Solid-state Recycling Research Articles

A new sustainable direct solid state recycling of AA1090 aluminum alloy chips by means of friction stir back extrusion process

Friction stir extrusion is an innovative process designed to recycle metal chips from various machining operations. In this research, the feasibility of solid-state recycling of pure aluminum AA1090 machining chips using FSE process is investigated. In the early stage, a FE simulation was conducted in order to optimize the die design (spiral scroll of the plunge, hole size and bearing distance) and the process parameters in terms of plunge rotational speed and extrusion rate. The AA1090 aluminum chips were produced by turning off an as-received bar without lubrication. The chips were compacted on a MTS machine up to 150KN of load. The resulting chip-billets had a diameter of 40mm and 30mm high. The chip-based billet was FS Extruded at 1000rpm rotational speed and 0.8mm/s of plunge displacement. The extruded samples were analyzed by optical microscope in order to see the material flow and to characterize the microstructure. Finally, micro-hardness Vickers profiles were carried out, in both longitudinal and transversal direction, in order to investigate the homogeneity of the mechanical properties of the extrudate.

Microhardness and Microstructure of Hot Extrusion Parameters in Direct Recycling of Aluminium Chip (AA 6061) by ANOVA Method

Microhardness and microstructure become a main concern in direct recycling of metal chips. Products by solid-state recycling of aluminum chips in hot extrusion process are controlled by many factors such as die geometry, extrusion ratio, temperature related parameters and etc. in which each of them could influence the extrudate’s quality. This study investigates the effects of preheating temperature and preheating time on the response variable (microhardness and microstructure). The various settings of preheating temperature were taken as 450 °C, 500 °C, and 550 °C. On the other hand, three values of preheating time were chosen (1, 2, 3) hours. The influences of the process parameters (preheating temperature and time) were analyzed using Design of Experiments (DOE) approach whereby full factorial design with center point analysis was adopted. The total runs were 11 and they comprise of two factors of full factorial design with 3 center points. The results found that the preheating temperature is more important to be controlled rather than the preheating time in analysis of microhardness and decreasing of temperature led to the high microhardness. The profile extruded at 450 °C and 1 hr had gained the optimum microhardness and it can be concluded that setting temperature at 550 °C for 3 hour resulted in the highest responses for average of grain sizes in analysis of microstructure.

Energy Demand Reduction Of Aluminum Alloys Recycling Through Friction Stir Extrusion Processes Implementation

Abstract Aluminum alloys are characterized by high-energy demands for primary production. Recycling is a well-documented strategy to lower the environmental impact of light alloys production. Despite that, conventional recycling processes are still energy-intensive with a low energy efficiency. Also, permanent material losses occur during remelting because of oxidation. Recently, several solid-state recycling approaches have been analyzed; in fact, by avoiding the remelting step both energy and material can be saved and, therefore, the embodied energy of secondary production can be substantially reduced. In this paper, the solid-state approach Friction Stir Extrusion (FSE) is analyzed for aluminum alloys recycling, the primary energy demand of such recycling strategy is quantified. Comparative analyses with both conventional and direct extrusion based processes are developed.

Aluminium sheet metal scrap recycling through friction consolidation

Abstract In the last decades, several direct-recycling techniques have been developed and investigated in order to avoid material remelting, typical of the conventional aluminum alloys recycling processes. Moreover, the remelting step for aluminum recycling is affected by permanent material losses. Solid-state recycling processes have proven to be a suitable strategy to face such issues. Friction Consolidation is an innovative solid state-recycling technology developed for metal chips. During the process, a rotating die is plunged into a hollow chamber containing the material to be processed. The work of friction forces decaying into heat soften the material and, together with the stirring action of the die, enable solid bonding phenomena producing a consolidated metal disc. This technology has been successively applied to metal chips; in this paper, the feasibility of the process to recycle sheet metal scrap is investigated. The quality of the obtained billets is evaluated through morphological observation and hardness measurements.

Solid state recycling of aluminium AA6061 alloy chips by hot extrusion

In the present study, aluminium AA-6061 alloy turning chips recycled using solid-state recycling technique through cold compaction followed by hot extrusion. The effect of the extrusion ratio (ER) and extrusion temperature (ET) on the microstructure evolution, the physical properties and the mechanical properties of the produced samples were studied. Moreover, the results were compared with those of the as-received samples extruded under the same conditions (reference samples extruded under ER = 5.2 and different ET = 350, 450 and 500 °C). Approximately dense samples with reasonable tensile strength, elongation, and microhardness were obtained. The tensile properties (strength and elongation) of recycled samples increased with the increase of the ER and ET those improve the bonding between chips. The chip samples processed under ER of 12.8 and ET of 500 °C have superior mechanical properties compared to the reference samples. The fracture surfaces of the recycled samples consisted of a combination of micro-cracks and equiaxed dimples according to the degree of the bonding of the chips.

Adaptive neuro-fuzzy and regression models for predicting microhardness and electrical conductivity of solid-state recycled EN AW 6082

In the last few years, there is a demand for developing new technologies in order to increase scrap reuse potential and CO2 emission savings. In this paper, aluminum was recycled from chips obtained by machining without any remelting in order to reduce environmental pollution and to increase material yield during the process. This process is called solid-state recycling (SSR) or direct recycling. SSR process consists of chips cleaning, cold pre-compaction, and hot direct extrusion followed by equal channel angular pressing (ECAP) at different temperatures. Influence of direct extrusion temperature, ECAP temperature, and number of ECAP passes on electrical conductivity and microhardness of the recycled EN AW 6082 aluminum chips was investigated. Microhardness and electrical conductivity of the recycled samples were comparable with commercially produced EN AW 6082. Experiments were planned utilizing design of experiments approach. Both adaptive neuro-fuzzy interference system (ANFIS) and regression models were developed and compared to describe the influence of input SSR process parameters on electrical conductivity and microhardness. Density and metallographic analysis of the recycled samples were also performed.

Microstructure and Mechanical Properties of AZ31B Magnesium Alloy Prepared by Solid State Recycling

Abstract Machined chips of AZ31B magnesium alloy were consolidated by cold pressing followed by hot extruding at different processing temperatures and extrusion ratios. Compared with ingot extruding alloy, the influences of processing technique on mechanical properties of recycled alloys were analyzed from two main aspects involving the dynamic recrystallization microstructure and the bonding condition between chips. Results show that with the increasing extrusion temperature, the ultimate tensile strength and elongation to failure of the recycled alloys firstly increase and then decrease. The combined action of grain growth and improvement of the bonding between chips with the increasing extrusion temperature results in the variations of mechanical properties of the recycled alloy. With the extrusion ratio increasing from 4:1 to 44:1, the grain is refined, and the bonding between chips is enhanced. This contributes to the increased ultimate tensile strength of recycled alloy. Whereas elongation to failure decreases when extrusion ratio is higher than 25:1 due to the significant deformation strengthening.

Effect of extrusion temperature on the surface roughness of solid state recycled aluminum alloy 6061 chips during turning operation

Nowadays solid state recycling of aluminum chips provides a competitive market for the aluminum foundry. Chip recycling requires an efficient pretreatment system in terms of optimizing consolidating technique to achieve a high metal recovery with rapid return of investment and hence drive down fixed cost and increase productivity without compromising on metal quality. Solid state recycling through hot extrusion is a promising technique to recycle machining chips without re-melting. In this article, aluminum chips were cold compacted followed by hot extrusion under extrusion ratio of 5.2 and at three different extrusion temperatures of 350°C, 425°C, and 500°C. The effect of extrusion temperature on the surface roughness (Ra and Rz) and machining time needed to remove a unit volume of aluminum alloy 6061 machined by turning were investigated. The effect of extrusion temperature was investigated using the Box–Behnken design; three machining parameters (depth of cut, feed rate, and cutting speed) on the surface roughness of the machined specimens were also examined. Obtained results were in favor of the presence of significant effect of those four parameters on specimen surface roughness with dominating effect of feed rate. Moreover, the optimum cutting conditions for minimum machining time per unit volume (Tm) while Ra is kept below an industrially accepted value of 0.8 μm was found for the original alloy for the following cutting conditions: cutting speed of 116 m/min, depth of cut of 0.33 mm, and feed rate of 0.11 mm/rev.

On the Role of Processing Parameters in Producing Recycled Aluminum AA6061 Based Metal Matrix Composite (MMC-AlR) Prepared Using Hot Press Forging (HPF) Process.

Solid-state recycling, which involves the direct recycling of scrap metal into bulk material using severe plastic deformation, has emerged as a potential alternative to the conventional remelting and recycling techniques. Hot press forging has been identified as a sustainable direct recycling technique that has fewer steps and maintains excellent material performance. An experimental investigation was conducted to explore the hardness and density of a recycled aluminum-based metal matrix composite by varying operating temperature and holding time. A mixture of recycled aluminum, AA6061, and aluminum oxide were simultaneously heated to 430, 480, and 530 °C and forged for 60, 90, and 120 min. We found a positive increase in microhardness and density for all composites. The hardness increased approximately 33.85%, while density improved by about 15.25% whenever the temperature or the holding time were increased. Based on qualitative analysis, the composite endures substantial plastic deformation due to the presence of hardness properties due to the aluminum oxide embedded in the aluminum matrix. These increases were significantly affected by the operating temperature; the holding time also had a subordinate role in enhancing the metal matrix composite properties. Furthermore, in an effort to curb the shortage of primary resources, this study reviewed the promising performance of secondary resources produced by using recycled aluminum and aluminum oxide as the base matrix and reinforcement constituent, respectively. This study is an outline for machining practitioners and the manufacturing industry to help increase industry sustainability with the aim of preserving the Earth for our community in the future.

Weld strength in solid–state recycling of aluminum chips

AbstractIn solid‐state recycling, chip morphology related parameters such as size fraction, surface topography and geometry are important factors in resulting final bond strength. Analyzing deformation parameters together with chip morphology can provide an insight of which factors are very crucial to mechanical performance of the recycled chips. This work investigates the effect of chip morphology and in particular chip roughness and surface area on the weld strength of direct recycled aluminum chips. The influence of these factors were compared with the influences of temperature and pressure. Full factorial design with center point analysis was adopted to rank the factors effects. The chips of AA6061 were cold compacted at 10 tonnes and subsequently hot forged through the dog bone shape‐die at different operating regimes. The elastic and plastic behavior and ultimate tensile strength of the hot‐pressed samples were analyzed and compared. It was found that temperature and pressure are more important to be controlled rather than the chip morphology. Low chip roughness incorporated with high temperature revealed a very significant influence over the weld strength attainment. Regardless of the chip roughness, the bond strength can still be maximized when other deformation factors were controlled within the minimum specified limit.

Solid-State Recycling of Light Metal Reinforced Inclusions by Equal Channel Angular Pressing: A Review

Solid-state recycling of light metals reinforced inclusions through hot Equal Channel Angular Pressing (ECAP) is performed to directly recycle metal scraps and reduce cost of material in engineering applications. The ECAP is one of the most important method in severe plastic deformation (SPD) that can convert light metals into finished products. This paper reviews several experimental and numerical works that have been done to investigate the effects of the ECAP parameters such as die angles, material properties, outer corner angle, friction coefficient, temperature, size of chips, pressing force, ram speed and direct effects of number of passes on the strain distributions. It also includes the performance enhancement of aluminium matrix composite reinforced ceramic-based particles that derived from direct recycled aluminium chips for sustainable manufacturing practices.

Solid-State Recycling of Aluminum Alloy (AA-6061) Chips via Hot Extrusion Followed by Equal Channel Angular Pressing (ECAP)

Aluminum alloy (AA-6061) chips were recycled using hot extrusion followed by equal channel angular pressing (ECAP) process at room temperature. AA-6061chips were cold compacted into billets, then extruded into rods under extrusion ratio of 5.2 at different extrusion temperatures (ET). Finally, the rods were processed through ECAP die with inner angle Φ of 90°, and outer arc angle Ψ of 32.8°, which impose strain ɛ of 1 per each pass up to different number of passes. The effects of the ECAP number of passes and extrusion temperature on the microstructure evolution and mechanical properties were fully investigated. Furthermore, the microstructure and the mechanical properties of the ECAPed samples were compared with those of the extruded samples. Grain refinement were noted after the ECAP process. Moreover, the ECAPed samples revealed higher mechanical properties than those of the extruded samples. The extrusion temperature (ET) and the number of the ECAP passes have an obvious effect on both the microstructure and mechanical properties of the solid state recycled chips samples.

The influence of temperature and preheating time in extrudate quality of solid-state recycled aluminum

Recovering waste metal without the need for remelting in solid-state recycling of metal chips can create green production. The overall process of solid-state recycling should be run at the lowest possible cost to remain competitive. High temperature and prolonged preheating time for billet’s homogenization in hot extrusion to consolidate the chips conflicts with the aim of minimizing energy usage. Therefore, optimizing the effect of preheating temperature and time prior to hot extrusion is important. This study investigates the effects of preheating temperature and preheating time on the extrudates’ quality. Milling chips of AA6061 were cold compacted and hot extruded through a flat-face die using preheating temperatures of 400, 500, and 550 °C for 1–6 h of preheating time. The mechanical and physical properties and microstructure of the extruded profiles were compared. The results revealed that higher acceptable strength and ductility were obtained at 500 °C with 2 h of preheating time. On top of that, temperature increase was the main criterion that results in the highest tensile strength; nevertheless, it was subjected to trade-off in ductility. The profile extruded at 500 and 550 °C had gained a close tensile strength. The study includes the prediction of the chip’s welding quality through the damage evolution on the extrudate’s structure. It was implemented with the help of DEFORMTM 3D finite element method (FEM) software, and the normalized Cockcroft and Latham (C & L) fracture criterion was chosen. The results of the simulations were compared and validated by the experimental results.

Solid-state recycling of light metals: A review

This article provides an intensive review of the past and current research work on the solid-state recycling of light metals. The review includes an experimental aspect of the relevant works that clearly clarify the effects of several critical factors noted as chip preparation, reinforcing phases, die geometry, process parameter selection and performance of miscellaneous methods over the quality of the extruded profiles. Likewise, reviews of numerical and analytical works on the solid-state recycling were presented to understand the strengthening phenomena of chip-based billet through the plastic deformation. Finally, concluding remarks underline challenges of direct recycling method and subsequently highlight the potential future work on making the method as a promising alternative for sustainable manufacturing agenda.

Solid state recycling of pure Mg and AZ31 Mg machining chips via spark plasma sintering

This work investigates the applicability of spark plasma sintering (SPS) as a solid state recycling technique for magnesium alloy scrap. In this respect, machining chips from pure Mg and AZ31 Mg alloy ingots are chemically cleaned, cold compacted and SPSed directly into bulk specimens. It is found that SPS can successfully establish full densification and effective metallurgical bonding between chips without altering compositional constituents. This is attributed to the dynamic compaction during sintering as well as to the disruption of the chips' surface oxide film due to SPS electric current based joule heating. Apart from the successful consolidation, microstructural analysis of the initial Mg ingots, chips and SPS recycled material reveals that the SPS microstructure was finer than that of the original ingots due to significant deformation induced grain refinement during machining. As a result, the recycled materials had a higher compression and shear strength than that of the starting ingot material. The findings indicate that SPS is an effective alternative method for solid state recycling of magnesium alloy scrap.

Effects of chip conditions on the solid state recycling of Ti-6Al-4V machining chips

Five different types of Ti-6Al-4V machining chips were recycled in their solid state using equal channel angular pressing (ECAP). The chips were produced by either turning or milling with or without the application of a coolant. All the as-recycled materials achieved full density but their microstructures, including the prior chip boundary spacing and grain size, are dependent on the starting chip morphologies and dimensions. The differences, however, were largely removed in the subsequent mill-annealing processing consisting of β homogenisation, hot deformation and final annealing, leading to near-identical mill-annealed microstructures for all different types of chips consisting of equiaxed α+β grains with lamellar α, typical following such a treatment. The recycled and mill-annealed materials possess high yield strengths of >900MPa and satisfactory tensile ductility of >11%, comparable or superior to the original commercial mill-annealed Ti-6Al-4V and independent of the initial chip conditions.

Mathematical modeling of solid-state recycling of aluminum chips

This paper deals with the direct recycling of EN AW 2011 aluminum alloy turning chips into semiproducts utilizing forward hot extrusion process. The main goal of this study is to obtain mathematical models that will relate the process parameters of direct recycling to mechanical properties of final semiproducts. Influence of chip size, compaction force, and extrusion temperature on mechanical properties was investigated using Box-Behnken experimental design. The study has shown that the most influential factor on mechanical properties was extrusion temperature. Chip size does not affect both ultimate tensile strength and yield strength and on the other hand, compaction force does not influence on both ultimate tensile strength and percent elongation. Optical microscopy revealed cracks and inhomogeneities for all samples extruded on 300 °C in contrast to those extruded on 500 °C which showed no voids or cracks. Optimal recycling process parameters that would give maximal mechanical properties were found.

Evolutionary in Solid State Recycling Techniques of Aluminium: A review

This paper provides an intensive review on past and current research works in solid state recycling of aluminium and its alloys. The review relates the extrudates quality of the solid state recycled aluminium to certain aspects noted as chips preparation, reinforced materials addition, die geometry, processing parameters, and performance of miscellaneous solid state recycling techniques. Finally, concluding remarks underline challenges for aluminium recycling by the solid state and also highlight the potential future work on making the method as a promising alternative for sustainable manufacturing and hence technologically feasible for industrial implementation.

Experimental Analysis and Microstructure Modeling of Friction Stir Extrusion of Magnesium Chips

In this work, the feasibility to recycle pure magnesium machining chips is first investigated experimentally with a solid-state recycling technique of friction stir extrusion (FSE). Heat generated from frictions among the stirring chips, die, and mold facilitates the extrusion process. Mechanical tests, optical microscopy (OM), and scanning electron microscopy (SEM) analysis are conducted to evaluate the mechanical and metallurgical properties of extruded wires. Mechanical tests show that almost all recycled specimens can achieve higher strength and elongation than original material of magnesium at room temperature. Due to a refined grain microstructure, good mechanical properties are obtained for samples produced by the rotational speed of 250 rpm and plunge rate of 14 mm/min. A metallo-thermo-mechanical coupled analysis is further conducted to understand the effects of process parameters. The analysis is carried out with a multistep two-dimensional (2D) coupled Eulerian–Lagrangian finite-element (FE) method using abaqus. The material constitutive model considers both work hardening and strain softening. Material grain size evolution is modeled by dynamic recrystallization (DRX) kinetics laws. The deformation process and its consequential microstructural attributes of grain size and microhardness are simulated. Physics principles of the microstructure evolution are discussed based on both experimental and numerical analyses.

Solid State Recycling of Aluminium Sheet Scrap by Means of Spark Plasma Sintering

Various solid state or ‘meltless’ recycling techniques have recently been developed for light metal scrap in form of chips. Main objective of all approaches is to bypass the need for remelting in order to reduce the overall required energy, and to avoid the material losses that occur during this step. Within this paper, the use of Spark Plasma Sintering (SPS) is proposed as a novel solid state recycling/welding technique for sheet metal scrap. Aluminium 5182 alloy scrap, derived from sheet metal, was successfully consolidated into a fully dense billet via SPS. The use of pulsed electric current heating, in temperatures well below the alloy melting point, combined with mechanical pressure, enchased the densification process resulting into a void-less material. The recycled SPS sample was fully densified and microstructural investigation has been performed in order to confirm effective oxide film breakage. The results illustrate the effectiveness of SPS in aluminium scrap consolidation, also in form of sheet scrap, providing additional means in solid state recycling. The involved mechanisms that contribute to oxide film fracture and scrap consolidation in SPS are being discussed.Keywords: Aluminium, recycling, spark plasma sintering (SPS)