Life cycle assessment of direct recycling hot press forging of aluminium AA7075 metal matrix composite

The primary objective of this research is to investigate the process of direct recycling of AA7075 aluminium alloy, which is extensively utilised in the aerospace and flight sectors due to its exceptional strength and lightweight characteristics. Alumina (Al2O3) is used as a reinforcing agent and the effect of hot press forging (HPF) parameters on the mechanical characteristics and surface integrity of the metal matrix composite (MMC) constructed of AA7075 alloy with 1% Al2O3 has been studied. Furthermore, the utilisation of an integrated life cycle assessment (LCA) approach is implemented to assess the environmental impacts and economic expenses associated with the recycling of aluminium via high-pressure forming for both the metal matrix composite and AA7075 alloy. Response surface methodology (RSM) is applied to ascertain the optimal parameters for high-performance filtration. The findings suggest that employing a forging temperature of 532.34 °C and a holding time of 60 min produces favourable results. When comparing the characteristics of the MMC and recycled aluminium, it can be observed that they both demonstrate similar essential process attributes. The utilisation of HPF in conjunction with the Multi-Material Composite has the potential to yield a reduction of up to 24.97% in Global Warming Potential (GWP). This research demonstrates the efficacy of HPF as a viable approach for environmentally conscious and economically efficient recycling of AA7075 aluminium scrap, thereby improving product performance and promoting sustainability.


Introduction
The aluminium industry is at the forefront of advocating for environmental sustainability, as it addresses the increasing energy consumption requirements and actively encourages the utilisation of recycled metals.Aluminium recycling offers substantial energy and emission reductions while maintaining the material's inherent properties without being compromised.Nearly three-quarters (75%) of the aluminium produced in the United States remains in active use [1].A significant amount of aluminium continues to be disposed of in landfills, which can be ameliorated by implementing post-consumer recycling practises.The objective is to achieve a 45 percent decrease in carbon dioxide (CO 2 ) emissions by 2030 [2].Numerous techniques are employed in industrial settings to recycle aluminium, with the melting technique, also known as the conventional technique, being the most widely utilised.The conventional technique is widely utilised, yet it exhibits environmental inefficiencies stemming from its inherent losses and impact on the surrounding environment.
A novel methodology has been suggested as a substitute for traditional recycling practises.The direct recycling method can reduce average metal loss during the remelting phase by approximately 20%.This can be achieved by implementing measures to prevent intensive oxidation on the molten metal surface, minimise burning, and avoid mixing with the slag that is removed from the surface of the ladle.Solid-state or direct aluminium recycling has the potential to mitigate environmental risks and minimise energy consumption compared to traditional recycling methods.Moreover, this measure is anticipated to effectively mitigate the Global Warming Potential associated with carbon emissions.The removal technique induces metal loss and oxidation, generating a reduced-density chip.During each stage of the process, there will be a certain degree of material loss, resulting in a maximum recovery rate of 54 percent for the metal.
Aluminium metal matrix composites (AMMCs) refer to incorporating other metals, organic compounds, or ceramics into the aluminium matrix to enhance its properties.Based on the reference findings [3], it was reported that the worldwide production of MMCs in 2008 amounted to 2700 metric tons.The growing demand for lightweight and durable alternative materials in the aerospace and ground transport sectors is a significant factor driving the MMCs market.The potential cost savings for airlines can be significant when reducing the weight of commercial aircraft, with each kilogram reduction corresponding to a savings of $700.Consequently, the investigation of MMCs emerges as a viable option in pursuing cost reduction and environmental sustainability within the aviation sector.The Metal Matrix Composite utilised in this study involves the amalgamation of Aluminium Alloy 7075 with a 1% alumina concentration.Alumina, called aluminium oxide (Al 2 O 3 ), can refine grains, and achieve homogenous distribution within the aluminium matrix.Incorporating alumina as a reinforcing agent can potentially enhance the material's hardness and Ultimate Tensile Strength (UTS).
This paper will additionally encompass the LCA of the recycling process for aluminium alloy by using hot press forging.The LCA is a comprehensive analysis method that evaluates the environmental impacts of a product throughout its entire life cycle, encompassing all stages from the extraction of raw materials to processing, manufacturing, distribution, and eventual use [4].The achieved outcome is compared to the recycled aluminium alloy AA7075, which has undergone the direct recycling, hot press forging (DR-HPF) process without reinforcement, specifically alumina.The characteristics of AA7075 are presented in table 1.
This study examines the impact of different hot press forging parameters on the mechanical properties and surface integrity of a metal matrix composite of aluminium alloy 7075 and 1% Al 2 O 3 .The examination entails the utilisation of the RSM to optimise the hot press forging parameters and ascertain their influence on the mechanical properties.In addition, this study aims to compare the LCA of two distinct aluminium recycling routes: the direct recycling hot press forging method applied to the metal matrix composite, and the direct recycling hot press forging method applied to aluminium alloy AA7075.The influence of temperature is widely acknowledged as a crucial variable in examining and managing aluminium alloys [5,6].In order to substantiate these assertions, the present study aims to examine the impact of parameter configurations within the direct recycling hot press forging (DR-HPF) technique on both mechanical characteristics and surface integrity.The optimisation of parameter settings in the high-pressure forming process is of utmost importance to improve the mechanical properties and surface integrity of recycled AA7075 aluminium billets.

Materials and methods
The present study provides a detailed outline of the experimental procedures in the Materials and Methods section.The main emphasis lies in recycling MMC (aluminium alloy AA7075 + 1% Al 2 O 3 ) through the HPF method.The process parameters under investigation include variations in the operating temperature and holding time, as shown in table 2. A series of tests were conducted to assess the recycled specimens' mechanical properties and surface integrity to improve their overall quality.The analysis of mechanical properties encompassed the assessment of two key parameters: the UTS and the elongation to failure (ETF).The surface integrity analysis encompassed the examination of the microstructure, the conductance of grain analysis, the evaluation of fractures, the determination of density, and the measurement of microhardness.To acquire the most favourable parameter settings, the RSM was utilised, employing Design Expert 13.0 software.The variables utilised in the optimisation procedure encompassed UTS, microhardness (MH), and GWP.Furthermore, the life cycle assessment of the direct recycling DR-HPF process was determined using SimaPro 9.2.0.2 software.A comparative analysis was conducted on the GWPvalues derived from using DR-HPF in the MMC and recycled aluminium alloy to evaluate the environmental impact.

• Material Preparation and Processing
To ensure the quality of recycled aluminium chips derived from the machining of semi-finished products, appropriate cleaning and preparation are required.The chips were meticulously cleansed with a solution of 99.5% pure acetone before being dried in a 60 °C thermal oven [1][2][3]5].This cleansing procedure conforms to the ASTM G131-96 standard practises.Mixing Aluminium AA7075 with alumina (Al 2 O 3 ) powder was the subsequent phase in enhancing the recycling process.Using a 3D mixing machine, the AA7075 Aluminium Alloy was mixed with 99.99% pure alumina powder with a particle size of 1 μm and a percentage of 1.0%.The subsequent recycling procedure utilised a laboratory hot press forging machine with a constant pressure of 35 tonnes and a four-time pre-compaction cycle (figure 1).To accomplish the desired properties, various temperatures and holding times were employed during the HPF process (figure 2).The operating temperatures specified ranged from 450 to 550 °C, and the holding times ranged from 60 to 120 min.
Based on the recommendations of previous researchers and AA7075's solidus and recrystallization temperatures, these parameter settings were determined [3][4][5]7].The resultant recycled specimens, known as T6-temper, were compared with values obtained from DR-HPF aluminium alloy to evaluate alumina's potential properties.
During the HPF process, the mould was loaded with cleaned, dried, and mixed aluminium chips and the plunge was fixed to undertake the forming operation.The HPF operating cycle consisted of 120 min of heating the mould and 30 min of holding to ensure even heat distribution.
During the pre-compacting cycle (PCC), where the plunge was repeatedly struck with 350 KN force for several cycles, the temperature and force remained constant.After turning off the heat, the plunge was maintained at maximal force to initiate the cooling phase [2,6,8].

• Mechanical Characterization
The strain was measured using a universal testing machine (UTM), the Shimadzu Servopulser, Kyoto, Japan, EHF-EM0100K1-020-0A, at room temperature with a 25 kN load and a gauge length of 25 mm [9,10].The experiments were conducted at a rate of 0.5 mm per second.The surface microstructure of the specimen was examined using a SU1510 scanning electron microscope (SEM) manufactured by Hitachi in Ibaraki, Japan.The specimens underwent metallographic preparation for analysis.This involved a sequential grinding process using SiC paper with 240, 600, and 1200 grit sizes.Subsequently, the specimens were polished using DIAMAT polycrystalline diamond cloths with 6 and 1 μm particle sizes.The final step involved finishing the specimens with SIAMAT non-crystalline colloidal silica with a particle size of 0.02 μm.To reveal the grain boundary resulting from dynamic recovery or recrystallization, the specimens underwent etching using Baker's reagent.The grain boundary analysis was performed using the intercept methods at basic magnification at 100x on ASTM-E112 [10,11].
The breakage mode of the recycled specimens exhibited variation based on the parameter setting.The fracture analysis was performed utilising a JSM-7600F field emission scanning electron microscope (FESEM) manufactured by JEOL in Akishima, Japan.The fracture specimens were sectioned into cubic units measuring 1 cm × 1 cm × 1 cm to facilitate testing.The cause of breakage can be determined by analysing the fracture surface of the fractured material using Field Emission Scanning Electron Microscopy (FESEM).The analysis of fracture surfaces is a valuable method for ascertaining the underlying factors contributing to fractures.
The Vickers macrohardness test was selected as the method for assessing the hardness of the recycled specimens by applying small, controlled loads.The Vickers macro hardness tester manufactured by Shimadzu in  Kyoto, Japan was utilised in the present investigation.The experimental procedure involved the application of controlled force (9.807N) on the surface metal using an indenter for 10 s.Subsequently, the indenter was removed, and the size of the indentation was measured.Each tested specimen was subjected to a minimum of three indentations.The density of the material was determined using the Archimedean method, wherein small pieces extracted from the specimens were weighed in the ambient air and when submerged in water.The specimen's density was determined by employing a density balance (HR-250A, A&D, Seoul, Korea).The measurement data for the DR-HPF process is collected and analysed on the HPF process refers to the aggregate energy encompassing the process.Hence, power and energy are the fundamental parameters that necessitate measurement during the experimental procedure.
Power and energy consumption are quantified for each environmental configuration.The energy, denoted as E (kWh), represents the inventory data obtained after evaluating the input data.The data is inputted into the inventories and processed using Simapro 9.2.0.2.The measurement of the five gases that are associated with Global Warming Potential (GWP) is conducted.The gases present in the context are CO 2 , methane (CH 4 ), sulphur dioxide (SO 2 ), nitrous oxide N 2 O, and perfluorocarbon (PFC-14).The hot press forging aluminium process primarily resulted in the emission of carbon dioxide gases.In contrast, it is not feasible to utilise N 2 O and PFC-14 in this procedure.

Results and discussion
Table 3 represents the response data for the DR-HPF MMC process with different operating temperatures and holding times run in Design Expert 13.0.This data being optimized using RSM to obtain the optimum parameters to carry out DR-HPF for MMC.As part of this study, the data included ultimate tensile strength UTS, HV and GWP.This data was then grouped into mechanical properties and surface integrity.
As shown in figure 3, the study revealed a positive correlation between the UTS and both temperature and duration.The control of temperature and time during processing plays a crucial role in determining the mechanical properties of aluminum alloys.The HPF process requires the application of high temperatures that are carefully controlled to prevent surpassing the melting point for an extended duration.This controlled temperature environment ensures the complete dissolution of the copper and magnesium, phase-forming nearly homogeneous solid solution [8,12].
The structure of the materials is the primary determinant of all physical and mechanical properties.The variance in the UTS between the reference specimen and the recycled specimen is thought to be caused by the different grain sizes.By demonstrating that, at constant holding times, the value of UTS increased up to 26.27-349.13MPa by raising the operating temperature from 450 °C to 550 °C, the results on the influence of operating temperature demonstrate the significant impact of operating temperature on the forging process.The operating temperature was higher than the solvus temperature, leading to redissolving precipitates and the solute solution hardening.This is the main reason for the increase in strength.The solidus and recrystallization temperatures were chosen as the operating forging temperatures for this investigation.The procedure needs high temperature to generate a virtually homogeneous solid solution, but it shouldn't be more significant than liquidus temperature.It also needs enough time for copper or magnesium phases to dissolve fully.The temperature and timing must be carefully managed because either insufficiency or excesses may result in coarsening and lowering precipitation [11,13].Conducting at lower temperatures will result in inferior characteristics since solubility rises as temperature rises [14,15].Furthermore, solid solution strengthening is anticipated to play a significant role at high temperatures.According to [16,17], the tensile tests show considerable hardening brought on by extreme plastic strain created in the materials, in the deformed state.Analysis of the yield strength was based on the independent contributions of intrinsic strength, precipitation hardening, and solid solution hardening.
It was observed that during the process of solutioning, solute atoms were dissolved in the aluminium matrix and maintained in a supersaturated state.Additionally, it was acknowledged that the increase in heating temperature impacted the enhancement of UTS.These minute clusters, known as precipitates, enhance the initial strength by impeding the movement of dislocations.Moreover, a study conducted by researchers reached a similar conclusion, stating that when the temperature is below 470 °C, the resulting strength and hardness are at their minimum.This finding suggests that at such a low temperature, a significant portion of the solute does not fully dissolve into the solid solution.Greater strength and hardness were demonstrated at high temperatures up to 500 °C.
The microstructure study of the operating temperature effect (450, 500, and 550 °C) at maximum holding period 120 min is then shown in table 4 and figure 4. The average grain diameter at the minimum temperature of 450 °C was found to be bigger at 62.8 μm.With an increase in operating temperature, grain fineness reduced.Figure 4(a) shows the porosity of the chips at 450 °C, which means that at this low temperature, the chips are only interlocking rather than being consolidated with one another.
Similar patterns can be observed when examining the impact of holding time on UTS, as shown in figure 3.At an operating temperature of 450 °C, there was a slight increase in UTS (7.55 MPa) when the holding time was extended from 60 min (26.27MPa) to 120 min (33.82MPa).Similarly, at the maximum operating temperature of 550 °C, the UTS value rose 28.33 MPa from 320.80 MPa at 60 min to 349.13 MPa at 120 min.Table 5 and figure 5 provide microstructural evidence regarding the influence of holding time at the maximum operating temperature of 550 °C.
Variations in temperature and holding time significantly influence the microstructure and mechanical properties of metal matrix composites (MMCs) such as Al-7075 [12,18].At this higher temperature, the average grain diameter ranges from 65.8 μm to 46.9μm across different holding times.At the lowest temperature and a holding time of 60 min, the porosity between the chips is barely noticeable, as depicted in figure 5(a).The profiles mentioned above indicate that when the maximum holding time is set at 120 min, the consolidation of the grain boundary is observed to be more proximate among the chips.This observation suggests that the effectiveness of solid-state welding of the chips is higher at the maximum temperature of 550 °C, as depicted in figure 5(c).These findings confirm that at 550 °C, the temperature is sufficient for chip recrystallization, and the holding time has a minimal impact.In contrast, upon comparing the minimum-maximum parameter range (450 °C and 60 min − 550 °C and 120 min), it is evident from figure 6 that the ultimate tensile strength (UTS) exhibits a significant increase from 26.27 MPa to 349.13 MPa.As for comparison the recycled AA7075 with alumina (Al2O3), the maximum UTS peaked at 349.13 MPa (550 °C/120 min) which shows an increment of 33.04% from the value of AA7075 without the presence of alumina; 233.76 Mpa [8,19].The increase in ultimate tensile strength (UTS) indicates that incorporating alumina reinforcement enhances the ability of metal matrix composites to serve as a viable option for recycling aluminium alloy [13].
Table 6 and figure 7 present the grain analysis and microstructure for the minimum (450 °C/60 min) and maximum (550 °C/120 min) parameter settings.The average grain diameter calculated from the number of grains intercepted per unit length (mm) of test lines [10].At the maximum parameter of 550 °C and a holding time of 120 min, the average grain diameter measures 46.9 μm.In contrast, the grain diameter is larger at the minimum parameter, reaching 83.2 μm. Figure 7(a) shows significant porosity between the chips at the minimum parameter.This indicates that the chips merely interlock at low temperatures without proper  consolidation.Conversely, the analysis of recycled aluminium profiles at different operating temperatures reveals that higher temperatures lead to recrystallized grain boundaries.As demonstrated in figure 7(b), the porosity between the chips is less noticeable at 550 °C/120 min.Moreover, the grain boundaries between the chips become closer, highlighting the enhanced solid-state welding of the chips at the maximum parameter.The specimens that underwent cold deformation and were subsequently subjected to solutioning at 550 °C exhibited recrystallization.It is known that higher strains result in smaller recrystallized grain sizes.Variations in   temperature and holding time significantly influence the microstructure and mechanical properties of metal matrix composites (MMCs) such as Al-7075 [8,20] .Higher temperatures and longer holding times lead to finer grain sizes and decreased in porosity, while lower temperatures and shorter holding times result in bigger grain sizes and reduced porosity.The results from tensile tests indicate that the material's structure affects its mechanical properties.A porous structure resulted in lower yield and tensile strength than finer microstructures.This is supported by the Ultimate Tensile Strength (UTS) value of only 26.27 MPa observed at the minimum parameter (450 °C/60 min).The greatest grain size, at 83.2 mm, was discovered.As previously noted, it was decided that the operating temperatures for hot working would fall between the solidus and recrystallization temperatures.The recrystallization temperature for the AA7075 aluminum alloy starts at 450 °C, while the solidus area begins at 580 °C.The second phase of precipitation occurs in the microstructure recrystallization.This precipitation happens by heating the alloy below the solidus line for the appropriate amount of time and temperature.Atoms of the solute disperse to produce minute precipitates.The initial strength is increased because these tiny precipitates prevent dislocation movement.
Figure 8 presents an investigation of the impact of varying temperature and duration parameters, specifically a minimum of 450 °C for 60 min and a maximum of 550 °C for 120 min, on the resulting fractured surface.The fracture behavior of metallic aluminum is contingent upon the parameter configuration employed in the Hot Press Forging process.At lower operating temperatures and holding times, the fractured surfaces appear flat, shallow, and devoid of any dilates.There seems to be no noticeable gross plastic deformation, and fractures do not occur along the crystallographic plane where the normal stress is maximized.Upon closer inspection, these flat areas exhibit fine grooves, ridges, and small cups, as depicted in figure 8(c).Conversely, numerous micro voids and dimples are observed at the maximum parameter, indicating a ductile fracture mode.The image shows that fracture occurs along the crystallographic plane, known as the cleavage plane, with no plastic deformation where the normal stress is maximized [15,17,21,22].
The formation of metallic bonding between chips is incomplete due to inadequate temperature and duration of holding.The reduction in temperature has been observed to result in minimal or negligible oxidation and enhanced precision in dimensional measurements.However, it also induces an elevation in flow stress and a reduction in ductility, thereby facilitating the occurrence of cracking phenomena [11,18,19].
Figure 9 shows the results of density with different operating temperatures and holding times while figure 10 shows the results of hardness and UTS with different operating temperatures and holding times.In comparing the minimum-maximum parameter range (450 °C/60 min − 550 °C/120 min), the density value increased from 2.663 g/cc to 2.791 g/cc This can be attributed to the microstructure analysis: the noticeable porosity in figure 4(a) results in a lower density value at the minimum parameter, while the closer grain boundary structure observed in figure 5(b) at the maximum parameter leads to a higher density.The maximum density value observed (2.791 g/cc) is higher than the reference value of 2.81 g/cc for AA7075 undergoes DR-HPF.There is no clear evidence in previous studies that forging aluminium alloy 7075 directly affects porosity.However, found   that during forging, micro and macro flaws such as folding, underfilling, broken streamline, fracture, and coarse grain can occur due to the deformation properties of aluminium alloys.
The similar increasing trend for hardness influenced by holding time.At an operating temperature of 450 °C, the hardness value marginally increased from 80.43 HV with a holding time of 60 min to 104.77 HV with a holding time of 120 min.Conversely, at the maximum operating temperature of 550 °C, the hardness value increased from 109.23 HV at 60 min to 122.97 HV at 120 min.Below 550 °C, there was a tendency for hardness to increase with longer holding times, but the trend was opposite for temperatures above, which is consistent with the findings of various researchers [23].Furthermore, when comparing the minimum-maximum parameter range (450 °C/60 min − 550 °C/120 min), the hardness value increased from 80.43 HV to 122.97 HV.Notably, the maximum hardness value peaked at 122.97 HV (550 °C/120 min), representing a 43.87% increase compared to the value of hardness of recycled aluminium AA7075 without alumina, which is 69.02HV.The observed changes in Ultimate Tensile Strength (UTS) and hardness can be attributed to variations in microstructure induced by different forging parameters.Studies by Amanda et al (2023) highlight the influence of grain size on mechanical properties, where a finer grain structure typically results in higher UTS and hardness [20].This is in line with the findings of various researchers [23], as evident in our study.Furthermore, Amanda's work emphasizes that grain refinement significantly affects the properties of magnesium and its alloys, leading to improved strength as expected from the Hall-Petch trends.
• Global Warming Potential Figure 11, shows the results of GWP for different operating temperatures and holding times.The highest temperature, 550 °C and holding time of 120 min gives the maximum GWP value, 22.63 kg CO2-eq/kg, while the minimum value is 18.99 kg CO2-eq/kg at the lowest temperature and holding time, 450 °C and 60 min.On the other hand, when the temperature increased, the GWP values subsequently increased.The same trend goes to the holding time.

• Modelling and Optimization Using Response Surface Methodology
The data of UTS, Hardness value, and GWP collected are optimized in Design Expert 13.0 to obtain the optimum pa-rameter to conduct the DR-HPF for MMC aluminium alloy AA7075 + 1% Al2O3.The suggested operating temperature is 532.34 °C for 60 min with 74.6% desirability.The predict-ed UTS is 331.05MPa, the Hardness value (105.92V) and GWP is 20.34 kg CO2-eq kg −1 .The prior analyses focused on the individual responses of UTS, HV, and GWP.The aim of obtaining the feasible optimization region for all responses and factors can be accomplished through overlaid contour plots.
This approach entails superimposing contour plots of individual responses and identifying the region that optimiz-es each response variable.The overlay plot illustrating the implementation of RSM for streamlining is depicted in figure 12.The shaded regions depicted in the graphical optimiza-tion plot fail to satisfy the specified criteria for determination.The lines demarcate the upper and lower thresholds of the reactions.The highlighted yellow area illustrates the feasible range for adjusting the components to meet all reactions' requirements.Based on the information presented in the figure 12, it can be inferred that the desirable attribute would be achieved    • Comparison of Life Cycle Assessment Figure 13, shows the endpoint categories, while figure 14, shows the midpoint impact categories of the HPF the optimum parameter HPF process for MMC aluminium alloy + 1% Al 2 O 3 and 0% Al 2 O 3 .As previously stated, the endpoint impact indicator aggregated all midpoint environmentally stress into three categories.Both recycling routes gave the highest endpoint impact on human health followed by resources and ecosystems.There are 18 midpoint categories in total, however only 6 that give the most significant midpoint impact to this study based on figure 14 that are climate change followed by fossil depletion, particulate matter, human toxicity, climate change and metal depletion.
The effect of climate change on human health is the main cause of most of the endpoints in this category as per impact category.The climate change issue, as defined by equivalent greenhouse emissions, is currently regarded as one of the most significant environmental issues.Carbon dioxide emissions from the combustion of fossil fuels are primarily responsible for aluminium extraction and recycling [21,22].The emission of CO 2 from the melting process of aluminium is a significant contributor to climate change and can have indirect impacts on human health endpoints within the context of LCA.

Conclusions
The direct recycling of AA7075 aluminium alloy + 1% Al 2 O 3 by employing DR-HPF significantly influences the material's mechanical properties, surface integrity, and global warming potential.The following is a summary of the findings.The UTS and ETF values increased as the forging temperature and holding time increased.Minimum 450 °C/60 min to the maximum (550 °C/120 min) process parameter, the UTS values increased by 92.5%.The UTS had the highest values at the maximum process parameter 550 °C/120 min, with 349.13MPa.
The maximum UTS peaked at 349.13 MPa representing an increase of 33.04% over the recycled aluminium through DR-HPF without the presence of alumina.The grain and voids size between the aluminium chips were reduced by increasing the forging temperature and holding time.At the low parameter, the porosity between the chips is visible where the chip is simply interlocking rather than consolidated.Meanwhile, the maximum forging temperature resulted in a crystallized grain boundary, with less porosity between the chips and a closer grain boundary orientation between the chips.
Fracture analysis at minimum parameter appeared flat, shallow, and without any dimples indicating brittle deformation.The fractured surfaces revealed numerous micro voids and dimples at the maximum parameter, indicating the ductile mode.The hardness and density values increased with the increasing operating process parameter.The percentage of hardness and density increased from the minimum to maximum process parameter by 4.59% and 34.6%, respectively.The hardness values reach their highest peak at the maximum parameter (122.97HV) comparable to the reference specimen.The maximum density at the maximum parameter exhibited 2.791 g cm −3 , higher than the reference specimen value (2.81 g cm −3 ).The higher grain boundaries, less porosity in microstructure, smaller grain size, and higher density resulted in higher hardness value.The ANOVA analysis from statistical modelling and optimization RSM on the effect of process parameters on mechanical properties (UTS and HV) shows that the operating forging temperature is the most significant factor influencing the investigated response variables.The holding time parameter gives the investigated GWP response variables the most significant factor.RSM shows the optimal parameter of the HPF process at 523 °C operating temperature and 60 min holding time with desirability of 0.746.
Both recycling routes gave the highest endpoint impact on human health, followed by resources and ecosystems.Meanwhile, the six environmental stress that gave the most significant midpoint impact on both recycling routes is climate change followed by fossil depletion, particulate matter, human toxicity, climate change, and metal depletion.

Figure 3 .
Figure 3. Results at different operating temperatures and holding time for ultimate tensile strength.

Figure 4 .
Figure 4. Microstructure on the effect of operating temperature at maximum holding time 120 min at 100x; magnification for temperature (a) 450 °C, (b) 500 °C; and (c) 550 °C.

Figure 5 .
Figure 5. Microstructure on the effect of holding time at maximum operating temperature 550 °C at 5x at time (a) 60 min, (b) 90 min; and (c) 120 min.

Figure 9 .
Figure 9. Results of density with different operating temperature and holding time.

Figure 10 .
Figure 10.Results of hardness and UTS with operating temperature and holding time.

Figure 11 .
Figure 11.Global warming potential (GWP) for different operating temperature and holding time.

Figure 13 .
Figure 13.The endpoint categories of the optimum parameter HPF process for MMC aluminium alloy + 1% Al 2 O 3 and 0% Al 2 O 3 .

Figure 14 .
Figure 14.The midpoint impact categories of the HPF the optimum parameter HPF process for MMC aluminium alloy + 1% Al 2 O 3 and 0% Al 2 O 3 .

Table 1 .
The properties of AA7075 aluminium alloy that undergoes hot press forging.

Table 2 .
The HPF process parameter for operating temperature and holding time.

Table 3 .
Data responses at different operating temperatures and holding times run in design expert 13.0.

Table 4 .
Grain analysis on the effect of operating temperature at maximum holding time 120 min at 100x magnification.

Table 5 .
Grain analysis on the effect of holding time at maximum operating temperature 550 °C at 100x magnification.
). Final response equation for UTS, HV and GWP.