Material Characterization and Remelting Behavior of Recycled Aluminum Briquettes Produced from Machining Chips

Abstract

This study presents a material-level characterization of recycled aluminum briquettes produced by cold pressing Al–Si–Mg machining chips and investigates their behavior during subsequent remelting. The study evaluates density, porosity, chemical composition, and metallurgical yield before and after remelting, with the aim of assessing material-related prerequisites for potential metallurgical reuse applications. The cold-pressed briquette (Sample A) exhibited a bulk density of 2.29 g·cm⁻³ and an estimated porosity of 14.6%, attributed mainly to intergranular voids and residual surface contaminants. After melting and resolidification (Sample B), the density increased to 2.388 g·cm⁻³, while the estimated porosity decreased to 10.9%. Handheld ED-XRF analysis indicated no substantial compositional variation within the instrumental uncertainty range after remelting. SEM–EDS observations revealed Al-rich surface regions containing minor oxygen contributions associated with naturally formed surface oxides, while no pronounced intermetallic features were observed at the analyzed surface locations. The remelting process achieved a metallurgical yield of 94.2% with low dross generation. The results indicate that appropriately preprocessed and compacted aluminum machining chips can form mechanically stable briquettes with favorable remelting characteristics and potential applicability in secondary metallurgical processing. However, the present study does not evaluate deoxidation efficiency under molten steel conditions, which remains a subject for future investigation.

1. Introduction

Circular economy concepts and reduced carbon emissions are key priorities of the contemporary metallurgical industry. One of the most effective ways to achieve these objectives is the efficient processing of secondary aluminum resources, particularly machining chips. Aluminum machining chips represent a valuable but technologically challenging feedstock due to residual lubricants, surface oxide layers, and ferromagnetic contaminants. Their direct remelting leads to increased metal losses, intensive dross formation, and less metallurgical stability. Aluminum recycling is among the most energy-efficient metallurgical operations, as secondary production requires up to 95% less energy than primary aluminum production.

Cold pressing of machining chips into compact briquettes is a widely applied approach that improves handling, reduces melting losses, and enables the production of recycled aluminum materials with defined physical properties. Cold-pressed briquettes typically exhibit increased density, reduced bulk porosity, and improved handling characteristics, which may be beneficial for subsequent metallurgical processing. Aluminum is widely used as a deoxidizing agent in steelmaking due to its high affinity for oxygen. The applicability of recycled aluminum-based materials in metallurgical processing depends on factors such as density, remelting behavior, contamination level, and material recovery efficiency. Increasing requirements for material traceability, recycled content declaration, and environmental reporting further enhance the industrial relevance of recycled aluminum-based deoxidizers.

A key prerequisite for the use of recycled aluminum briquettes in steelmaking is their chemical homogeneity and compositional stability. Machining chips often originate from different alloy systems (e.g., AlSi or AlMgSi), which may lead to variations in the content of alloying elements and trace impurities. Collection, cleaning, and compaction processes can further affect elemental distribution, oxide content, and residual steel particle presence. Verification of chemical composition before and after remelting is essential for assessing suitability as deoxidizing additives, since variability in Fe, Mn, or Cr contents may promote intermetallic formation and affect metallurgical behavior.

Aluminum chip recycling, briquetting technologies, oxidation phenomena, and remelting behavior have been widely investigated. Aluminum chips are known to undergo significant oxidation already during machining and handling, which strongly influences their recyclability. Brocq et al. [1] demonstrated that oxide layers develop complex morphologies and progressively increase in thickness with air exposure, while Chen et al. [2] showed that appropriate chip pretreatment and controlled melting conditions can substantially improve the metallurgical quality of secondary aluminum alloys. Briquetting has been shown to effectively improve melting behavior [3]. Pavlásek et al. [4] reported reduced dross generation for high-density briquettes, and Penchev et al. [5] demonstrated that impact-controlled briquetting enhances compaction and metallurgical efficiency. The influence of recycled material content on alloy performance has also been investigated; Vlach et al. [6] showed that an appropriate fraction of recycled chips does not impair mechanical properties provided that material cleanliness and compositional homogeneity are maintained.

Composition-related aspects remain critical, particularly for secondary aluminum alloys. Oxidation, inclusion formation, and their interaction with the melt strongly influence metallurgical quality. Vallejo Olivares et al. [7] demonstrated that compaction reduces oxidation reactions during melting, while Cinkilic et al. [8] highlighted the importance of controlling Mg and Si levels in recycled Al–Si–Mg alloys. Iron contamination has been extensively studied by Nunes et al. [9], who showed that excessive Fe promotes brittle intermetallic phase formation and increased porosity. Similar conclusions regarding intermetallic evolution and impurity control were reported by Javidani and Larouche [10].

From the steelmaking perspective, aluminum deoxidation and inclusion behavior have been widely investigated. Sasai [11] described the kinetics of Al₂O₃ inclusion formation in molten steel, Zhang and Thomas [12] analyzed oxide behavior in melt streams, and Wu et al. [13] demonstrated the influence of Mg on oxide morphology in Al-based deoxidation systems. Additional studies emphasize the importance of stable and compositionally uniform deoxidizers for controlling inclusion characteristics and minimizing reoxidation phenomena [14,15,16,17,18]. In this context, recycled aluminum briquettes may represent a viable alternative to primary aluminum additives, provided that sufficient chemical purity and stability are ensured.

Despite this body of literature, most studies focus on mechanical properties, oxidation losses, or melting yield of recycled aluminum briquettes. Systematic post-remelting verification of spatial chemical homogeneity within individual recycled aluminum briquettes remains largely unaddressed, despite its importance for predictable metallurgical behavior and reliable use as chemically stable deoxidizing additives. In particular, studies combining spatially distributed bulk chemical analysis with localized microstructural verification at the scale of an individual briquette are scarce.

The novelty of this work lies in the systematic, spatially resolved verification of chemical homogeneity within individual recycled aluminum briquettes after remelting, achieved by combining spatially resolved ED-XRF measurements with local SEM–EDS analysis, rather than in the evaluation of melting yield or density alone. Accordingly, this study provides a material-level assessment of recycled aluminum briquettes in the pressed and remelted states. The specific objectives are to analyze the chemical composition of cold-pressed aluminum briquettes produced from recycled machining chips, evaluate spatial chemical homogeneity after remelting by comparing elemental distributions between core and peripheral regions, identify trace elements and potential contaminants, and assess physicochemical prerequisites for their potential use as deoxidizers in steelmaking. The current work is positioned as a pre-screening investigation, establishing a necessary foundation for future laboratory-scale or industrial steelmaking studies. Recent studies further emphasize the importance of controlled remelting and compositional stability of recycled aluminum materials, highlighting the influence of pretreatment, remelting conditions, and chemical uniformity on the metallurgical behavior of secondary aluminum alloys and their performance in steel deoxidation applications [19,20].

Although recycled aluminum materials may be considered for metallurgical reuse applications, the present study does not investigate direct deoxidation behavior under molten steel conditions. Instead, the work focuses on multi-scale characterization of physical properties, chemical composition, and microstructure, including density, porosity, and remelting behavior of cold-pressed aluminum chip briquettes, which represent key parameters for further metallurgical evaluation.

2. Materials and Methods

Aluminum briquettes produced by cold pressing recycled aluminum chips without binding additives were characterized using the procedures described below. The analyzed briquettes were selected from a larger batch produced under identical industrial processing conditions. For characterization of the pressed state (Sample A), representative briquettes prepared under identical conditions were analyzed. For remelting experiments (Sample B), three briquettes were independently remelted and cast under identical laboratory conditions.

Chemical analysis was performed to determine the elemental composition of the recycled aluminum briquettes, including major alloying elements (Mg, Si, Mn, Fe) and selected trace impurities.

The experimental workflow comprised material characterization in the pressed state and after remelting under controlled laboratory conditions. Measurements were conducted to evaluate elemental composition and local compositional variability after remelting.

2.1. Materials

The recycled aluminum briquettes were supplied by FENEGA Ltd. (Presov, Slovakia), which specializes in processing aluminum machining chips for metallurgical applications. The briquettes were manufactured from chips generated during the machining of AlSi- and AlMgSi-type alloys. The input material was collected from machining facilities and mechanically pretreated. Lubricants and coolant residues were removed by centrifugation, while ferromagnetic contaminants were eliminated using magnetic separation. This procedure significantly reduces the content of oils and steel particles that could otherwise negatively affect remelting behavior and increase dross formation.

After mechanical pretreatment, a limited amount of residual lubricants, coolants, and moisture may remain, which is typical for recycled machining chips. The applied centrifugation and magnetic separation correspond to standard industrial practice and significantly reduce organic and ferromagnetic contaminants. Although the exact quantitative determination of residual hydrocarbons and moisture was not the primary objective of this study, their influence is indirectly reflected in the measured metallurgical parameters, particularly the evolution of porosity, dross formation, and metallurgical yield during remelting.

Any remaining organic residues may contribute to gas evolution and increased oxidation during remelting, which can influence dross formation and overall metallic recovery.

After mechanical pretreatment, the chips were cold-pressed at 20–30 °C using a hydraulic press (FENEGA Ltd., Presov, Slovakia) operating at 100–160 MPa. Pressing without binding agents contributed to increased material compactness and stable briquette geometry. The resulting cylindrical briquettes measured 60.5 × 62.9 × 45.7 mm and had an average mass of 322.8 g.

The compaction process was carried out in a single-stage hydraulic cycle consisting of the following steps:

• Filling of the pressing chamber with loose chips;
• Precompression at a low pressure of approximately 10–20 MPa;
• The main pressing step at 100–160 MPa;
• Maintaining the pressure for 3–5 s to stabilize the shape and reduce elastic springback;
• Pressure release and ejection of the briquette from the chamber.

The ram speed ranged from 5 to 15 mm·s⁻¹, which is typical for chip compaction and ensures uniform venting and gradual densification. The total duration of a single pressing cycle was 12–20 s, depending on the chip type and bulk density.

The compaction cycle was designed to promote uniform briquette geometry and consistent densification.

Cold pressing was selected as an energy-efficient compaction method that limits additional oxidation and minimizes alteration.

The compaction process directly influences briquette density, porosity, and metallurgical yield. Compacting chips into a consolidated form significantly affects the melting behavior and overall metal recovery. Higher density and lower porosity lead to [7,20]:

• Reduced oxidation during heating;
• Limited metal loss in the form of fine particulate matter;
• Improved handling and remelting characteristics;
• Potentially higher metallic recovery depending on the melting conditions.

2.2. Sample Preparation

Two sample types were prepared for chemical composition and local compositional variability analysis:

• Sample A—the cold-pressed briquette in its original state

This sample represents an industrial briquette and was used for chemical composition, microstructural, and porosity analyses (Figure 1). For Sample A, three individual briquettes were analyzed in the pressed state to evaluate variability of the measured chemical composition and physical parameters.

The dimensions of the pressed briquette were 60.5 × 62.9 × 45.7 mm with an average mass of 322.8 g.

• Sample B—the remelted and resolidified block

The briquette was melted in a laboratory induction furnace at approximately 850 °C, held for 8–10 min after complete melting, and cast into a simple steel mold. No refining additions, degassing, fluxes, or protective atmospheres were applied during melting. The starting mass of the pressed briquette prior to remelting corresponded to the average briquette mass reported for Sample A. The difference between the average briquette mass (322.8 g) and the input mass used for remelting (290.25 g) reflects the selection of a representative specimen after surface cleaning and removal of loosely bound contaminants prior to melting.

Remelting was performed using representative briquettes prepared under identical processing conditions to evaluate post-remelting compositional variability under comparable laboratory conditions. A laboratory induction furnace operating in ambient air was used, and no inert gas bubbling, rotary degassing, or chemical refining agents were applied. Remelting was performed to evaluate the behavior of recycled chip-based briquettes during melting and resolidification under laboratory conditions representative of simplified remelting practice. Sample B had dimensions of 116.2 × 41.4 × 23.8 mm. The representative remelted sample used for mass balance calculations had a mass of 273.41 g.

Table 1. Mass balance and metallurgical yield of the remelting experiment.

Initial briquette mass [g]	290.25
Recovered metal mass [g]	273.41
Total process loss (including dross and oxidation losses) [g]	16.84
Metallurgical yield [%]	94.2

summarizes the mass balance of the remelting experiment.

The mass balance presented in Table 1 corresponds to a representative remelting experiment. Although multiple briquettes were produced and tested to verify process consistency, a detailed mass balance is shown for one representative sample to ensure clarity. Six measurement points (four corners and two mid-side positions) were defined on the polished cross-section to evaluate local compositional variability after remelting.

2.3. Chemical Composition Analysis (XRF)

The chemical composition of the samples was analyzed using energy-dispersive X-ray fluorescence spectroscopy (ED XRF). Measurements were performed using a handheld XRF spectrometer (DELTA INNOV.X, Olympus Innov-X Systems Inc., Waltham, MA, USA). The instrument is equipped with a 4 W X-ray tube and a large-area SDD detector optimized for sensitivity to light elements (Mg, Al, Si). Handheld ED-XRF was applied as a bulk, non-destructive screening technique suitable for assessing spatial compositional variability at the millimeter scale of industrial briquettes, rather than for high-precision microanalytical determination. Local micro-scale assessment of local compositional variability and potential intermetallic phases was therefore carried out using SEM–EDS (JEOL JSM-7000F, JEOL Ltd., Tokyo, Japan). According to the manufacturer’s specifications, the effective X-ray interaction area (spot size) on the sample surface in Alloy mode was approximately 8 mm in diameter. Measurements were conducted in Alloy mode with an acquisition time of 60 s per point. For each measurement location, three consecutive XRF measurements were performed, and the reported value represents their arithmetic mean. Considering the instrumental uncertainty of the handheld ED-XRF method (approximately ±0.1–0.5 wt.%), the observed compositional variations fall within the expected measurement error. The variability between repeated measurements was low and consistent with the expected experimental uncertainty, confirming acceptable repeatability of the results. Therefore, the results indicate compositional uniformity within the limits of the applied method rather than strict chemical homogeneity. This procedure was independently applied to three different briquettes of the same sample type, resulting in a total of nine measurements per analyzed position. For Sample A, a central point in the core region was analyzed to minimize surface effects related to pressing. For Sample B, six measurement points were evenly distributed at the corners and mid-side positions of the polished cross-section (Figure 2). The measurement points represent different regions of the sample, including central, edge, and surface locations, to capture potential spatial variability in chemical composition. This layout enabled evaluation of local compositional variability between core and peripheral regions after remelting.

The surfaces of the samples were ground using abrasive papers with grit sizes P600 → P1200 → P2500 and subsequently polished to minimize surface effects typical of XRF analysis. The surface was degreased with ethanol. According to the manufacturer’s specifications, detection limit for light elements (Mg, Al, Si) ranged from several to several tens of ppm. Spectra were evaluated using the manufacturer’s software with standard FP corrections and aluminum alloy calibration models. Measurements affected by pronounced surface contamination, unstable detector positioning, edge irregularities, or mechanically damaged surface regions were excluded according to predefined criteria. A total of 2 measurements out of 30 were discarded. No measurements were excluded solely on the basis of compositional deviations.

For each measurement location, the reported chemical composition represents the average value of three repeated measurements. The observed variability remained within the manufacturer-specified instrumental uncertainty range of the handheld ED-XRF instrument. The handheld ED-XRF technique provides semi-quantitative elemental analysis with limited spatial and compositional resolution compared with laboratory analytical methods such as ICP-OES or spark-OES.

2.4. Microstructural and Local Chemical Analysis (SEM–EDS)

Microstructural and local chemical analyses were performed using a field-emission SEM (JEOL JSM-7000F, JEOL Ltd., Tokyo, Japan) equipped with an SDD-type EDS detector (energy resolution 129 eV at Mn Kα). The measurements were performed at an accelerating voltage of 15 kV, a working distance of 10–12 mm, and an electron beam current of 10–15 µA.

Samples were ground, polished, degreased with ethanol, and analyzed without carbon coating. EDS acquisition was carried out with an exposure time of 60 s per point, and each measurement was conducted in duplicate.

EDS spectra were processed using the manufacturer’s software with standard deconvolution and normalization procedures. SEM–EDS was used for local microstructural and compositional assessment, including evaluation of surface oxidation and potential intermetallic phase formation.

2.5. Surface Topography and 3D Profilometry

Macroscopic surface morphology was analyzed using a Keyence VHX-6000 digital microscope (Keyence Corporation, Osaka, Japan). Measurements were performed at magnifications between 50× and 200×, with a vertical resolution of approximately 1 µm.

Prior to analysis, sample surfaces were cleaned to ensure stable optical imaging. 3D models and height profiles were processed using the software supplied with the Keyence VHX-6000 digital microscope (Keyence Corporation, Osaka, Japan).

The method was used to identify surface pores and visible defects associated with briquette morphology and remelting behavior.

2.6. Density, Porosity and Compaction Analysis

Density, porosity, and the degree of compaction are important physical parameters influencing oxidation losses, dross formation, and metallurgical yield of recycled aluminum briquettes.

The reported porosity values reflect remelting without refining or degassing and therefore represent inherent characteristics of secondary aluminum rather than optimized casting conditions.

Multiple volume determination approaches, including geometrical measurement, hydrostatic weighing, and 3D CAD-based estimation, were used to verify density and porosity values. Repeated density determinations using independent methods showed deviations below ±1.5%, indicating good agreement between the applied measurement procedures for both pressed briquettes and remelted samples despite residual microporosity typical of secondary aluminum materials. The use of several methods was not motivated by discrepancies between results, but rather by the need for cross-validation and increased measurement reliability. All applied methods yielded density values consistent within the expected experimental uncertainty range.

• Determination of the bulk density of the briquettes:

When evaluating density, a distinction was made between the theoretical density of the fully dense alloy and the experimentally determined bulk density of the briquette, which is reduced due to residual porosity. The theoretical alloy density used for porosity and compaction calculations was determined using a rule-of-mixtures approach based on the experimentally measured chemical composition obtained by ED-XRF analysis. The calculation considered the major alloying elements (Al, Mg, Si, Mn, Cr, and Fe) using literature density values of the corresponding pure elements. Based on the average chemical composition determined for the pressed briquettes (Sample A), the theoretical density used for porosity calculations was estimated to be approximately 2.68 g·cm⁻³, which is consistent with literature values reported for Al–Mg–Si alloys of comparable composition.

The theoretical density was calculated according to the rule of mixtures using the reciprocal relationship:

$${\mathsf{\rho }}_{\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{o}\mathrm{r}\mathrm{e}\mathrm{t}\mathrm{i}\mathrm{c}\mathrm{a}\mathrm{l}}={\left({\sum }_i{\displaystyle \frac{w_i}{{\rho }_i}}\right)}^{-1}$$

(1)

where $w_i$ is the weight fraction (expressed as a decimal fraction) and ${\rho }_i$ is the density of the respective pure alloying element.

The density of Sample B was determined as the mean value obtained from three independently remelted and cast specimens prepared under identical laboratory conditions, with each specimen measured using the same procedure.

• Geometrical method (for regular shapes):

The geometrical method for determining the volume and density of a cuboid (referred to in the context of metallurgical briquettes as the determination of the apparent bulk density) is a non-destructive technique based on precise measurement of the linear dimensions of the body. The method assumes a regular specimen shape, with surface irregularities minimized through repeated measurements. The dimensions of each specimen (length, width, height, or diameter and height) were measured using a digital caliper at three points along each axis, and the final value was taken as the arithmetic mean. The volume of cylindrical briquettes was calculated according to the following equation:

$$\mathrm{V}=\mathsf{\pi }{\displaystyle \frac{{ d}^2}{4}} \times h$$

(2)

For cuboid-shaped bodies according to the following relationship:

$$\mathrm{V}=a\times b\times c$$

(3)

The bulk density was subsequently calculated as:

$${\rho }_{br}={\displaystyle \frac{\mathrm{m}}{\mathrm{V}}}$$

(4)

where m is the mass of the briquette determined using analytical scales. The resulting bulk density provides an indirect indication of the degree of compaction and residual void content formed during the chip pressing process.

• Hydrostatic weighing:

To increase measurement accuracy, the volume of selected samples was also determined by hydrostatic weighing according to Archimedes’ principle:

$$V={\displaystyle \frac{m_{air}-m_{water}}{{\rho }_{water}}}$$

(5)

The hydrostatic measurements were based on bulk sample behavior and did not allow separate quantification of open and closed porosity. Possible liquid penetration into surface-connected pores may have influenced the measured values.

• Calculation of porosity and degree of compaction:

The volumetric porosity was determined by comparing the measured density with the theoretical density of the alloy:

$$P=\left(1-{\displaystyle \frac{{\rho }_{br}}{{\rho }_{alloy}}}\right)\times 100 (\%)$$

(6)

The reported porosity values should therefore be interpreted as approximate bulk estimates and do not distinguish between open porosity, closed porosity, oxide films, residual contaminants, or non-metallic inclusions.

The degree of compaction was defined as a dimensionless coefficient:

$$C={\displaystyle \frac{{\rho }_{br}}{{\rho }_{alloy}}}$$

(7)

Interpretation of the values:

• C < 0.70—low compaction, high porosity;
• 0.70 ≤ C ≤ 0.90—standard industrial compaction;
• C > 0.90—highly compact briquettes.

• Digital metrology and 3D correlation:

Volume determination was further verified using digital metrology in the PTC Creo CAD (PTC Inc., Boston, MA, USA) environment. The volume was calculated based on 3D scanning and numerical integration, which allowed the elimination of errors caused by surface irregularities. The results for density and porosity were subsequently compared with 3D profilometry and microstructural observations indicating localized surface irregularities and regions of increased apparent porosity.

• Determination of metallurgical yield:

The metallurgical yield of aluminum was determined using a melting test in which the briquette was melted in a ceramic crucible and, after dross removal, cast into a mold. The yield was calculated as:

$$\eta ={\displaystyle \frac{m_{solidifield}}{m_{Input}}}\times 100 (\%)$$

(8)

Reported metallic recovery values for compacted aluminum briquettes commonly depend on melting conditions, briquette density, and processing parameters. A comprehensive analysis of density, porosity, and compaction provides useful information for evaluating briquette quality and optimizing processing.

3. Results

3.1. Analysis of Sample A

3.1.1. Chemical Composition Analysis (XRF)

Sample A represents a cold-pressed aluminum briquette produced from recycled Al–Si–Mg machining chips at pressures of 100–160 MPa. This type of briquette corresponds to an industrially produced recycled aluminum material intended for metallurgical processing applications. The chemical composition of Sample A is summarized in

Table 2. Chemical composition of the cold-pressed briquette (Sample A) determined by ED-XRF. Major alloying elements are reported in wt.%, while trace elements are expressed in ppm.

Measurement	Position	Al [wt.%]	Mg [wt.%]	Si [wt.%]	Cr [ppm]	Mn [ppm]	Fe [ppm]
AM 1	Center	96.70	1.00	1.15	1500	3100	-
AM 2	Center	96.62	1.07	1.12	1400	3300	200
AM 3	Center	96.78	0.96	1.18	1600	3000	100
Average	–	96.70	1.01	1.15	1500	3133	150
Std. deviation	–	0.08	0.06	0.03	100	153	50

. The measured composition of Sample A did not reveal unusually elevated concentrations of alloying or trace elements within the detection limits of the applied ED-XRF method. The observed fluctuation of aluminum content within ±0.1 wt.% is typical for heterogeneous recycled chip-based materials.

The measured composition falls within the typical compositional range of recycled Al–Si–Mg-based materials. The fluctuation of aluminum content within ±0.1 wt.% is typical for heterogeneous recycled mixtures composed of chips of various origins. The elements magnesium and silicon exhibit natural variability due to the non-uniform distribution of individual chips within the pressed volume. Chromium and manganese exhibited comparatively lower variability within the analyzed measurement set. Iron was detected locally in trace amounts, potentially associated with machining-related contamination or residual ferromagnetic particles.

3.1.2. Density, Porosity, and Degree of Compaction of Sample A

The physical parameters of Sample A are important indicators of cold-pressed briquette quality and may influence its behavior during subsequent melting. The density, porosity, and degree of compaction were determined using a combination of precise geometrical measurements, 3D CAD modeling, and surface morphology analysis performed with a Keyence VHX digital microscope.

Geometrical parameters and density determination:

The dimensions of Sample A (60.5 × 62.9 × 45.7 mm) and its average mass of 322.8 g were complemented by an accurate volume approximation derived from a 3D CAD model, which eliminated the effects of surface irregularities on the briquette.

The calculated volume was 141.1 cm³, enabling determination of bulk density:

$${\rho }_A={\displaystyle \frac{\mathrm{m}}{\mathrm{V}}}={\displaystyle \frac{322.8}{141.1}}\approx 2.29 \mathrm{g}·{\mathrm{c}\mathrm{m}}^{-3}$$

(9)

Porosity and degree of compaction:

Comparison of the measured bulk density with the theoretical alloy density enabled estimation of porosity and degree of compaction. The theoretical density used for these calculations (2.68 g·cm⁻³) was determined using the rule-of-mixtures approach described in Section 2.6 based on the average ED-XRF composition of the pressed briquettes.

The porosity was calculated according to the following relationship:

$$P=\left(1-{\displaystyle \frac{{\rho }_A}{{\rho }_{alloy}}}\right)\times 100=\left(1-{\displaystyle \frac{2.29}{2.68}}\right)\times 100\approx 14.6\%$$

(10)

The degree of compaction was 85.4%, corresponding to standard industrial compaction of recycled aluminum briquettes and indicating the presence of intergranular voids within the compacted structure. Such structural discontinuities are typical for briquettes pressed at room temperature and may be reduced by elevated-temperature compaction or additional processing steps.

Surface morphology and pore characteristics:

Figure 3 illustrates surface-accessible pores and microstructural features of the cold-pressed briquette. Due to the heterogeneous lamellar structure formed during chip compaction, this analysis is intended to provide a descriptive assessment of surface morphology rather than a full volumetric pore characterization. The micrograph provides an overview of the macroscopic texture and surface characteristics of the material. Surface analysis performed using the Keyence VHX digital microscope revealed a markedly heterogeneous lamellar texture with a high degree of surface complexity, directly reflecting the origin of the chip-based precursor material.

The image shows a heterogeneous lamellar structure formed by overlapping aluminum chips compressed during cold briquetting. Distinct surface features can be observed, including flattened chip segments, microcracks generated by mechanical deformation, and numerous pores and cavities at the micrometer scale irregularly distributed across the surface. The darker regions may correspond to oxidized surface areas or regions with increased surface roughness, whereas the lighter regions correspond to exposed metallic surfaces. The morphology reflects the relatively high intergranular porosity (14.6%) and confirms the presence of mechanically induced discontinuities that contribute to the reduced bulk density of the briquette.

The physical properties of the pressed briquette may influence its remelting behavior and material recovery during thermal processing, as lower density and increased porosity have a dual effect on its melting behavior. On the other hand, such a briquette tends to remain on the melt surface for a longer time, exhibits a higher susceptibility to surface oxidation (so-called burn-off), and may contribute to increased dross formation. Lower density and higher porosity may increase surface exposure during remelting, potentially contributing to increased oxidation and dross generation. These effects are generally associated with the geometry and compactness of recycled chip briquettes during thermal processing.

Sample A therefore represents a material with a typical degree of compaction found in recycled aluminum chips pressed at room temperature, exhibiting physical parameters relevant to subsequent remelting and metallurgical reuse applications.

3.2. Analysis of Sample B

3.2.1. Chemical Composition of the Remelted Briquette (Sample B)

The chemical composition of Sample B is summarized in

Table 3. Assessment of local compositional variability of the remelted briquette (Sample B) determined by ED-XRF. Major alloying elements are reported in wt.%, while trace elements are expressed in ppm. Reported values are not normalized to 100%, as only selected detectable elements are listed; the remaining balance includes oxygen associated with surface oxides and minor trace impurities not quantified by ED-XRF.

Measurement	Position	Al [wt.%]	Mg [wt.%]	Si [wt.%]	Cr [ppm]	Mn [ppm]	Fe [ppm]
BM 1	Bottom-left corner	97.21	0.49	0.77	910	3600	4100
BM 2	Top-left corner	97.29	0.38	0.85	510	4100	4400
BM 3	Center—left face	97.51	0.55	0.78	880	3800	5300
BM 4	Center—right face	97.50	0.41	0.67	430	3600	6300
BM 5	Top-right corner	97.50	0.33	0.78	380	3600	3800
BM 6	Bottom-right corner	97.46	0.38	0.84	540	3800	5900
Average	-	97.45	0.42	0.80	560	3700	4970
Std. deviation	-	0.11	0.07	0.07	172	132	1010

. The reported values represent averages obtained from representative briquettes remelted under identical conditions and indicate limited compositional variability of the main alloying elements across the remelted material within the uncertainty range of the applied ED-XRF method. No major compositional differences between the analyzed measurement locations were observed after remelting.

To quantify compositional variability after remelting, median values and minimum–maximum ranges were evaluated. Aluminum exhibits a median content of 97.48 wt.% with a narrow range of 97.21–97.51 wt.% (total range 0.30 wt.%), indicating limited compositional variability within the analyzed measurement locations.

Magnesium and silicon show median values of 0.395 wt.% and 0.780 wt.%, with ranges of 0.22 wt.% and 0.18 wt.%, respectively. These variations are consistent with the heterogeneous nature of recycled Al–Si–Mg chip-based materials and do not indicate substantial compositional differences within the analyzed regions.

For trace elements, chromium exhibits a particularly narrow range of approximately 200 ppm, while manganese varies within about 500 ppm. Iron shows the highest variability (0–0.39 wt.%), which can be attributed to local remnants of ferrous contamination from the input scrap rather than systematic compositional redistribution during remelting.

Figure 4 presents the median chemical composition of Sample B based on measurements at six locations on three specimens. Aluminum is the dominant element, with its content exceeding 97 wt.%, while the remaining elements are present in trace to low concentrations. The use of a logarithmic Y-axis enables visual enhancement of the differences between elements occurring at distinct orders of magnitude, providing a clear interpretation of their relative proportions.

The data indicate a low level of compositional variation across the analyzed locations. Compared with Sample A, a moderate increase in iron content is observed after remelting. This increase is attributed to minor ferrous contamination introduced during melting and handling, rather than to elemental segregation within the remelted material. The concentrations of Mg, Si, Mn, Fe, and Cr fall within the ranges typical for secondary Al–Si–Mg alloys produced from recycled chips. These results suggest that remelting reduced large local compositional differences within the analyzed regions and produced a relatively uniform distribution of the measured alloy.

The quantitative assessment of chemical variability is based on the data summarized in Table 3. Figure 4 provides a visual representation of the median elemental composition using a logarithmic y-axis to enable comparison of elements present at different concentration levels. The aluminum content fluctuates within ±0.15 wt.%, which remains within the expected uncertainty range of the handheld ED-XRF method. Similarly stable are the values of Mn (0.36–0.41 wt.%) and Mg (0.33–0.55 wt.%). The slightly higher variability in Fe content may be associated with localized residual ferromagnetic contamination or compositional heterogeneity typical of recycled materials.

Silicon, a key alloying element in AlSiMg type alloys, exhibits stable concentrations within the range of 0.67–0.85 wt.% with no major compositional differences observed between the analyzed core and peripheral regions.

Based on the measured data, Sample B exhibited limited compositional variability within the analyzed measurement locations and within the uncertainty range of the applied handheld ED-XRF method. Differences between the corner and mid-side regions are minimal, suggesting that remelting and resolidification reduced several of the larger compositional differences initially present in the pressed chip-based material.

The highest relative variability was observed for Fe; however, even in this case, the values fall within the typical range for secondary AlSiMg alloys produced from recycled chips. The presence of trace Cr (0.038–0.091 wt.%) is consistent with minor contamination of the input scrap and did not exhibit major localized compositional deviations within the analyzed regions.

The obtained results indicate that the remelted briquettes exhibit compositional characteristics typical of recycled Al–Si–Mg materials and may be considered suitable for further metallurgical evaluation.

3.2.2. Microstructural and Local Chemical Analysis (SEM–EDS) of Sample B

SEM–EDS analysis of Sample B revealed a heterogeneous surface morphology composed of flattened chip segments, microcracks, and surface-accessible pores formed during cold pressing and retained after remelting. Point EDS analysis indicated the dominant presence of aluminum and a secondary oxygen signal, consistent with surface oxidation typical of recycled Al–Si–Mg materials. Figure 5 shows the typical surface texture with local discontinuities and fine pore structures.

The second measurement point (Figure 6) exhibits a similar morphology but with a slightly lower oxygen signal, suggesting local variations in surface oxidation.

The third measurement point (Figure 7) exhibited a similar surface morphology and comparable local chemical characteristics. The slightly elevated oxygen signal may be associated with a local microcrack or pore where oxidation can occur more readily.

Observations in secondary electron (SE) mode (Figure 5, Figure 6 and Figure 7) revealed that the surface of the remelted and resolidified block exhibits:

• A fine-grained metallic structure typical of AlSiMg-type aluminum alloys;
• A regular texture with oriented grinding and polishing traces resulting from sample preparation;
• Local discontinuities, fine micropores, and small material pullouts that may represent preferential sites for oxidation;
• Occasional darker features potentially associated with localized compositional variability or oxidized regions.

The obtained micrographs indicate the formation of a continuous yet slightly porous microstructure characteristic of secondary aluminum, without large voids or segregation defects. The detected micropores are typical for secondary aluminum materials, particularly after remelting of chip-based feedstock with increased specific surface area.

Point EDS analysis was performed at an accelerating voltage of 15 kV with an exposure time of 60 s in spot analysis mode. The evaluation was carried out using the ZAF correction algorithm, which compensates for matrix effects related to absorption, fluorescence, and atomic number.

SEM–EDS analysis indicated oxygen enrichment at the surface of Sample B, consistent with the presence of surface oxide regions on recycled aluminum materials. All three EDS spectra show the presence of oxygen in the range of 4.34–5.94 wt.%, which is typical for naturally formed oxide layers on aluminum surfaces. Aluminum is the dominant element at all analyzed surface locations, with normalized Al contents ranging from 94.06 to 95.66 wt.%. These values are consistent with the bulk chemical composition determined by ED-XRF (97.5–97.7 wt.%), with the difference attributed to the influence of the surface oxide layer detected by EDS, whereas XRF probes a larger material volume. Magnesium, silicon, manganese, and iron were not detected in the analyzed EDS spectra. However, this should not be interpreted as evidence of their absence or actual depletion in the remelted material. Rather, the discrepancy between SEM–EDS and ED-XRF results is most plausibly explained by the surface-sensitive nature of EDS combined with enrichment of the naturally formed Al-rich oxide layer. Because EDS probes only a shallow near-surface volume, the oxide film can effectively mask contributions from alloying elements present at concentrations close to the practical detection limits of the technique. The consistent detection of oxygen at all investigated locations (4.34–5.94 wt.%) further supports the interpretation that the analyzed microlocalizations correspond predominantly to oxidized aluminum-rich surface regions. The measured iron content should also be considered in relation to the requirements for aluminum-based deoxidizers used in steelmaking. Literature sources indicate that increased Fe content may influence inclusion formation and steel cleanliness [14,15,16,17,18].

In this context, the observed Fe levels are consistent with values reported for recycled aluminum materials; however, their suitability for specific steelmaking applications requires further validation. SEM observations further indicate a fine-grained microstructure typical of secondary Al–Si–Mg alloys, with a generally uniform surface texture, the presence of fine micropores, and localized discontinuities that may act as preferential sites for oxidation. Overall, the analyzed surface locations exhibited relatively consistent microstructural features and comparable local EDS responses within the limits of the applied SEM–EDS methodology.

EDS point analyses were performed at three representative surface locations selected on the basis of characteristic microstructural features observed in the SEM images. The corresponding elemental compositions are summarized in

Table 4. Normalized SEM–EDS point analyses of Sample B.

Point	O [wt.%]	Al [wt.%]	Mg	Si	Mn	Fe
EDS-1	5.94	94.06	ND	ND	ND	ND
EDS-2	4.34	95.66	ND	ND	ND	ND
EDS-3	5.54	94.46	ND	ND	ND	ND

Note: ND = not detected under the applied SEM–EDS operating conditions. Values are normalized to 100 wt.%.

. The selected sites included regions of the metallic matrix adjacent to micropores and local surface discontinuities considered representative of the remelted material. The objective was to characterize the local surface chemistry of representative microstructural features rather than to perform a statistically random surface survey of the remelted surface.

3.2.3. Density, Porosity, and Degree of Compaction of Sample B

Sample B represents a remelted and resolidified block obtained by melting the pressed briquette (Sample A) in an induction furnace at approximately 850 °C, followed by unidirectional solidification in a simple mold. This process enables evaluation of metallurgical yield and determination of physical parameters characterizing the remelted material. Density, porosity, and the degree of compaction were determined using a combined geometrical–mass method analogous to that used for Sample A. Although the estimated porosity reached approximately 11%, no pronounced macroscopic voids were observed on the analyzed cut surfaces of the samples. The applied bulk density method does not allow a strict distinction between open and closed porosity. Possible penetration of the immersion liquid into interconnected pores may slightly influence the calculated porosity values. In addition, this approach does not explicitly account for microstructural features such as oxide films, inclusions, or residual contaminants typical for recycled aluminum materials. Therefore, the reported porosity values should be interpreted as approximate bulk porosity estimates rather than absolute pore volume fractions and should be considered an indicator of overall material compactness rather than a precise measure of internal pore structure. The theoretical alloy density (ρ_alloy) was estimated from the measured Al–Si–Mg chemical composition using literature density values of the principal alloying elements and a rule-of-mixtures approach. Minor contributions of trace elements and oxide inclusions were not explicitly included in the calculation.

• Geometrical parameters and density determination:

After casting, Sample B had dimensions of 116.2 × 41.4 × 23.8 mm and an average mass of 273.41 g. The sample volume calculated from the stereometric dimensions was 114.5 cm³.

Bulk density:

$${\rho }_B={\displaystyle \frac{\mathrm{m}}{\mathrm{V}}}={\displaystyle \frac{273.41}{114.5}}\approx 2.388 \mathrm{g}·{\mathrm{c}\mathrm{m}}^{-3}$$

(11)

The value is higher than the density of briquette A (2.29 g·cm⁻³), which is consistent with reduced porosity and consolidation of the chip-based structure during remelting.

• Porosity and degree of compaction:

The volumetric porosity was estimated by comparing the measured bulk density with the theoretical alloy density determined using the rule-of-mixtures approach described in Section 2.6 (ρ_alloy = 2.68 g·cm⁻³).g·cm⁻³

$$P=\left(1-{\displaystyle \frac{{\rho }_B}{{\rho }_{alloy}}}\right)\times 100=\left(1-{\displaystyle \frac{2.388}{2.68}}\right)\times 100\approx 10.9\%$$

(12)

After remelting, the briquette transforms into a continuous metallic volume and the nature of porosity fundamentally changes. In contrast to Sample A, image-based pore counting is no longer representative of the overall material state. Therefore, the porosity of Sample B was quantitatively evaluated using bulk density measurements, providing a volumetric porosity estimate relevant to bulk remelting behavior. The revised porosity value should be interpreted as an approximate bulk estimate derived from density measurements and does not distinguish between open and closed porosity.

The density of the casting therefore reaches:

$$C={\displaystyle \frac{{\rho }_B}{{\rho }_{alloy}}}\approx 89.1\%$$

(13)

which represents a higher degree of compaction compared with the pressed briquette (85.4%), indicating that remelting substantially reduced the void fraction and transformed the chip-based structure into a more consolidated metallic material.

• Weight loss, yield, and dross fraction:

Based on the input mass of the briquette prior to remelting (290.25 g) and the recovered mass of Sample B, the following process-related parameters were calculated:

• weight loss:

$$\mathrm{L}=290.25-273.41-6.54=10.3\mathrm{g}$$

(14)

• metallic recovery (yield):

$$Y={\displaystyle \frac{273.41}{290.25}}\times 100\approx 94.20\%$$

(15)

• dross fraction:

$$Dr={\displaystyle \frac{6.54}{290.25}}\times 100\approx 2.25\%$$

(16)

These values fall within the range commonly reported for remelting of recycled aluminum briquettes under comparable laboratory conditions.

• The results show that:

• The estimated porosity decreased from 14.6% (Sample A) to 10.9% (Sample B);
• The density increased to 2.388 g·cm⁻³, indicating substantial consolidation of the chip-based material into a compact metallic volume; g·cm⁻³;
• The metallic recovery of 94.2% indicates efficient material recovery during remelting;
• The dross fraction of 2.25% is consistent with oxidation effects typically observed during remelting of recycled chip-based aluminum.

The combination of increased compaction and reduced porosity suggests improved remelting characteristics of the recycled material after consolidation and resolidification. The remelted material exhibited relatively consistent physical and compositional characteristics within the limitations of the applied experimental methods.

The application of different pore-evaluation approaches for Samples A and B reflects their fundamentally different material states—cold-pressed chip aggregates versus remelted compact cast material—rather than an inconsistency in methodological rigor.

A concise comparison of the key physical, chemical, and metallurgical properties of the cold-pressed briquette (Sample A) and the remelted material (Sample B) is provided in

Table 5. Summary comparison of key properties of Sample A (cold-pressed briquette) and Sample B (remelted material).

Property	Sample A—Cold-Pressed Briquette	Sample B—Remelted and Resolidified Material
Material state	Chip-based compact	Continuous cast aluminum block
Density [g·cm−3]	2.29	2.388
Porosity [%]	14.6	10.9
Degree of compaction [%]	85.4	89.1
Dominant elements	Al–Mg–Si	Al–Mg–Si
Chemical homogeneity	Variability due to chip mixture	Improved compositional uniformity after remelting
Surface/internal pores	Intergranular voids between chips	Gas-induced microporosity
Metallurgical yield [%]	—	94.2
Dross fraction [%]	—	2.25
Pore Type	Interparticle/intergranular	Gas microporosity

Note: Porosity and degree of compaction values were recalculated using the composition-based theoretical density of 2.68 g·cm⁻³ determined by the rule-of-mixtures approach.

.

4. Discussion

The results from Samples A and B enable assessment of physical, chemical, and microstructural changes occurring during recycling of aluminum chips. This discussion synthesizes the key findings and compares them with established knowledge in the literature, particularly in the context of oxidation, remelting behavior, and compositional variability of secondary aluminum materials.

4.1. Changes in Density and Porosity During the Transition from Pressed Briquette to Casting

Sample A exhibits a relatively low density (2.29 g·cm⁻³) and high porosity (14.6%), typical of cold-pressed briquettes produced from aluminum alloy machining chips. This behavior is primarily associated with mechanical porosity between individual chips and residual processing contaminants. Such constituents do not contribute to the consolidated metallic fraction of the material—upon heating, gas evolution and decomposition of residual contaminants may contribute to mass loss and increased microporosity during remelting.

After remelting, Sample B reached a density of 2.388 g·cm⁻³ with reduced porosity (11%). This decrease suggests that a substantial portion of mechanically formed voids was reduced during remelting; the material becomes compacted, while residual porosity remained present within the remelted material. The remaining porosity may be associated with gas evolution and oxidation effects commonly occurring during chip remelting. This interpretation is consistent with literature describing rapid surface oxidation of aluminum chips and retention of gases within recycled feedstocks.

The density of the cold-pressed briquette (2.29 g·cm⁻³) obtained in this work lies well within the range reported for industrially produced aluminum chip briquettes, where values between approximately 2.1 and 2.4 g·cm⁻³ are typically achieved depending on pressing pressure, chip morphology, and residual lubricant content [4,5]. Similar density levels and porosities of 10–15% for cold-pressed aluminum chip briquettes have been reported by Pavlasek et al. and Penchev et al., who also attribute the remaining porosity primarily to intergranular voids formed during room temperature compaction [4,5]. After remelting, the density increases to 2.388 g·cm⁻³ and the corresponding reduction in porosity to 11.% are consistent with literature data for secondary AlSiMg alloys remelted without melt refining, where residual gas-related porosity typically remains in the range of 5–10% [7]. These results are consistent with densification and porosity reduction trends reported for recycled aluminum briquettes produced from machining chips.

However, the density of the casting remains lower than that of the theoretically fully dense alloy (2.68 g·cm⁻³), indicating that the secondary material may retain remnants of the original structural discontinuities and residual contamination. This phenomenon is also highlighted in studies on recycled AlSiMg alloys, where porosity persists as one of the limiting factors affecting the physical integrity and remelting behavior of secondary aluminum materials. Therefore, the measured porosity of approximately 10.9% in Sample B should not necessarily be interpreted as evidence of improper melting practice, but rather as a technologically expected consequence of non-refined remelting of recycled aluminum chip briquettes with residual gas-forming contaminants.

While surface-based pore analysis is meaningful for characterizing pressed chip briquettes, volumetric density-based porosity provides a suitable quantitative metric for evaluating remelted secondary aluminum materials.

4.2. Mechanisms of Oxidation and Dross Formation

The formation of dross and process-related metal losses represents an important aspect of remelting behavior of recycled aluminum briquettes. Sample B exhibits a dross fraction of 2.25%, a relative melting loss of 3.55%, and an overall metallurgical yield of 94.2%.

The metallurgical yield of 94.2% achieved in this study lies at the upper end of values reported for briquetted aluminum machining chips, where yields in the range of approximately 90–96% are typically observed depending on briquette density, chip pretreatment, and melting conditions [4,7,20]. Pavlasek et al. reported that aluminum briquettes with densities above ~2.2 g·cm⁻³ generally exhibit lower oxidation losses and improved remelting behavior compared with loose chips, which is in good agreement with the results obtained in this work [4]. Similarly, Ulus et al. demonstrated that increasing briquette density leads to a substantial improvement in aluminum recovery, particularly when residual lubricants are effectively reduced prior to remelting [20]. The measured dross fraction of 2.25% is consistent with literature data for cold-pressed aluminum briquettes remelted without fluxing or degassing, where oxide-related losses are commonly associated with the specific surface area of the original chips and the presence of pre-existing Al₂O₃ surface layers [7,20].

These values fall within the typical range reported for remelting of briquettes with densities between 2.15 and 2.40 g·cm⁻³ and are consistent with oxidation effects commonly observed during remelting of aluminum chips. During remelting, surface oxides originating from the chip surfaces are expected to contribute predominantly to dross formation rather than to the bulk metallic material.

The literature describes how oxide layers on aluminum chips can develop during air exposure and contribute to metal losses during remelting. SEM–EDS analysis indicated oxygen signals in the range of 4.34–5.94 wt.% at the remelted surface, consistent with oxidized aluminum surfaces. Although primarily surface-related, such oxidation effects may contribute to observed dross formation and metallic losses during remelting. From a sustainability perspective, recent life-cycle assessments indicate that optimized recycling routes, including solid-state processing, can significantly reduce environmental impact compared with conventional remelting processes [21].

4.3. Chemical Homogeneity After Remelting and Its Significance for Metallurgical Applications

Chemical analysis of Sample B indicates reduced compositional variability after remelting compared with the initially heterogeneous chip-based material. Importantly, the observed compositional ranges are comparable to, or lower than, the manufacturer-specified uncertainty of handheld XRF analysis (±0.1–0.5 wt.% for major elements and approximately ±0.01 wt.% for trace elements). This suggests that the observed compositional scatter is comparable to the expected uncertainty range of handheld ED-XRF analysis and that no major compositional differences were detected within the analyzed measurement locations after remelting.

Accordingly, the conclusions are limited to internal chemical homogeneity within individual briquettes produced under stable industrial conditions. A statistical evaluation of batch-to-batch variability across different production campaigns or chip sources was beyond the scope of this work and will be addressed in future studies. Aluminum content (97.21–97.51 wt.%) shows minimal variation (±0.15%), with similarly low variability observed for Mg, Si, Mn, and Cr. The slight variability in Fe content may be associated with localized residual ferromagnetic contamination or compositional heterogeneity typical of recycled materials.

Unlike most previous studies reporting average chemical composition, the present approach evaluates local compositional variability within a single remelted briquette using multiple measurement locations. In the context of this study, the term chemical stability refers specifically to the relatively limited compositional variability observed within individual briquettes after remelting of an initially heterogeneous chip-based feedstock.

The applied analytical approach was therefore focused on evaluating local compositional variability within a single briquette and across the remelted volume. A systematic assessment of batch-to-batch compositional variability resulting from different machining shops, scrap sources, or production campaigns represents an important industrial aspect; however, such an analysis would require a substantially broader sampling strategy and lies beyond the scope of the present work.

EDS analyses did not reveal pronounced Fe-, Mn-, or Si-rich phases within the analyzed surface regions and no major compositional differences were observed between the analyzed edge and core regions. Relatively limited compositional variability may be beneficial for secondary metallurgical processing of recycled aluminum materials, although the present study does not evaluate behavior under real steelmaking conditions. These findings are consistent with current circular economy research, which highlights that alloy mixing, contamination, and compositional variability represent key limitations in maintaining high-quality secondary aluminum streams [22].

Compared with literature reports describing segregation-related challenges in recycled materials, the analyzed sample exhibited relatively limited compositional variability within the investigated regions.

4.4. Influence of Process-Related Contaminants and Gas Porosity

A significant factor influencing the density of Sample A as well as the quality of the remelted material in Sample B is the presence of residual cutting emulsions and oils entrapped between the chips. These residual organic substances may reduce the effective density of the briquette and contribute to gas evolution, oxidation effects, and metallic losses during remelting.

This phenomenon is consistent with the conclusions of modern research, which indicate that proper pretreatment of chips (drying, centrifuging, and degreasing) is essential for minimizing melting losses and improving the remelting behavior of secondary aluminum materials. The surface visualization of Sample A (Keyence VHX digital microscopy) confirms the presence of a highly complex lamellar structure formed by overlapping aluminum chips, multiple surface oxide sites, and a high density of surface-accessible pores characteristic of cold-pressed secondary aluminum materials.

Therefore, the porosity observed in Sample B is likely associated with residual contamination, gas evolution during remelting, and the original chip-based microstructure rather than with pronounced solidification segregation effects.

4.5. Industrial Implications for the Use of Recycled Briquettes

From the perspective of potential metallurgical reuse applications, two aspects are of primary importance:

• The compaction level of the briquette (85.4% for Sample A);
• The density after melting (2.388 g·cm⁻³ for Sample B).

According to literature reports and industrial practice, aluminum briquettes with densities between 2.20 and 2.40 g·cm⁻³ may exhibit favorable handling and remelting characteristics during metallurgical processing:

• Improved compactness during handling;
• Reduced tendency for excessive surface oxidation;
• Favorable remelting behavior;
• Metallic recovery values commonly reported for compacted recycled aluminum feedstocks.

These considerations represent general industrial observations related to remelting and handling behavior of compacted recycled aluminum. In this context, recent studies demonstrate that the choice and consistency of deoxidizing agents strongly influence the type, size, and distribution of non-metallic inclusions, which directly determine the final steel quality and mechanical performance [23]. It should be emphasized, however, that the present study evaluates only material-level characteristics such as density, porosity, compositional variability, and remelting yield, rather than direct deoxidation behavior under molten steel conditions.

Although deoxidation performance under molten steel conditions was not evaluated in the present study, the physical characteristics of recycled aluminum briquettes may influence their behavior in steel baths. Elevated residual porosity may reduce the effective density of the briquette, increase its tendency to float, and limit the contact efficiency between aluminum and molten steel. Consequently, lower porosity and higher compaction may be advantageous for improving dissolution kinetics and promoting more efficient interaction with the melt. Verification of these effects, however, requires dedicated steelmaking experiments and remains a subject for future investigation.

At the same time, excessively high porosity and elevated oil content may contribute to unstable remelting behavior, increased gas evolution, and higher metallic losses. The results therefore highlight the need for thorough chip pretreatment and optimization of the pressing pressure to achieve higher briquette density and reduced porosity. It should be noted that aluminum briquettes are not intended to replace wire injection systems, which provide superior dosage and compositional control, but rather represent a potential form of recycled aluminum feedstock for selected secondary metallurgical processing applications where controlled addition practices are employed.

5. Conclusions

This study evaluated cold-pressed recycled aluminum briquettes produced from Al–Si–Mg machining chips and their behavior during subsequent remelting, with particular emphasis on density, porosity, compositional variability, and metallurgical yield. The pressed briquettes exhibited a bulk density of 2.29 g·cm⁻³ and an estimated porosity of 14.6%. After remelting, the density increased to 2.388 g·cm⁻³, while the estimated porosity decreased to 10.9%, indicating substantial material densification during remelting. The remelting process achieved a metallic recovery of 94.2% with a low dross fraction of 2.25%, demonstrating favorable remelting characteristics under the applied laboratory conditions.

Handheld ED-XRF analysis indicated limited compositional variability after remelting within the uncertainty range of the applied analytical method. SEM–EDS observations revealed Al-rich surface regions containing only minor oxygen contributions associated with naturally formed surface oxides and did not indicate pronounced intermetallic features within the analyzed surface regions. These findings suggest that appropriately preprocessed and compacted aluminum machining chips can be transformed into mechanically stable briquettes with favorable remelting behavior and potential applicability in secondary metallurgical processing.

The findings further underscore the importance of chip pretreatment, particularly the removal of residual oils and the reduction in surface oxidation, as well as the optimization of pressing conditions to increase briquette density and reduce residual porosity. However, the present work represents a material-level characterization rather than a direct assessment of steelmaking performance. Given the limited number of specimens investigated and the spatial nature of the measurements, the results should not be interpreted as population-based descriptors of industrial process variability, and their generalization to broader industrial populations should be made with caution.

Although deoxidation behavior under molten steel conditions was not evaluated, residual porosity and compaction may influence flotation tendency, dissolution kinetics, and contact efficiency in steel baths. Verification of these effects, together with the establishment of acceptance criteria for aluminum-based deoxidizers under practical steelmaking conditions, requires dedicated experiments and remains a subject for future investigation.