Microstructural evolutions and mechanical properties of 6082 aluminum alloy part produced by a solution-forging integrated process

https://doi.org/10.1016/j.jmatprotec.2022.117715Get rights and content

Highlights

  • 6082 aluminum alloy parts were manufactured using an integrated forging process.

  • Abnormal grain growth was suppressed by using the integrated forging process.

  • The integrally processed parts obtained uniform mechanical properties.

Abstract

The conventional forging process for 6082 aluminum alloy entails the following steps: preheating, hot forging, solution heat treatment (SHT), and artificial aging. In this study, 6082 aluminum alloy automobile parts were successfully manufactured under industrialized conditions using a novel forging process that integrated SHT and hot forging into one operation, resulting in the cost savings compared with the conventional forging process. Mechanical properties were determined using tensile tests, while microstructural evolutions were revealed using optical microscopy (OM), scanning electron microscope (SEM), electron backscatter diffraction (EBSD), high-resolution transmission electron microscopy (HRTEM), and finite element method (FEM) simulation. It is found that the abnormal grain growth (AGG) induced by post-forging SHT occurred in the surface layer of conventionally processed parts, resulting in a decrease in the strengthening effects of grain boundary and dislocation, as well as a decrease in tensile strength, yield strength, and elongation. However, benefiting from the SHT before forging and the immediate quenching after forging, the AGG triggered by secondary recrystallization was suppressed by using the novel forging process. Along with achieving mechanical properties comparable to those obtained through conventional forging in the interior, the novelly processed parts' surface layer exhibits relatively consistent properties due to their homogeneous grain structure. The results above demonstrate the efficacy of the novel solution-forging integrated process, which is well-suited for industrial production, application, and promotion.

Introduction

Aluminum alloy forgings are widely applied and exhibit outstanding specific strength, recyclability, and fatigue resistance, especially for safety-critical structural components, as reviewed by Deng et al. (2018). The 6082 aluminum alloy is the most commonly used Al-Mg-Si alloy and is extensively utilized in vehicle chassis and steering forgings.

Conventionally, 6082 aluminum alloy forgings are manufactured using a series of operations that can be divided into two distinct stages: forging to acquire the desired shape and heat-treating to achieve the required mechanical properties. To guarantee proper plasticity, the raw extruded bars are preheated to the forging temperature and kept there for a short time until thoroughly heated. The preheated bars are then pressed between one or more sets of dies, depending on their pressability, to create specific geometric shapes. Lai et al. (2019) summarized that the superior properties of the heat-treatable Al-Mg-Si alloys are primarily attributable to the dispersed nano-sized precipitates resulting from the post-forging heat treatment. This is why the solutionizing, quenching, and aging treatments must be performed sequentially on the forged parts. Shao et al. (2020) demonstrated that solutionizing results in a uniform distribution of alloying elements, which is then quenched to form a supersaturated solid solution (SSSS). Mrówka-Nowotnik and Sieniawski (2005) concluded that the 6082 aluminum alloy is sensitive to cooling conditions and samples cooled in water have the highest hardness compared to samples cooled in oil or air. Therefore, water-based quenchants are extensively used in the heat treatment of aluminum alloy forgings. Finally, the SSSS is precipitated in an orderly manner by artificial aging to form precipitates, which play a crucial role in the final mechanical properties, as pointed out by Chen et al. (2020).

Conventionally processed forgings often suffer from an inhomogeneous grain structure caused by abnormal grain growth (AGG), also known as discontinuous grain growth or secondary recrystallization, in the surface layer of forgings after SHT. Birol (2015) elaborated that AGG defects occurred in a conventionally forged part after SHT, and these abnormally grown grains were observed to extend from the surface layer to the interior of the part to a certain depth. AGG phenomenon is not limited to forgings. Sun et al. (2019) investigated that secondary recrystallization also occurrs on extruded aluminum alloy due to post-forming SHT. On friction stir welds, Lezaack and Simar (2021) concluded that post-welding SHT is also the main reason leading to AGG, but it can be improved by optimizing process parameters. The coarse grains greatly affect the mechanical properties of forgings, such as tensile strength, elongation, fatigue behavior, and so on. For example, Chang et al. (2019) studied that the grain growth occurred after SHT reduces the elongation and tensile strengths of aluminum alloy forgings. Shou et al. (2016) discussed that the coarse grains have a negative effect on the fatigue crack propagation resistance. Therefore, automobile manufacturers limit the thickness of the coarse-grained layer on aluminum alloy parts to ensure service performance.

To reduce the high energy and equipment costs and to improve the quality of forgings, there has been great interests in designing novel forging processes. Jin et al. (2016) proposed a single-step hot stamping-forging process to produce pan-shaped shell aluminum alloy parts, which improved the production efficiency and reduces the cost compared to conventional manufacturing methods. He et al. (2020) introduced an improved thermomechanical treatment for aluminum forgings that comprises hot deformation, cold deformation, and a subsequent heat treatment to improve grain structure and mechanical properties. Manjunath et al. (2021) reviewed that multi-directional forging/multi-axial forging are adequate processes for obtaining fine equiaxed grains in aluminum alloys. Kumar et al. (2022) proposed a multi-axial forging process at liquid nitrogen temperature and investigated the grain refinement mechanism in 6082 aluminum alloy. Recently, Zhao et al. (2022) demonstrated an integrated forging process via uniaxial hot compression tests that combined SHT and hot forging into one operation followed by artificial aging. Notably, one of the characteristics of this integrated forging process is the preservation of the deformed grain structure by immediately quenching following forging. Güzel et al. (2012) concluded that post-forming quenching could be used to prevent static recrystallization and grain growth. Considering the post-forging SHT plays a key role in AGG defects during conventional forging, it is possible to avoid the AGG induced by the secondary recrystallization effect if the forgings are not treated with SHT after quenching. As a result, aluminum alloy forgings with a more homogeneous and uniform microstructure may be achievable. Therefore, it is worthwhile to investigate the utilization of the integrated forging process on an industrial scale that potentially combines the advantages of short processing and fine microstructure.

With regard to the application of the forging process of 6082 aluminum alloy, automobile chassis parts were produced on an industrial scale with a conventional forging process (forging, solution treatment, and artificial aging) and an integrated forging process (forging and artificial aging only) to investigate their influences on the microstructure and mechanical properties. The mechanical properties were evaluated by tensile tests. The microstructure and strengthening mechanisms were illustrated by optical microscopy (OM), scanning electron microscope (SEM), electron backscatter diffraction (EBSD), high-resolution transmission electron microscopy (HRTEM), and finite element method (FEM) simulations.

Section snippets

Materials

In this work, 160-mm-long 6082 aluminum alloy extruded bars in fabricated state with a diameter of 110 mm were prepared for industrial scale forging experiments; the chemical composition was Al-0.65Mg-0.93Si-0.06Cu-0.17Fe-0.12Cr-0.03Ti-0.45Mn (wt %).

Forging procedure

Fig. 1 depicts the schematic diagrams of conventional and integrated forging processes under industrialized conditions. In the conventional forging process, the preheated 6082 aluminum alloy bar is sequentially forged, solutionized, quenched, and

Mechanical properties

The mechanical properties of the conventionally and integrally processed 6082 aluminum alloy automobile parts are listed in Table 2. It can be concluded that the internal mechanical properties of the parts processed by the conventional and integrated forging processes are comparable. However, significant sacrifice of tensile strength, yield strength, and elongation occurs in the conventionally processed parts’ surface layer compared to those of the interior. Specifically, the tensile strength

Effect of processing impact on the evolution of grain structure

To understand the grain evolution during pre-forging and final-forging of the 6082 aluminum alloy parts, the Zener-Hollomon parameter (Z), established by Zener and Hollomon (1944), is used to express the strong connection between the forming parameters and microstructure characterization. The Z is defined as follows:Z=ε̇exp(Q/RT)where ε̇ is the strain rate (s−1), R represents the gas constant (8.314 J/K·mol), Q is the deformation activation energy that equals 173.54 kJ/mol as calculated by Li

Conclusions

This paper reports the microstructural evolution and mechanical properties of 6082 aluminum alloy parts produced using a novel forging process that integrates forging and SHT in one operation, along with comparisons to the conventionally processed parts. Based on the observations and analyses of representative identical areas in the parts via tensile tests, OM, SEM, EBSD, HRTEM, and FEM simulation, the following conclusions are proposed:

  • The novel forging process is capable of producing 6082

CRediT authorship contribution statement

Ning Zhao: Investigation, Formal analysis, Visualization, Writing – original draft. Huijuan Ma: Formal analysis, Investigation. Qian Sun: Formal analysis, Investigation. Zhili Hu: Methodology, Writing – review & editing, Resources. Yang Yan: Validation, Investigation. Tianfu Chen: Validation. Lin Hua: Supervision, Resources.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Key Research and Development Program of China [grant number 2019YFB1704500]; the National Natural Science Foundation of China [grant numbers 52075400, 51805393]; the 111 Project [grant number B17034]; and the Key Research and Development Program of Hubei Province [grant number 2021BAA200].

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