate the intensity of the destruction by cavitation, regardless of the state (semi-finished or with heat treatment); the structures of the three samples, during exposure to cavitation, have a similar behavior, the values of the standard error σ tending to decrease from 4.197 (for the U-state of the semi-finished product, for which the average algebraic cumulative mass loss is 36.447 mg) to 0.486 (for the UOI condition, for which the cumulative algebraic average mass loss is 8.837 mg), with increased resistance to cyclic cavitation stresses. -A time interval exists between 0 and 30 min, depending on the state of the sample, in which the mass losses are reduced, following that from here until the end (165 min), the cumulative mass loss is, as is natural, continuously increasing. The small losses in the first part show the dependence of the surface resistance, at the impact pressures with the microjets of the structure, on the grain sizes and on the number and sizes of intermetallic compounds.
Obviously, this resistance, according to older studies by Garcia a.o [40], in the initial phase of the generation of networks of cracks and breaks with expulsions, is primarily dependent on the hardness, but also on the values of the mechanical properties (see Table 2), which determines the level of intercrystalline bonding forces. However, unlike ferrous alloys, with very high resistance to cavitation erosion, such as stainless steels with martensitic structures and surfaces hardened by various technologies [8,9], where hardness is the determining factor in surface resistance to the demands of cavitational microjets, in the case of these samples, as shown by the values of the MDEmax parameter (the highest for the UOI sample (8837 mg- Figure 16) and the lowest for the U sample (36,447 mg- Figure 5)), this leads to the opinion that the strength of the alloy structure is strongly dependent on the mechanical properties that determine high capacity for plastic deformation (resilience). - The decrease in the slopes of the M(t) curves, after 30 (60)  After 45 (60) min, the averaging/approximation curves v(t) decrease asymptotically towards the value vs. This evolution is determined by the decrease in mass losses (see the related diagrams) as a result of the reduction in the energy absorbed to break the bonds between the grains and the expulsion.
From our studies [2,7,9], this decrease is caused by the mechanical hardening over time of the superficial layer, by the number and geometric dimensions of the caverns in which, during the compression phase of the sonotrode, air enters that acts as a shock absorber in the expansion phase of the sonotrode, a phenomenon that mitigates the impact force of the cavitational microjets. - The evolution of the curve v(t), from the maximum value to the minimum value, is slower for structures with high resistance (see UOI sample, Figure 18).
To evaluate the resistance, the histogram in Figure 27 compares the values of the parameters recommended by ASTM G32 standards and used in the Cavitation Erosion Research Laboratory custom. From our studies [2,7,9], this decrease is caused by the mechanical hardening over time of the superficial layer, by the number and geometric dimensions of the caverns in which, during the compression phase of the sonotrode, air enters that acts as a shock absorber in the expansion phase of the sonotrode, a phenomenon that mitigates the impact force of the cavitational microjets. - The evolution of the curve v(t), from the maximum value to the minimum value, is slower for structures with high resistance (see UOI sample, Figure 18).
To evaluate the resistance, the histogram in Figure 27 compares the values of the parameters recommended by ASTM G32 standards and used in the Cavitation Erosion Research Laboratory custom. The parameters MDEmax and MDERs were calculated with the following relation: where dp = 15.8 mm represents the diameter of the surface exposed to cavitation. The diagram also shows the parameter Hcav, which represents the value of the maximum depth of the cavern, measured with a stereomicroscope (see Figures 19d, 20d, 21d, 22d, 23d, 24d and 25d). Mentioning this depth is intended to prove that its value is dependent on the arbitrary place where the section for measurement was made and that, compared to MDEmax, which is an average value on the cavitated surface, it cannot, in any case, serve as a parameter credible in comparing the resistance of a structure to cavitation erosion.
The histogram in Figure 27 shows that, regardless of the parameter we want to refer to (including Hcavb), all heat treatment regimes led to higher strengths than that obtained for sample U (corresponding to the state of delivery (laminated semi-finished product)). The resistance increases compared to the U condition, regardless of the parameter used, being from 20% (UL sample) to more than 4 times (UOI sample). This leads us to support the necessity of volume heat treat"ents'in increasing the cavitation resistance of the structure of aluminum-based alloys such as alloy 6082.
After the heat treatment regime, we find that the structure of the UOI sample (temperature of 140 °C and holding time of 24 h) confers the highest resistance to cavitation erosion. Among all the treatment regimens, the lowest resistance was recorded for the UL sample (temperature of 180°C and duration of 12 h) Although they are regimes with different temperatures and durations, the treatments applied to the UI and UOL samples show that the differences between the values of the mentioned parameters, regardless of which we report (Mmax, MDEmax, vs or MDERs), are about 1%. This aspect confirms that different volume heat treatment regimes can create structures with similar cavitation erosion resistances.
Among the regimes with the temperature of 180°C, the values of the parameters in the histogram indicate the one with a duration of 24 h (sample UI) as the one that leads to The parameters MDEmax and MDERs were calculated with the following relation: where d p = 15.8 mm represents the diameter of the surface exposed to cavitation.
The diagram also shows the parameter H cav , which represents the value of the maximum depth of the cavern, measured with a stereomicroscope (see Figures 19d, 20d, 21d, 22d, 23d, 24d and 25d). Mentioning this depth is intended to prove that its value is dependent on the arbitrary place where the section for measurement was made and that, compared to MDE max , which is an average value on the cavitated surface, it cannot, in any case, serve as a parameter credible in comparing the resistance of a structure to cavitation erosion.
The histogram in Figure 27 shows that, regardless of the parameter we want to refer to (including H cavb ), all heat treatment regimes led to higher strengths than that obtained for sample U (corresponding to the state of delivery (laminated semi-finished product)). The resistance increases compared to the U condition, regardless of the parameter used, being from 20% (UL sample) to more than 4 times (UOI sample). This leads us to support the necessity of volume heat treatments in increasing the cavitation resistance of the structure of aluminum-based alloys such as alloy 6082.
After the heat treatment regime, we find that the structure of the UOI sample (temperature of 140 • C and holding time of 24 h) confers the highest resistance to cavitation erosion. Among all the treatment regimens, the lowest resistance was recorded for the UL sample (temperature of 180 • C and duration of 12 h) Although they are regimes with different temperatures and durations, the treatments applied to the UI and UOL samples show that the differences between the values of the mentioned parameters, regardless of which we report (M max , MDE max , v s or MDER s ), are about 1%. This aspect confirms that different volume heat treatment regimes can create structures with similar cavitation erosion resistances.
Among the regimes with the temperature of 180 • C, the values of the parameters in the histogram indicate the one with a duration of 24 h (sample UI) as the one that leads to the structure and mechanical properties that determine the best resistance to cavitation erosion (higher than the strength of the sample structure UP by about 53% according to the values of M max , MDE max , v c , and MDER and by about 19% according to the values of H cav , and compared to the strength of the sample structure, UL is higher by about 70% after M max and MDE max , by about 73% after vs and MDER s , and by about 17% after H cav ).
Among the regimes with a temperature of 140 • C, the values of the parameters in the histogram also indicate the one with a duration of 24 h (the UOI sample) as being the one that leads to the structure and mechanical properties that determine the best resistance to cavitation erosion (higher compared to the strength of the structure of the sample UOP by 53% according to the values of M max and MDE max , by about 51% according to the values of vs and MDER s , and by 26% according to the values of H cav , and compared to the strength of the structure of the sample, UOL is about 2 times higher according to the values of M max , MDE max , v c , and MDER s and by about 17% according to the values of H cav ).
The fact that for both temperatures the holding time of 24 h led to structures with high strengths suggests that of the three times (1 h, 2 h, and 24 h), this is the one that can be used for parts made of alloy 6082, type pump rotors, cooling radiators, and boat propellers, which will be operated in cavitation mode.

Conclusions
The experimental research carried out in the present work allowed the formulation of the following conclusions:

•
The structure of the alloy in the T651 state is constituted by solid solution α in which fine compounds of Mg 5 Si 6 dispersed in the matrix are evident. By applying heat aging treatments, the structural aspect does not change much, the amount of compounds increasing considerably. • Applying heat aging treatments to aluminum alloy type 6082 products can lead to an increase in mechanical strength, yield strength, and a decrease in necking and resilience, as follows. When aging at 140 • C, the breaking strength can increase by 7-13% (differentiated according to the duration of holding at the temperature), the yield strength increases by 18-34%, the necking increases by 17-77% and the resilience decreases by about 18-36%; when applying an aging at 180 • C, the fracture resistance can increase by 18-94% (differentiated according to the duration of holding at the temperature), the yield strength increases by 39-186%, and the resilience decreases by about 4-43%.

•
The dispersion mode of the experimental values compared to the averaging curves M(t) and v(t), as well as the evolutions of the two curves, show similar and distinct/specific elements of the behavior of the surface structure to the erosion created by the cyclic stresses of the microjets produced by vibratory cavitation. Basically, these diagrams show the effect of the volumetric heat treatment regime (temperature and holding time) on the mechanical response to cyclic fatigue stresses, through the structures and values of the mechanical properties obtained.
The conclusions drawn from the experimental research and the analysis of the results obtained, based on the specific parameters and the photographic images from significant times, as well as the microscopic ones from the end of the cavitation test, are as follows:

•
The cavitation resistance of the structures resulting from artificial aging heat treatments, regardless of the parameters of the heat treatment regime, is superior to the structure of the control sample, cast (sample U) from 30% to more than 4 times according to the values of the M max and MDE ma parameters and from 21% to more than 4 times according to the values of the parameters v s and MDER s . • For all samples, regardless of whether it is blank or thermally treated, in the first 15 (30) min of vibratory cavitation, material losses are reduced due to compliance with the specific erosive mechanism, through which, in the attacked surface, more elastoplastic deformations and networks are produced of cracks and the tips of roughnesses and abrasive dust are removed. Material ejections, with the creation of pittings, are significantly reduced in the mass values recorded by weighing.

•
In all samples, starting from minute 30 (45) and until the end of the test, the losses are of close order, which is why the curve M(t) becomes linear, the differences be-tween the experimental values of the speeds are small, and the curve v(t) decreases asymptotically towards the stabilization value v s . • In all samples, the evolutions of the v(t) averaging curves have an asymptotic tendency towards stabilization at the maximum value (v s ). Based on previous experiences, we believe that this mode of evolution is the effect of the mechanical properties, of the hardening over time, of the applied layer and of the reduction in the stress/impact force, as a result of the cushioning effect of the air penetrating into the formed caverns.

•
The thermal treatment regime by artificial aging that gives the best resistance to vibrating cavitation erosion is the one with a temperature of 140 • C and a holding time of 24 h.

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The thermal treatment regime that led to the structure with the lowest resistance to cavitation is the one with a temperature of 180 • C and a holding time of 12 h. • The differences between the experimental values and their dispersions in front of the averaging curves are expressions of the influence of the structure on the resistance to cavitation by the regime parameters of the heat treatment, by the microstructural changes, and by the values of the mechanical properties, a fact confirmed by the microscopic and photographic images from the significant times.

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The increase in resistance to cavitation, conferred by heat treatment regimes, justifies the possible use of aluminum alloy type 6082 in order to expand applications to parts that work in cavitation regimes, such as pump rotors and boat propellers.