Effects of equal-channel angular pressing and equal-channel multi-angular pressing on the microstructure and mechanical performance of aluminum strips

Metals are rarely used in their raw, cast form. To make them strong enough for airplanes, cars, or buildings, engineers must change their internal structure. Imagine a block of metal as a collection of tiny crystals, or grains, packed together. In a soft, untreated piece of aluminum, these grains are relatively large and loosely organized. To make the metal harder and stronger, scientists use a technique called severe plastic deformation. This involves forcing the metal through a confined space without changing its overall shape, essentially squeezing the internal grains until they are crushed into much smaller pieces. The smaller the grains, the stronger the metal becomes, because the boundaries between them act as barriers that stop the material from bending or breaking easily. This process is crucial for industries that need lightweight materials that can withstand immense stress.

In a recent study, researchers set out to compare two specific methods of this squeezing process to see which one works better for thin strips of aluminum. The two methods are known as equal-channel angular pressing and equal-channel multi-angular pressing. Both involve pushing a metal strip through a die, or mold, that has intersecting channels. The difference lies in the design: one method uses a single intersection point, while the other uses two. The team wanted to know which design would create the finest, most uniform grain structure and result in the strongest material. They chose a common, commercially pure aluminum alloy, often used in insulation and general manufacturing, and subjected it to these processes up to three times to see how the metal changed with each pass.

The researchers began with aluminum strips that had been softened by heating them in an oven, a step that made the internal grains large and uniform. They then fed these strips through the two different presses at room temperature. The first press, the single-channel version, forced the metal through a sharp bend. The second press, the multi-channel version, guided the metal through two successive bends. After each pass, the team examined the metal under powerful microscopes and tested how much force it could take before breaking. They also measured the metal's resistance to scratching, a property known as hardness, and looked at the surface of the metal after it snapped to see how it failed.

The results showed that both methods successfully broke down the large grains into much smaller ones, but they did so in different ways and with different levels of success. The single-channel press proved to be the more aggressive tool. After three passes, it reduced the grain size from a range of up to eight micrometers down to a range of just 2.4 micrometers. This process created a mix of very fine grains and some ultra-fine, almost nanoscale grains. The metal became significantly stronger and harder. Its ability to resist pulling forces, known as tensile strength, increased by nearly 94 percent, and its resistance to permanent bending, or yield strength, rose by about 21 percent. The metal also became harder to the touch, with a hardness increase of over 63 percent in the direction of the pressing.

The multi-channel press also improved the metal, but the changes were less dramatic. While it did refine the grains, the final size remained larger, settling at a range of up to 5.5 micrometers after three passes. The metal did become stronger, with a tensile strength increase of roughly 129 percent and a yield strength increase of about 31 percent, but the hardening effect was not as intense as with the single-channel method. The hardness of the metal increased by about 54 percent in the direction of the press, which was substantial but still lower than the single-channel result. The researchers observed that the multi-channel process sometimes led to a slightly coarser structure in certain areas, likely because the metal experienced a different pattern of stress as it moved through the two bends.

When the team looked at how the metal broke, they found clear evidence of these internal changes. The original, soft aluminum broke with large, deep pits on its surface, a sign that it stretched and deformed easily before snapping. As the metal was processed, these pits became smaller and shallower. This shift indicated that the metal had become stronger but less flexible. The single-channel processed metal showed the most significant reduction in pit size, confirming that its internal structure had been altered the most drastically. The team noted that while the metal became much stronger, it also lost some of its ability to stretch, a common trade-off in this type of strengthening.

The study concluded that while both techniques are effective for refining the structure of aluminum strips, the single-channel method is superior for achieving maximum hardness and strength in this specific material. The single-channel press created a more uniform and finer grain structure, leading to a more pronounced strengthening effect. The multi-channel press, while still beneficial, produced a slightly coarser result. These findings provide a clear guide for manufacturers: if the goal is to make thin aluminum strips as strong and hard as possible, the single-channel pressing route is the more efficient choice. The research highlights how the specific geometry of a tool can dictate the final quality of a material, offering a precise way to tailor metals for demanding industrial applications.