Recycling of the Al Scrap: The Effect of Adding Inoculant Nb+B and Mg with Subsequent Heat-Treatment in the Mechanical Behavior of Al Alloy

Review Article

Ann Materials Sci Eng. 2021; 5(1): 1039.

Recycling of the Al Scrap: The Effect of Adding Inoculant Nb+B and Mg with Subsequent Heat-Treatment in the Mechanical Behavior of Al Alloy

Carlos Narducci Jr1,2,3*, Junior N3 and Abdalla AJ2

¹Aeronautics Institute of Technology-ITA, São José dos Campos, SP, Brazil

²Institute for Advanced Studies-IEAv, São José dos Campos, SP, Brazil

³Federal Institute of São Paulo-IFSP, Itaquaquecetuba, SP, Brazil

*Corresponding author: Carlos Narducci Junior, Aeronautics Institute of Technology-ITA, São José dos Campos, 1249, Rua Alto Garças, Cidade Patriarca, São Paulo - SP, Brazil

Received: July 27, 2021; Accepted: August 18, 2021; Published: August 25, 2021


A new perspective for the use of Al-Si alloys produced with recycled Al (with Fe>1%) in Gravity Die Casting (GDC) processes. To study the morphology of ß-Fe precipitates and the material’s mechanical properties were added the inoculate via Nb+B and the element Mg with subsequent heat treatment. The samples were cast in Al10Si1Fe0.35Mg alloy in a metal mould according to ASTM B108. The microstructure was analyzed with BSE-SEM and EDS. The work investigated the morphology of ß-Fe precipitates and their effects and interactions on the material’s mechanical properties. The combined effect resulted in reduced size and shape of ß-Fe precipitates, thereby improved higher yield strength (YS = 207.71 MPa), ultimate tensile strength (UTS = 300.35 MPa), and elongation of 4.66%, exceeding the strength and elongation limit values found in commercial alloys, such as ASTM A357 alloy, where the Fe content is low (max. 0.2%).

Keywords: Recycled Al-Si alloys; NbB; Casting; Heat treatment; Mechanical properties; Refinement of the grain; Intermetallic precipitates


Even in small amounts, iron (Fe) degrades the mechanical properties of aluminum alloys, such as tensile strength, fatigue, fracture toughness, and especially the material’s elongation. The work of Taylor [1] reports the formation of the intermetallic phase ß-Al5FeSi (ß-Fe), with thick morphology in the form of plate or needle, it becomes rigid points in the soft structure of the material that, when subjected to external stresses, initiate small fissures extend into cracks that eventually decrease its mechanical properties [1]. As a result, recycled Al, due to the contamination of the material with Fe during its production process, is not recommended for applications where resistance and elasticity are required in the same product. Some examples of products with automotive applications that could benefit from this process are Master Cylinders, Calipers, Camshaft Rockers, and Suspension Brackets components, among other automotive and aerospace parts.

Mahta [2] says the commonly accepted method to alleviate the harmful influence of iron is adding one or more corrective elements [2]. Such additions generally convert the ß-Fe platelets into a-Fe dendrites. One example of modification mechanisms is adding chemical elements to the alloy, Mn being the most commonly used today. However, Ebhota [3] states that with this mechanism, a new problem can arise with so-called sludge formation, which reduces the material’s mechanical properties [3]. Another approach to combat the level of iron impurity in Al alloys is diluting the recycled aluminum using primary aluminum, but this makes the material more expensive. Basak [4] proposed the fragmentation theory as the mechanism for the ß phase’s refinement, based on experimental evidence. With a suitable heat treatment, one can change the morphology of the ß-phase [4]. However, it is only recommended in recycled Al-Si alloy castings with low Si concentrations and high Fe concentrations, which is not the case for castings made by the gravity die casting process because the Si element is essential for the fluidity material in the mold. BASAK also proposed gravitational segregation, but this is not an economically productive process. Thus, there is a real need to search for a viable alternative to deal with the high iron concentration in recycled aluminum alloys. In this sense, this work studies the simultaneous use of two techniques to enhance the material’s strength: the phenomenon of heterogeneous nucleation and precipitation hardening.

Grain reduction through inoculation via chemical elements (heterogeneous nucleation) has been studied for decades. Easton [5] already confirmed that Ti is an element with an excellent growth restriction factor of the a-Al grain, proving that inoculation with the addition of a master alloy Al-Ti-B acts as powerful refiners the production of Al stamping parts [5]. Al alloys for the stamping process generally use quantities lower than 2 Wt.% of Si. However, in the case of Al-Si alloys for casting, studies with much higher Si levels would be necessary to ensure the reliability of the final products. Nowak [6] report that the inoculants’ efficiency based on the master alloy Al- Ti-B is doubtful when using Si concentrations higher than 4 Wt.%, proving through microstructural analyzes that Ti and Si interact to form titanium silicates, which are ceramic compounds and end up damaging the alloy’s mechanical properties, this phenomenon has been treated as a poisoning effect on the alloy [6]. The same results were reported for Li [7] about understanding grain refining and the anti-Si-poisoning effect [7].

Thus, the alternative found by engineering among the elements available in the periodic table was Nb as explained in Bolzoni [8], reporting that Nb because it has a higher melting point and a slightly lower network parameter with lower incompatibility than Ti, which favors the solidification of the molten Al [8]. Bolzoni [9,10] report that niobium-based compounds are highly effective in refining a-Al dendrite grains of Al-Si cast alloys with silicon levels greater than 6 Wt.% of Si, demonstrating through the underlying mechanism of heterogeneous nucleation the clusters of AlB2 and Al3Nb substrates found in the nucleus of the a-Al grains, being revealed as crystals that grow along a non-specific direction and also without a specific orientation relationship, initiating several nucleation points at the same time, limiting the size of the a-Al grain dendrites [9,10]. This phenomenon causes the refinement of the grain and creates a mechanism that ends up modifying the morphology of intermetallic precipitates. Narducci [11,12] reports that the addition of Nb+B caused an exponential increase of the nucleation points (heterogeneous nucleation), forming a large number of simultaneous grains that ended up restricting the mobility of the solute elements and thus decrease their interconnectivity and also the formation of agglomerates and finally present a refined grain structure with reduced and spheroidal ß-Fe intermetallic precipitates [11,12]. Theoretically, the yield strength values are correlated to the grain size a-Al. According to the Hall-Petch equation, the smaller the average grain size, the higher the grain boundaries’ density and, therefore, the greater the obstruction to the movement of disagreements. Thus, a larger load will then be required to stimulate disagreements, increasing the yield strength.

Precipitation hardening is a technique used in Al-Si cast alloys generally with a Fe content within the recommended limits (e.g., ASTM A357 alloy) to strengthen the material’s mechanical properties. According to Apelian [13], the process consists of adding an alloying element (Mg or Cu) to the material, with the subsequent heat treatment of solubilization and precipitation (T6) [13]. Mcqueen [14] explains that in this heat treatment process, the material is heated within the monophasic region for a sufficient time for solubilization of the solute atoms, followed by quick cooling to obtain a supersaturated solid solution [14]. Then the alloy is reheated at a temperature below the monophasic region to allow for the precipitation of finely dispersed particles from the supersaturated solid solution. The formation of a fine precipitate dispersion hinders discordance’s movement.

Experimental Procedure

The materials used were: Primary Al (supplied by HYDRO), Si (supplied by LIASA), Fe (supplied by MEXTRAMETAL), Mg (supplied by RIMA), and an Al4Nb0.05B master alloy (supplied by Companhia Brasileira de Metalurgia e Mineração - CBMM). Manufactured the tensile test standard (TTS) was in the Sunny foundry in Itaquaquecetuba.

The metal was melted in an electric crucible oven with a capacity of 60 kg of material. After adding each element, the temperature was stabilized at 850±10 °C, with a 1-hour hold was applied to ensure complete dissolution. Degassing was then carried out by adding hexachloroethane tablets (supplied by ALFA TREND). Each sample collection, achieved homogenization by 30 s of stirring and a new temperature stabilization at 720±10 °C, followed by a second homogenization at the same temperature, with 30 s of stirring and sample collection. The chemical composition of each batch produced is shown in Table 1. The verification of the base alloy was done by atomic absorption spectrometry.