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. 2025 Feb 21;18(5):941.
doi: 10.3390/ma18050941.

The Influence of Rare Earth Metals on the Microstructure and Mechanical Properties of 220 and 356.1 Alloys for Automotive Industry

Herbert W Doty  1 , Shimaa El-Hadad  2 , Ehab Samuel  3 , Agnes M Samuel  3 , Fawzy H Samuel  3
Affiliations

The Influence of Rare Earth Metals on the Microstructure and Mechanical Properties of 220 and 356.1 Alloys for Automotive Industry

Herbert W Doty et al. Materials (Basel). .

Abstract

Application of rare earths (RE) as grain refiners is well-known in the technology of aluminum alloys for the automotive industry. In the current study, Al-2.4%Cu-0.4%Mg alloy (coded 220) and Al-7.5%Si-0.35%Mg alloy (coded 356.1), were prepared by melting each alloy in a resistance furnace. Strontium (Sr) was used as a modifier, while titanium boride (TiB2) was added as a grain refiner. Measured amounts of Ce and La were added to both alloys (max. 1 wt.%). The alloy melts were poured in a preheated metallic mold. The main part of the study was conducted on tensile testing at room temperature. The results show that although RE would cause grain refining to be about 30-40% through the constitutional undercooling mechanism, grain refining with TiB2 would lead to approximately 90% refining (heterogenous nucleation mechanism). The addition of high purity Ce or La (99.9% purity) has no modification effect regardless of the alloy composition or the concentration of RE. Depending on the alloy ductility, the addition of 0.2 wt.%RE has a hardening effect that causes precipitation of RE in the form of dispersoids (300-700 nm). However, this increase vanishes with the decrease in alloy ductility, i.e., with T6 treatment, due to intensive precipitation of ultra-fine coherent Mg2Si-phase particles. There is no definite distinction in the behavior of Ce or La in terms of their high affinity to interact with other transition elements in the matrix, particularly Ti, Fe, Cu, and Sr. When the melt was properly degassed using high-purity argon and filtered using a 20 ppi ceramic foam filter, prior to pouring the liquid metal into the mold sprue, no measurable number of RE oxides was observed. In conclusion, the application of RE to aluminum castings would only lead to formation of a significant volume fraction of brittle intermetallics. In Ti-free alloys, identification of Ce- or La-intermetallics is doubtful due to the fairly thin thickness of the precipitated platelets (about 1 µm) and the possibility that most of the reported Al, Si, and other elements make the reported values for RE rather ambiguous.

Keywords: RE; aluminum alloys; automotive industry; grain refining; tensile properties.

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Conflict of interest statement

The authors declare no conflicts of interest.

Figures

Figure 1
Figure 1
(a) Casting set-up used for casting tensile bars; (b) 20 ppi ceramic foam filter placed at the bottom of the refractory pouring cup, and (c) Dimensions of the ASTM-B108 standard tensile bars produced from the permanent mold. All dimensions are in mm.
Figure 2
Figure 2
Optical microstructure of as-cast: (a) 220 alloy, 1—α-Al, 2—α-AlFeSi, 3—Al2Cu, (b) 356.1 alloy; and (c) Al-Si binary diagram.
Figure 3
Figure 3
Backscattered electron images of: (a) 220 alloy, (b) 356.1 alloy, revealing the co-existence of the white phase platelets and the gray-phase massive particles in both alloys-1 wt.%La, (c) 356.1 alloy +1 wt.%La-0.06 wt. %Ti, (d) 356.1 alloy + 1 wt.%La + 0.27 wt.%Ti, (e,f) morphology of the white phase in 356.1 alloy containing 1 wt.%La (cavity), (g) morphology of the gray-phase particles viewed in a cavity placed on top of a white platelet—note the clear boundaries between the two phases, (h) a high magnification image of (g) showing the boundaries between the particles of the two phases. White arrows in (a) point to the gray-phase particles, whereas yellow arrows point to the white-phase platelets.
Figure 3
Figure 3
Backscattered electron images of: (a) 220 alloy, (b) 356.1 alloy, revealing the co-existence of the white phase platelets and the gray-phase massive particles in both alloys-1 wt.%La, (c) 356.1 alloy +1 wt.%La-0.06 wt. %Ti, (d) 356.1 alloy + 1 wt.%La + 0.27 wt.%Ti, (e,f) morphology of the white phase in 356.1 alloy containing 1 wt.%La (cavity), (g) morphology of the gray-phase particles viewed in a cavity placed on top of a white platelet—note the clear boundaries between the two phases, (h) a high magnification image of (g) showing the boundaries between the particles of the two phases. White arrows in (a) point to the gray-phase particles, whereas yellow arrows point to the white-phase platelets.
Figure 4
Figure 4
EDS spectra obtained from alloys treated with 1 wt.%Ce; (a,b) Ti-free 220 alloy, (c) 220 alloy treated with 0.21 wt.%Ti, Al/Si ratio > 1, (d) 356.1 alloy + 1%Ce, Al/Si ratio~1.
Figure 5
Figure 5
(a) Microstructure of the 220 alloy in the SHT condition, (b) backscattered electron image of 356.1 alloy—SHT condition, (c) 220 alloy +1 wt.%Ce—SHT condition, (d) 356.1 alloy in the SHT condition.
Figure 6
Figure 6
DSC heating curves of 220 alloy containing 0%, 1%, and 5% Ce [24]. The red arrows highlight the disappearance of the Al2Cu phase with increase in Ce concentration.
Figure 7
Figure 7
STEM images of 220 alloy: (a) T6 condition-bright field, (b) same as (a) dark field, (c) HR image of precipitation in (a) showing the continuity of the matrix with precipitates, (d) T7 condition.
Figure 8
Figure 8
FESEM secondary electron images of 356.1 alloy: (a) T6 condition, (b) T7 condition.
Figure 9
Figure 9
Effect of Ce and La addition on the tensile properties of 220 alloy under different working conditions.
Figure 10
Figure 10
(a) Backscattered electron image of deeply etched unmodified 356.1 alloy, (b) backscattered electron image of deeply etched modified 356.1 alloy, (c) distribution of 0.5% La in the as-cast 356.1 alloy-backscattered electron image, (d) 1%La-backscattered electron image, (e) 1%La-optical image—note the disappearance of the modified Si eutectic particles, 100 ppm Sr, (f) partial fragmentation of RE platelets (1%La) following the solutionizing treatment.
Figure 10
Figure 10
(a) Backscattered electron image of deeply etched unmodified 356.1 alloy, (b) backscattered electron image of deeply etched modified 356.1 alloy, (c) distribution of 0.5% La in the as-cast 356.1 alloy-backscattered electron image, (d) 1%La-backscattered electron image, (e) 1%La-optical image—note the disappearance of the modified Si eutectic particles, 100 ppm Sr, (f) partial fragmentation of RE platelets (1%La) following the solutionizing treatment.
Figure 11
Figure 11
Effect of Ce and La additions on the tensile properties of 356.1 alloy under different working conditions.
Figure 12
Figure 12
Contribution of heat treatment and Ce addition to the tensile parameters of the 220 alloy; the as-cast sample (without RE) was used as a reference for all the data.
Figure 13
Figure 13
Contribution of heat treatment and La addition to the tensile parameters of the 220 alloy; the as-cast sample (without RE) was used as reference for all the data.
Figure 13
Figure 13
Contribution of heat treatment and La addition to the tensile parameters of the 220 alloy; the as-cast sample (without RE) was used as reference for all the data.
Figure 14
Figure 14
Contribution of heat treatment and Ce addition to the tensile parameters of the 356.1 alloy; the as-cast sample (without RE) was used as a reference for all the data.
Figure 14
Figure 14
Contribution of heat treatment and Ce addition to the tensile parameters of the 356.1 alloy; the as-cast sample (without RE) was used as a reference for all the data.
Figure 15
Figure 15
Contribution of heat treatment and La addition to the tensile parameters of the 356.1 alloy; the as-cast sample (without RE) was used as a reference for all the data.
Figure 16
Figure 16
Fracture surface of 356.1 alloy containing La: (a) 0.2%La-as cast condition, (b) 1%La-as cast, (c) 1%La-following T6 treatment, (d) 1%La following T7 treatment. The white arrows in (b) point to cracks; the yellow arrows in (c) point to a secondary crack through the matrix; the circled areas in (d) show fractured La-based platelets.
Figure 17
Figure 17
(a) Sr, (b) Mg, and (c) O distribution in a MgSiSr-phase particle, acting as nucleation sites for the formation of La-containing intermetallics.
Figure 18
Figure 18
Sr-metal interactions in 356.1 alloy: (a,b) Sr-Ti, (c,d) Sr-B, (e,f) Sr-CE.
Figure 18
Figure 18
Sr-metal interactions in 356.1 alloy: (a,b) Sr-Ti, (c,d) Sr-B, (e,f) Sr-CE.
Figure 19
Figure 19
Quality index charts obtained from 220 alloy as a function of applied heat treatment and added RE: (a) Ce and (b) La. The colored lines in the two plots represent the heat treatment condition, i.e. As-cast, SHT, T6 and T7 The dots on each colored line show the Ce and La additions of 0%, 0.2%, 0.5% and 1%, on going from left to right.
Figure 20
Figure 20
Quality index charts obtained from 356.1 alloy as a function of applied heat-treatment and added RE: (a) Ce, (b) La. The colored lines in the two plots represent the heat treatment condition, i.e. As-cast, SHT, T6 and T7. The dots on each colored line show the Ce and La additions of 0%, 0.5%, and 1%, on going from right to left.
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