Prospective, non-heat-treatable aluminum alloys for railway car manufacturing

A global trend of sustainable growth in the use of aluminum alloys in railway car manufacturing has been observed due to their significant advantages compared to steel. It has been demonstrated that in Russia, the primary alloy used in railway car manufacturing is alloy 1565ch, which is a modification of magnesium alloy AMg6. As an alternative, alloys containing approximately 2 % Cu and 2 % Mn are proposed, characterized by the presence of nanoscale dispersoids Al ₁₅ Cu ₂ Mn ₃ in an amount exceeding 7 % by volume. These proposed alloys exhibit high formability and increased strength properties in the annealed state.

Keywords: aluminum alloys, Al Cu Mn, system, railway car manufacturing, structure, phase composition, mechanical properties.

JEL Classification R01

Introduction

Aluminum alloys have been widely used in the construction and operation of rolling stock in railway transportation for quite some time. Global experience shows that freight cars made of aluminum alloys have gained the most popularity. This is because it is more cost-effective to transport various bulk cargoes, such as cement, coal, grain, mineral fertilizers, iron ore, and more, in aluminum wagons. Additionally, it is advantageous to transport petroleum products, various chemicals, and liquefied gases in aluminum tank cars. Such practices are widespread in European countries, Japan, China, and North America, where the share of aluminum wagons reaches fifty percent of the entire freight fleet.

It should be noted that in the Soviet Union, experimental operation of four-axle and six-axle gondola cars with aluminum bodies was conducted. It is important to highlight that the developers at that time assumed that aluminum gondola cars should mainly replicate their steel counterparts in terms of design. However, the weight of such a body was approximately four tons lighter than that of a steel gondola car. The aluminum cars were experimentally operated for about five years but were discarded due to the formation of cracks in the most heavily loaded areas. Some enterprises in Russia later developed aluminum wagons and conducted experimental operation of individual prototypes, but they did not enter mass production.

Nevertheless, today in Russia, there is demand for railway wagons made of aluminum alloys, including the products of the All-Russian Scientific Research Institute of Railway Transport [1–5]. Additionally, according to the Aluminum Association [6–10], OK RUSAL, Samara Metallurgical Plant, plant «Sespel» in collaboration with RM Rail and All-Russian Scientific Research Institute of Railway Transport, have developed and manufactured a so-called hopper (a bulk cargo wagon) of model 19–1244 (Figure 1). The use of aluminum semi-finished products in the construction of the wagon body has resulted in the innovative hopper having significantly better performance characteristics compared to its steel counterparts.

Fig. 1. Innovative aluminum alloy hopper wagon [1]

According to forecasts from the Aluminum Association, railway carriers will be procuring only innovative hopper wagons in quantities of at least four thousand per year in the near future. Additionally, the use of aluminum alloys for manufacturing railway cargo containers for transporting liquefied gases is becoming relevant. By replacing steel wagons with aluminum ones, transportation companies will reduce their rolling stock by at least fifteen percent without decreasing the volume of transportation.

Samara Metallurgical Plant (formerly Arkonik SMZ) has proposed the use of a new deformable aluminum alloy, 1565ch (State standard 4784–2019), in the design of railway wagon bodies. This alloy, while possessing the main advantages of well-known magnesium alloys such as AMg5 and AMg6, has higher strength, as reflected in Table 1.

Table 1

Mechanical properties under tensile testing of aluminum alloy sheets used in transportation [ 2–7 ]

The 1565ch alloy (its composition is provided in Table 2) can be considered the strongest not only among the grade-based magnesium alloys (excluding those containing expensive scandium additives) but also among the known deformable aluminum alloys that do not require heat treatment (so-called «thermally non-strengthenable alloys»). However, the potential for further improvement of magnesium alloy properties can be considered exhausted. This is due to the following reasons:

1. The concentration of magnesium (Table 2), which contributes significantly to the strengthening, cannot be increased as it would lead to an unacceptable reduction in deformability and corrosion resistance.
2. Complete dissolution of magnesium in the aluminum solid solution (referred to as (Al) ) requires homogenization of ingots at 430–440 °C, which can result in coarsening of dispersoids (primarily Al ₆ Mn ).
3. Strict limitations on the concentrations of zirconium, chromium, and titanium, as well as control over the temperature regime during melting and casting, must be strictly maintained to prevent the formation of primary aluminides containing these additives.

As an alternative to deformable magnesium alloys like AMg6 , alloys containing approximately 2 % Cu and 2 % Mn are proposed (variants also include small additions of Zr, Si, and other elements) [14, 15]. Since the cast structure of these alloys (referred to as ALTEK) contains a small amount of eutectic Al2Cu phase inclusions (Figure 1, a ), homogenization of the ingots is not required. During deformation, there is an improvement in the structure, particularly a more uniform distribution of Al2Cu particles (Figure 1, b ). Furthermore, the deformed semi-finished products do not require quenching, as the formation of Al15Cu2Mn3 dispersoids (Figure 2) occurs during the heating of the ingots prior to pressure processing and subsequent thermo-mechanical treatment (TMT). These dispersoids hinder recrystallization and provide higher thermal stability compared to grade-based alloys. This similarity with the 1564ch alloy lies in their optimized structure, not for achieving maximum solid solution strengthening, but for maximizing the effect of dispersoid strengthening. This is achieved through the optimization of the TMT regime, which allows for the formation of dispersoids in an amount exceeding 7 % in volume and with a size less than 100 nm (Figure 2) [1–6].

Fig. 2. Microstructure of ALTEK-2 alloy in the ingot ( a ) and cold-rolled sheet ( b ), obtained from a non-homogenized ingot

The compositions of two typical variants of such alloys are presented in Table 2. While the ALTEK-1 alloy is relatively complex in terms of alloying elements, resembling 1565ch, ALTEK-2, on the contrary, has a simpler composition. Despite this, the mechanical properties of the latter are quite high (Table 3) and comparable to the properties of the 1565ch alloy. In particular, after annealing at 400 °C, the strength properties of the ALTEK-2 alloy are significantly higher than those of the grade-based heat-treatable alloy 2219 (Table 3).

Fig. 3. Microstructure of the cold-rolled sheet of the ALTEK-2 alloy after a 3-hour annealing at 400 °C

Currently, research is being conducted to increase the strength of alloys in the proposed group while maintaining their basic advantage in terms of processability (i.e., without the need for homogenization and quenching). In particular, in the ALTEK-3 alloy (in the form of a cold-rolled sheet annealed at 400 °C) containing small additions of magnesium and zinc, the following values have been achieved: σ _v ~ 350 MPа, σ _0,2 ~280 MPа, ~7 % elongation. This makes it promising for the development of next-generation alloys for use in the railway industry.

Conclusion

1. A brief analysis of the global experience in the use of aluminum alloys in the railway industry has been conducted. The steady increase in their market share has been noted, which is attributed to significant advantages compared to steel.
2. It has been shown that in Russia, the main alloy used in railway construction is the 1565ch alloy, which is a modification of the AMg6 magnesium alloy. The 1565ch alloy can be considered as one of the most widely used deformable aluminum alloys that do not require quenching (excluding expensive alloys containing scandium additives).
3. As an alternative to deformable magnesium alloys (including 1965ch), alloys containing approximately 2 % Cu and 2 % M n are proposed. These alloys have a distinctive structure characterized by the presence of nanoscale Al ₁₅ Cu ₂ Mn ₃ dispersoids in quantities exceeding 7 % in volume. The proposed alloys exhibit high deformation processability and enhanced strength properties in the annealed condition.

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