RRA Treatment Research Articles

Possibilities to modify the properties of the AW7075 aluminum alloy for the automotive industry

The paper investigated the AW7075 aluminum alloy that is used in the automotive industry. The alloy is widely used, among others, in the production of heads and engine blocks. The possibility of obtaining various properties of the alloy (material states) by appropriate heat treatment (saturation and aging) was demonstrated. The results of strength, hardness, abrasion and fracture toughness tests of the alloy in the T73, RRA and HTPP aging treatments, in comparison with the T651 reference state, are presented. The need to select the appropriate parameters of heat treatment in relation to the load conditions of the structural element, especially in elements with notches, was indicated. Depending on the state of the AW7075 alloy, the results prove the wide and diverse possibilities of its use, and should be used consciously in the design and production processes of modern automotive drivetrain components.

The effect of RRA treatment on mechanical properties and wear behavior in vanadium micro-alloyed Hadfield's steel

To solve the ongoing problems of micro-alloyed Hadfield's steel, insufficient hardness and excessive deformation in the early working period, expensive alloying elements like Ti, Nb, and V have been utilized. However, traditional long-time, single-stage aging exacer-bates vanadium to assemble at the edge of initial precipitates, which diminishes estimated effects. For this, here we report the tremendous impact of retrogression and re-aging process (RRA) in this steel by booming the V (C, N) mean sized in 11 nm. The procedure enhances the micro-hardness, yielding strength, tensile strength, and impact-wear resistance for more than 27.4%, 29.5%, 22.2%, and 51.5%, respectively. Moreover, the wear mechanism of RRA samples has been investigated in that micro-sized precipitates surrounded by numerous nano V (C, N) stick in the matrix and hinder the abrasive embedding consistently, thus decreasing the micro-cutting and micro-plowing. This new process maximizes the utilization of costly and nonrenewable vanadium and magnifies the impact-wear resistance under either mild or medium-intensity working conditions.

Fractographic analysis of the AA 7075-T6 alloy subjected to fatigue in the RRA treatment condition and plasma nitride

RESUMO Este trabalho avaliou o comportamento de fratura da liga de alumínio AA7075-T6 submetida a carregamento de fadiga. Ambos os corpos de prova entalhados e não entalhados foram submetidos a um tratamento térmico de retrogression and re-aging (RRA) e nitretação a plasma antes dos ensaios mecânicos. A nitretação a plasma, que tem sido um método escolhido para aumentar a dureza superficial e as resistências à fadiga, desgaste e corrosão de diversas ligas, foi realizada com tela ativa com fonte DC de 20 Hz e ciclo de trabalho de 40 Hz. O tratamento RRA, que é utilizado para aumentar a resistência e a dureza da liga de alumínio AA7075 e obter melhorias nos comportamentos de fadiga e corrosão, foi realizado a 200 °C por 20 minutos com resfriamento a água (resolubilização), seguido de 24 horas a 120 °C (reenvelhecimento). Os ensaios de fadiga axial de corpos de prova entalhados e não entalhados foram realizados com R = 0,1, frequência de 14 Hz, e em nível de tensão de 150 MPa para corpos de prova entalhados e 500 MPa para corpos de prova não entalhados. A fratura por fadiga foi então realizada via estereomicroscopia e microscopia eletrônica de varredura. Para os corpos de prova não entalhados, as condições AA7075-T6 e AA7075-RRA apresentaram características de fratura semelhantes, com boa definição dos três estágios de falha por fadiga; a condição nitretada AA7075 não apresentou evidência de fratura por fadiga típica. Os corpos de prova entalhados apresentaram fractografias semelhantes à condição de concentração de tensão severa, com marcas superficiais convexas à região de nucleação.

Stress corrosion behavior and microstructure analysis of Al-Zn-Mg-Cu alloys friction stir welded joints under different aging conditions

The friction stir welding joints of Al-Zn-Mg-Cu alloy were prepared, then their stress corrosion evolution during slow strain rate tensile testing was studied by in-situ electrochemical impedance spectroscopy. Slow stress corrosion process can be divided into three stages according to the corrosion trend, the corresponding charge-transfer resistance also shows three evolutionary patterns. Grain boundary precipitates in thermal machine affected zone (TMAZ) of the pre-weld RRA treatment are more discrete and coarser, and have a higher Cu content than those of pre-weld double-stage aging treatment TMAZ, which reducing the potential difference with the surrounding substrate and exhibiting higher stress corrosion resistance.

Effect of Dual-stage Ageing and RRA Treatment on the Three-body Abrasive Wear of the AW7075 Alloy

The paper presents an analysis of the influence of the heat treatment state on the abrasive wear of the AW7075 aluminium alloy. To determine the hardening state, the material hardness was measured. It was found that hardness is not the only factor that influences this type of wear. For this reason, the influence of the emerging microstructure is also analysed in the considerations. After tribological tests, microscopic observations of surface features were carried out to determine the dominant mechanisms of surface damage. The results were extended by the hardness distribution carried out on the cross-section. There were no changes in hardness that could be related to either strain hardening or structural changes caused by friction.

Evaluation of the Effect of Dual-Stage Aging and RRA on the Hardening and Corrosion Resistance of AW7075 Alloy

The corrosion resistance of the AW7075 alloy after RRA treatment was evaluated. The corrosion rate in the NSS test was assessed and the electrochemical parameters were determined. The results were compared with the results obtained for the alloy subjected to conventional aging (T6) and dual-stage aging (DA). In order to determine the state of alloy hardening, the tests were carried out simultaneously with the material hardness measurements and microscopic examinations. The hardness of the AW7075 alloy increased in the following order: DA < T6 < RRA. The tests revealed that the achieved increase in strengthening was correlated with the preservation of high resistance to general corrosion of the alloy after RRA treatment. After the corrosion tests, SEM microscopic observations were also carried out to determine the corrosion features. The corrosion rate can be arranged in the following order: RRA < T6 ≈ DA. At the same time, the alloy after RRA was characterized by the lowest value of the corrosion potential and the open circuit. The corrosion potential value can be presented in increasing order: RRA < T6 ≈ DA. The corrosion behavior of the aluminum alloy after dual-stage aging was similar to that of T6. However, it was accompanied by a decline in hardening.

Influence of heat treatment on the microstructure, mechanical and tribological performances of particle reinforced aluminum matrix surface composite fabricated through friction stir processing

In this investigation, TiC reinforcements were incorporated into Al-Zn-Mg-Cu alloy by integrating reverse engineering and geometry reconstruction technique with a surface composite being developed through the recursive method of friction stir processing (FSP). The fabricated composite was further subjected to T6 and RRA heat treatments and its microstructural changes, mechanical and tribological properties examined. Microstructural studies revealed the formation of well refined equiaxed grain structure with a reduced average grain size of 2.40 µm for the FSPed composite and 2.90 and 3.03 µm for T6 and RRA heat treated composites, indicating that grain growth and recrystallization were inhibited effectively by the presence of uniformly dispersed TiC particles during FSP and heat treatments. Improved distribution of hardness with an average of 208.64 HV was observed for the FSPed composite. After T6 and RRA treatments, average hardness was enhanced to 244.29 and 230.26 HV due to the additional strengthening caused by the precipitates. Tribological investigations revealed better wear resistance for the heat treated composites compared to the as-FSPed composite. The presence of a hardened surface with TiC rich tribo-layer and finely distributed precipitates stifled the extent of plastic deformation and improved wear resistance.

Multistage aging treatment influenced precipitate characteristics improve mechanical and corrosion properties in powder hot-extruded 7055 Al alloy

The precipitation evolution, mechanical properties and corrosion behavior of powder hot-extruded 7055 Al alloy subjected to T6, RRA, T76 and T74 aging treatments were investigated. The TEM and DSC results reveal that the dominant intragranular precipitates under those conditions are GP (I, II) + η' phase, GP (II) + η' phase, η' phase and η' + η phases, respectively. The quantitative characteristics (size, interspacing and volume fraction) show that the small volume fraction of intragranular precipitates limits the yield strength of the T6 alloy, while the enlarging of interspacing caused by the coarsening of intragranular precipitates is the main reason for the reduction of yield strength of over-aged alloys. The continuously distributed grain boundary precipitates (GBPs) with low Cu content in the T6 alloy are responsible for higher corrosion susceptibility, while the separated and coarsened GBPs with high Cu content formed in the deepening aging process contribute to the better corrosion resistance. At the same time, the drop in corrosion potential of Al matrix also reduces the corrosion driving force of GBPs with the aging degree deepening. The optimal comprehensive performance of the 7055 Al alloy can be obtained after RRA treatment, namely the highest tensile strength of 739.7 MPa and the acceptable corrosion resistance comparable to that of the T76 state.

The Effect of Heat Treatments and Nickel Additive on The Microstructure and Tensile Properties of 7075 Aluminum Alloy

The aluminum alloys of the 7xxx series consist of Al with Zn mainly, Mg and Cu. 7xxx aluminum alloys has high mechanical properties making it distinct from other aluminum alloys. The effect of adding Nickel and heat treatments on the microstructure, formed phases and tensile properties of the 7075 aluminum alloy were studied in this paper. Different percentages of nickel [0.1, 0.5, 1] wt% was added to 7075 Aluminum alloy, and various heat treatments (artificial aging T6 and Retrogression and re-aging RRA) was applied on the 7075 alloys that containing nickel. The results obtained by applying of RRA treatment were better than the results of T6 treatment, the tensile properties increased and the microstructure became softer by adding nickel to the studied alloys. The maximum tensile strength of 7075 aluminum alloy was (UTS = 437 Mpa) when RRA heat treatment was applied and 0.5% nickel was added.

Modeling of Yield Strength in Retrogression of RRA Treatment of Spray Formed Al-Zn-Mg-Cu Alloy

7075 alloy is the most typical Al-Zn-Mg-Cu alloy and widely used in industry. In the present study the regularities of the change for the size and volume fraction of the precipitates in the retrogression process and the effects of aging time on strengthening of spray formed 7075 alloy were investigated based on the thermodynamics, aging kinetics and hardening theory. The results show that there was a relationship between the parameters of retrogression treatment and yield strength of the alloy. A unified model is presented to establish the quantitative relations between the retrogression process and the yield strengths of spray formed 7075 alloy from the perspective of combining micro and macro.

Investigation on effect of optimized RRA in strength and SCC resistance for aluminium alloy AA7010

7XXX series precipitation hardening alloys have a prominent position in high strength Aluminum alloys which are suitable especially for aerospace applications. Many efforts are put forward by the researchers to achieve increased tensile strength as well as SCC resistance in aluminium 7XXX aerospace alloys. Over aging is a solution for the SCC, but it considerably reduces the strength by 10–15%. As an alternate option, RRA treatment was carried out in AA7010 alloy to improve SCC resistance without significant reduction in strength. Two RRA cycles have been selected for SCC resistance evaluation. Tensile test was performed on RRA treated samples to understand the trend of strength. SCC resistance was evaluated by SSRT in 3.5% NaCl and air environments. A trend of increase in the tensile strength during the retrogression was observed but excessive retrogression temperature resulted in lowering the strength both in L and ST directions. No SCC susceptibility is witnessed in the RA2 samples proving better SCC resistance results with extended retrogression time.

The Effect of Heat Treatments and Nickel Additive on The Microstructure and Hardness of 7075 Aluminum Alloy

The aluminum alloys of the 7xxx series consist of Al with Zn mainly, Mg and Cu. 7xxx aluminum alloys has high mechanical properties making it distinct from other aluminum alloys. In this paper, we examine the effect of adding Nickel and heat treatments on the microstructure and hardness of the 7075 aluminum alloy. Were we added different percentages of nickel [0.1, 0.5, 1] wt% to 7075 Aluminum alloy, and applied various heat treatments (artificial aging T6 and Retrogression and re-aging RRA) on the 7075 alloys that Containing nickel. By applying RRA treatment, we obtained better results than the results obtained by applying T6 treatment, and we obtained the high values of hardness and a smoother microstructure for the studied alloys by the addition of (0.5 wt%) nickel to alloy 7075.

The Effect of Heat Treatments and Nickel Additive on The Microstructure and Hardness of 7075 Aluminum Alloy

The aluminum alloys of the 7xxx series consist of Al with Zn mainly, Mg and Cu. 7xxx aluminum alloys has high mechanical properties making it distinct from other aluminum alloys. In this paper, we examine the effect of adding Nickel and heat treatments on the microstructure and hardness of the 7075 aluminum alloy. Were we added different percentages of nickel [0.1, 0.5, 1] wt% to 7075 Aluminum alloy, and applied various heat treatments (artificial aging T6 and Retrogression and re-aging RRA) on the 7075 alloys that Containing nickel. By applying RRA treatment, we obtained better results than the results obtained by applying T6 treatment, and we obtained the high values of hardness and a smoother microstructure for the studied alloys by the addition of (0.5 wt%) nickel to alloy 7075.

Study of the influence of the RRA thermal treatment and plasma nitriding on corrosion behavior of 7075-T6 aluminum alloy

Aluminum alloys are mainly used in aircraft's structural components, which present the fatigue process as a predominant failure mode. The fact that there are many airports in coastal regions makes the study of aluminum alloy corrosive processes relevant as localized corrosion causes adverse effects on mechanical properties of the material. Plasma nitriding is an alternative method of raising surface hardness, fatigue resistance, wear and corrosion of alloys, there is a trend to expand the applicability of the plasma nitriding process through utilization of new materials and nitriding of aluminum and its alloys would be of greatest interest. In order to improve the corrosion behavior of 7075-T6 aluminum alloy, this work evaluated and compared the effect of RRA thermal treatment and plasma nitriding on the corrosion behavior of AA7075-T6 alloy. The tests were performed in 3.5%wt NaCl solution at room temperature (~25 °C) and pH ~ 6. The linear potentiodynamic polarization study showed that RRA thermal treatment and plasma nitriding were effective in improving corrosion behavior in saline solution. The protective efficiency of plasma nitriding was 64.2% and RRA treatment was 18.4%. The corrosive process at the T6 condition was deeper and generalized, with a high concentration of corrosion pits. For the RRA condition, the process was less generalized and not so deep, while in the plasma nitrided condition it was more localized and superficial, preserving much of the exposed surface in the corrosion tests. These results can be considered very promising by indicating an increase in the useful life of this material. This directly implies a consequent reduction in the preventive maintenance costs of the industries that use this alloy, as replacing damaged parts that have been treated by the plasma assisted technique used in this study may be performed at longer time intervals than the alloy parts without any kind of treatment.

The Effect of Multistage Aging on Mechanical Properties and Microstructure of Forged 7050 Aluminum Alloys

A new four-stage aging treatment, which is composed of pre-aging, retrogression, natural aging (NA) and re-aging, is designed to increase the tensile properties and the fatigue life of forged 7050 aluminum alloy in this work. As compared to traditional T74 and RRA treatments, the four-stage aging-treated sample exhibits the highest tensile properties and ductility. The tensile strength and yield strength are 627 and 608 MPa, respectively, and the elongation is 11.3%. The results of TEM and DSC show that an increased G.P zones and fine η′ phase distributed in the matrix of the alloy are subjected by this new aging processing as compared with T74 and RRA treatments. The natural aging in four-stage aging treatment accelerates the diffusion of Mg vacancies, and promotes the precipitation of G.P zones in grain interior during re-aging stage. On the basis of dislocation shearing mechanism, more dense and fine G.P zones of four-stage aging temper result in the superior tensile strength as compared to T74 and RRA temper. In addition, a combination of shearable precipitates, fine GBPs and narrow PFZs is beneficial to the slip reversibility in the plastic zone at the crack tip, thereby reducing fatigue damage accumulation and increasing the fatigue life.

Microstructure and Mechanical Properties of Spray-Formed 7075 Alloy During Retrogression

The mechanical properties, conductivity, microstructure, precipitation behavior and evolution process of elements and phases of spray-formed 7075 alloy during retrogression are studied by the tensile test, electric test, hardness test, transmission electron microscope (TEM), x-ray diffraction (XRD) analysis, electron-dispersive spectroscopy (EDS) and lattice parameter measurements. The results show that the mechanical properties and conductivity of the alloy can be effectively improved when treated at a suitable retrogression temperature and time. According to the TEM observations, XRD analysis, EDS and an accurate measurement of the lattice parameter, the redissolution of elements and precipitated phases can be readily observed. From the optimization selection of the RRA treatment for the 7075 alloy, the regularity of redissolution and precipitation of the elements and phases are evaluated. The ultimate tensile strength, yield strength, hardness, elongation and conductivity of the spray-formed 7075 alloy after preaging at 120 °C for 16 h, retrogression at 200 °C for 8 min and reaging at 120 °C for 24 h are 791, 736 MPa, 181 HBW, 8.5% and 22.6 MS/m, respectively.

Rhodospirillum rubruml-asparaginase targets tumor growth by a dual mechanism involving telomerase inhibition

Rhodospirillum rubruml-asparaginase mutant RrA E149R, V150P, F151T (RrA) was previously identified to down-regulate telomerase activity along with catalyzing the hydrolysis of l-asparagine. The aim of this study was to define the effect of prolonged RrA exposure on telomerase activity, maintenance of telomeres and proliferation of cancer cells in vitro and in vivo. RrA could inhibit telomerase activity in SCOV-3, SkBr-3 and A549 human cancer cell lines due to its ability to down-regulate the expression of telomerase catalytic subunit hTERT. Telomerase activity in treated cells did not exceeded 29.63 ± 12.3% of control cells. Continuous RrA exposure of these cells resulted in shortening of telomeres followed by cell death in vitro. Using real time PCR we showed that length of telomeres in SCOV-3 cells has been gradually decreasing from 10105 ± 2530 b.p. to 1233 ± 636 b.p. after 35 days of cultivation. RrA treatment of xenograft models in vivo showed slight inhibition of tumor growth accompanied with 49.5–53.3% of decrease in hTERT expression in the all tumors. However down-regulation of hTERT expression, inhibition of telomerase activity and the loss of telomeres was significant in response to RrA administration in xenograft models. These results should facilitate further investigations of RrA as a potent therapeutic protein.

Evaluation and Comparison of the Effects of RRA, T73 and T6 Heat Treatments on Hardness, Tensile and Bending Strengths of 7075 Aluminum Alloy

Evaluation and Comparison of the Effects of RRA, T73 and T6 Heat Treatments on Hardness, Tensile and Bending Strengths of 7075 Aluminum Alloy

Influence of RRA Treatment on the Microstructure and Stress Corrosion Cracking Behavior of the Spray-Formed 7075 Alloy

The effects of retrogression under pre-aging on the microstructure, mechanical properties, and stress corrosion cracking behavior of spray-formed 7075 aluminum alloys were investigated by the transmission electron microscopy, tensile tests, and slow strain rate tests. The results show that as a result of aging at 120°C for 16 h (as pre-aging), the strength of the alloy can be preserved on a high level and the grainboundary precipitates are discrete after retrogression and re-aging treatment. However, the retrogression treatment is uncontrollable by the shortened retrogression period. After retrogression at 200°C for 8 min and re-aging, the ultimate tensile strength, elongation, and the stress corrosion cracking index of the alloy are 791 MPa, 8.5% and 0.155, respectively.

Quantitative Investigation on Nano-Scale Precipitates in an Al-7.5Zn-1.7Mg-1.4Cu-0.12Zr Alloy under Various Aging Tempers

In the present study, a systematic study of nanoscale precipitates in an Al-7.5Zn-1.7Mg-1.4Cu-0.12Zr alloy have been carried out for various typical aging tempers, including T6, T76, T74, T73 and RRA treatments, by combining synchrotron-based small-angle x-ray scattering (SAXS) and transmission electron microscopy (TEM) techniques. Based on the TEM observations, quantitative and statistical structural information, including precipitate size, volume fraction, number density and inter-precipitate distance have been extracted from SAXS data through model fitting. The results show that the T6 peak-aged alloy with the smallest precipitate size has the highest number density and lowest volume fraction of precipitates. Under two-step T7X over-aged tempers, with the deepen of aging degree, the precipitate size, volume fraction and inter-precipitate distance increases, but the number density decreases. The size distribution of precipitates for the RRA-treated alloy is in between that of T6 and T76. The results also show that as the degree of over-aging deepens, the precipitate size distribution interval becomes broader.