Study on the mechanism of coarse grain formation of 6061 aluminium alloy forged wheels

The formation of coarse grains is easy to occur during the mass production of 6061 aluminium alloy forged wheels. The coarse grains and uneven grain structure will lead to the reduction and instability of the mechanical properties of the wheels, which will affect the fatigue strength, corrosion resistance and impact toughness of the wheels. In this paper, the effect of solution temperature and solution time on the coarse-grained formation of 6061 forged wheel was studied. The reasons and mechanism of coarse-grained formation were revealed by advanced materials characterization techniques such as scanning electron microscope, electron backscattering diffraction and transmission electron microscope. The results show that the deformation causes a large number of dislocation pile-up and entanglement in the grain, forming a large number of small angle sub-grains, and the heating process of sub-grains has more interfaces, which provides the driving energy for the abnormal grain growth.


Introduction
With energy conservation and emission reduction, 6061(AlMg1SiCu) aluminum alloy forged wheels have been extensively used in automobile industry due to the overall performance [1][2][3][4][5].The aluminum forged wheels made it possible to ensure grain flow parallel to the main force lines by precisely adjusting the part geometry to the forging process [6][7].The distribution of fiber-like microstructure has positive influence on the fatigue life and corrosion resistance [8][9][10].Generally, defects often occur in the surface of forged wheel [11].The coarse grain, the macrostructure as shown in figure 1 (Area A is fine grains, Area B is coarse grains), is a common defect in forged aluminum alloy wheels due to the complex shape and multi-step forming processes.Grain growth could appear during inter-pass time and cooling after deformation.Generally, it is noted some surface defects (large grains) which will grow during heat treatment (solution step) [12].After solid solution heat treatment (SSHT), coarse grains were obviously produced at the junction of spoke and rim.In order to eliminate or mitigate the detriment resulting from the coarse grain, some related research have been carried out.Although certain experts and technicians have carried out relevant research work [13][14], but because the entire forged aluminum alloy wheel production process has not been systematically studied, the formation and understanding of coarse crystals is only concentrated in a certain aspect.

Methodology
The tested 6061 alloy was direct chill (DC) casting bar with 203 mm diameters.The chemical compositions (wt.%) are Mg 0.9, Si 0.8, Cu 0.25, Fe 0.20, Mn 0.05, Cr 0.10, Zn 0.02, Ti 0.02, balance Al.The billets cut from the bar to desired size were hot die-forging by the hydraulic press with a press capacity of 6000 tonnes.The large samples as figure 1 were cut from the junction of spoke and rim of the wheels.In order to clarify the relationship between the solution temperature, holding time and the grain growth, SHT under 540 ℃ for 5-30 min and different temperature for 120 min were carried out.Area A and Area B without SSHT were investigated to reveal the microstructure evolution and the mechanism of coarse grain formation.
The microstructures were observed and characterized by using optical microscopy (OM, Axio observer, Carl Zeiss, Germany), scanning electron microscopy (SEM, EVO MA 15, Carl Zeiss, Germany) equipped with an electron backscattered diffraction (EBSD, EDAX-TSL) and transmission electron microscopy (TEM, Talos F200i, FEI, U.S.A.).The OM and SEM specimens cut into 10 × 10 × 10 mm 3 in dimension were ground and polished mechanically by diamond spray to a mirror surface.The samples were anode-coated at 18 V in a solution of 38 vol.%H2SO4+43 vol.%HNO3+19 vol.% deionized water for 1.5 min to determine the average grain size.The TEM specimens were mechanically thinned to 100 μm and punched into discs of 3 mm in diameter.On this basis disks with a diameter of 3 mm was punched out a thickness of 30 μm.Finally, the specimens were further thinned by Gatan PIPS691 ion thinning instrument under the vacuum degree lower than 1.1 × 10 -2 Pa.

Microstructure of SHT for different holding time
Figure 2 shows the anode-coated microstructure of Area A and B(as marked in figure 1)after finish forging.Clearly, there is no significant difference in the microstructure of either area prone to coarse grains (figure 2a) or the area with fine grains (figure 2b) before SSHT.However, a large number of small grains disappeared in the Area B, and the grains merged and grew up obviously during SSHT.The grains in the Area A did not grow significantly after SSHT and still retained the original deformation morphology.Figure 3 shows the macrostructure of SSHT at 540 ℃ for different holding time.As can be seen, the grain structure is very fine before SSHT, and the coarse grain area have grown slightly even within

Microstructure of SHT for different temperature
Figure 4 shows the macrostructure evolution under different SSHT temperature for same holding time.
It can be drawn that the grain growth trend is similar with figure 3. The original deformation morphology is still maintained after SSHT at 420 ℃.The driving force of grain merging and growth increases obviously with the increase of SSHT temperature.From the results it is known that the formation of coarse grains cannot be alleviated or eliminated by reducing the solid solution temperature on the premise of ensuring the mechanical properties of forged wheels.

Formation mechanism of coarse grains
To provide more detailed information from mechanism aspect, TEM have been carried out to reveal the main reason of the coarse grain of forged wheel.Figure 5 shows the TEM micrographs of fine and coarse grain regions after finish forging prior to SSHT.For the fine grain area (figure 5a,b), the metallic mobility during forging processes is smooth.The grain boundary is stretched and straightened along the metal flow direction.Scarcity of the dislocation entanglement and pile-up inside indicate that the entire matrix is in a low energy stable state, that is, the dislocation storage energy caused by deformation is low [14].In the subsequent heat treatment process, the driving force of grain growth is insufficient, so there is no coarse grain formation.On the contrary, the free metallic flow in the coarse grain area (figure 5c,d) is hindered due to the wheel shape limitation.A large number of slip or cross-slip dislocation present with the plastic deformation accumulation leads to dislocation proliferation and entanglement, which results in the appearance of cellular structures (sub-grains) [15].These cellular structures may become the nuclei of subsequent second recrystallization during SSHT.Figure 5d indicates that large amounts of high density dislocation make the 6061 alloy have sufficient driving force to promote the rotation of sub-grains, thus recrystallization occurrence.Figure 6 shows the EBSD IPF images of fine grain area and coarse grain area before and after SSHT as well as the percentage statistics of little angle grain boundary (LAGB) less than 15°.Deformation results in a large number of LAGB.Before SSHT, the LAGB ration of coarse grain area is as high as 68%, while that of fine grain area is only 43%.After SSHT, the proportion of LAGB in both two areas is greatly reduced.It is well known that the formation of LAGB is mainly due to the migration and accumulation of dislocation cells in the process of large deformation.These dislocation cells eventually evolve into sub-grains.The grain in region B is elongated more obviously along the deformation direction, and the grain orientation difference increases macroscopically.A very small number of fine recrystallized grains appear along with the elongated grains, and the grain boundaries of some grains are blurred.The serration of grain boundaries of some grains indicates that a small amount of geometric dynamic recrystallization occurs in the alloy during the deformation process.The deformation of 6061 forged aluminum alloy wheels during series production is in a nonuniform state.The driving force of growth is mainly the strain energy retained in the form of dislocation within the grain.With the increase of heating temperature and the extension of heating time, the cellular structures merge with each other through dislocation migration.

Conclusions
The formation mechanism of coarse grain in forged aluminum alloy wheel is studied.The key findings of the study are stated as follow: • At the same solution temperature of 540 ℃, the coarse grains grow abnormally within 10 min of holding time, and then prolongation of holding time has little effect on the coarse grains.The • Under the same solution holding time of 120 min, some coarse grains have been formed at 460 ℃, and the coarse grains have grown significantly above 500 ℃.On the premise of ensuring the mechanical properties of forged wheels, it is difficult to reduce the solution temperature to alleviate or eliminate the formation of coarse grains.• The coarse grain formation mechanism is non-uniform deformation forming a large number of sub-grains results in dislocation entanglement and pile-up, which provides driving energy for the abnormal grain growth.

Figure 2 .
Figure 2. Microstructure of Area A and Area B after finish forging: (a)(b) Area A and B prior to SSHT; (c)(d) Area A and B SSHT 540 ℃ for 30 min, respectively.
After 10 min, grain growth have basically completed in the coarse-grained region.Then further extending the holding time, the grains grow slightly, and the orientation difference between adjacent coarse grains increases.

Figure 5 .
Figure 5. TEM micrographs of fine grain region (Area A) (a, b) and coarse grain region (Area B) (c, d) before SSHT.

Figure 6 .
Figure 6.IPF images of Area A and Area B after finish forging: (a)(b) Area A and B before SSHT; (c)(d) Area A and B after SSHT, respectively; (e)percentage statistics of LAGB before and after SSHT.
on Recrystallization and Grain Growth Journal of Physics: Conference Series 2635 (2023) 012006 IOP Publishing doi:10.1088/1742-6596/2635/1/0120065 formation of coarse grains cannot be effectively alleviated or eliminated by shortening the holding time of solid solution.