In the
forging process, forgings go through complex thermoplastic deformation, which involves several mechanisms like intragranular slip, twinning, grain boundary sliding, and diffusion creep. These mechanisms are strongly influenced by the forging conditions such as temperature and strain rate. Especially under high temperatures and high strain rates, the way the forging deforms shows various characteristics. In this article, we will take a closer look at these thermoplastic deformation mechanisms and explore how they relate to the softening process of the
forging. This will help us better understand how deformation and microstructure evolve during forging.
During thermoplastic deformation, forgings experience a mix of different mechanisms that determine how they deform during the forging process. These mechanisms include intragranular slip, twinning, grain boundary sliding, and diffusion creep. Depending on the temperature and strain rate, each mechanism plays a different role, influencing the plasticity, strength, and final microstructure of the forging.
Intragranular slip is the most common and important deformation mechanism during forging, especially at lower temperatures. As the deformation temperature increases, atomic spacing becomes larger, and thermal vibrations and diffusion become stronger, making dislocation movement easier. Dislocations slip, climb, cross-slip, and unpin more easily at higher temperatures compared to lower temperatures. When more slip systems are activated, the forging's ability to deform improves, and the deformation between grains becomes more coordinated, leading to more uniform deformation across the entire forging.
Twinning generally happens at higher temperatures and strain rates, especially in metals with hexagonal crystal structures (like magnesium and zinc). In these metals, intragranular slip is harder, and twinning provides an additional path for deformation. This helps the forging maintain good plasticity at high strain rates.
Grain boundary sliding and diffusion creep are important in high-temperature forging. Grain boundary sliding occurs when the strength of the grain boundaries is lower than the grains themselves, making it easier for the grains to slide along their boundaries. Under high-temperature forging conditions, the effect of diffusion creep becomes more noticeable. Diffusion creep happens slowly, even under low stress, and becomes more pronounced as temperature increases, grain size decreases, and strain rate lowers.
Diffusion creep can happen in two ways: intragranular diffusion and grain boundary diffusion. Intragranular diffusion causes the grains to elongate in the direction of tensile stress or shrink in the direction of compressive stress. Grain boundary diffusion leads to grain "rotation." At high temperatures, diffusion speeds up, and this helps to reduce the negative effects of grain boundary sliding, improving the overall mechanical properties of the forging.
During the forging process, triaxial compressive stress helps repair cracks caused by grain boundary sliding at high temperatures. It works through plastic welding, which promotes deformation between the grains. However, under typical thermomechanical conditions, grain boundary sliding contributes less to overall deformation compared to intragranular slip. Only under superplastic deformation conditions, where grains are fine, does grain boundary sliding play a more dominant role. It usually works together with diffusion creep.
The softening process during thermomechanical deformation plays a crucial role in determining the forging's plasticity and resistance to deformation. The softening process is closely related to factors such as deformation temperature, strain rate, deformation level, and the metal properties of the forging. Common softening processes include dynamic recovery, dynamic recrystallization, static recovery, static recrystallization, and subdynamic recrystallization.
Dynamic recovery is a self-healing process that happens during hot forging. It occurs through dislocation climb and cross-slip, and is more common in metals with high stacking fault energy, such as aluminum alloys and ferritic steels. These metals allow dislocations to move more easily, which reduces dislocation density and lowers the forging's distortion energy.
Dynamic recovery usually happens during the forging process. Even if the deformation is large and the temperature is high, the forging tends to undergo dynamic recovery rather than dynamic recrystallization. In this case, the forging retains a subgrain structure with a higher dislocation density. If heat treatment is applied during this process, the forging can benefit from both deformation strengthening and heat treatment strengthening, improving its mechanical properties. This combined approach is called high-temperature deformation heat treatment.
Dynamic recrystallization is the process where new grains form through nucleation and growth during hot forging. This process is more common in metals with low stacking fault energy, such as copper, gold, silver, and stainless steel. These metals have slower dynamic recovery rates, which means the forging accumulates enough distortion energy in local areas, promoting dynamic recrystallization. After dynamic recrystallization, the grains are usually smaller, with irregular grain boundaries and many dislocation tangles. These characteristics help the recrystallized grains grow and spread.
The grain size after dynamic recrystallization depends on the deformation temperature, strain rate, and the level of deformation. By controlling these factors, the grain size after recrystallization can be adjusted to optimize the forging's mechanical properties. Smaller grains typically lead to higher resistance to deformation and stronger material properties.
After the forging process, when the forging is kept at high temperatures, static recovery and static recrystallization continue to take place. Static recovery usually happens in forgings with minimal deformation. Static recrystallization occurs in forgings that have undergone significant deformation. When the deformed forging is kept above the recrystallization temperature, static recrystallization begins. After an incubation period, the forging becomes equiaxed in structure, removing any distortions caused by deformation.
Static recrystallization is a slower process that requires time to complete. Factors such as the purity of the material, stacking fault energy, and cooling rate after deformation affect how quickly static recrystallization happens.
Sub-dynamic recrystallization refers to recrystallized nuclei that form during thermomechanical deformation but haven't fully grown yet, or grains that have partially grown during dynamic recrystallization. These continue to grow after deformation stops. Unlike dynamic recrystallization, sub-dynamic recrystallization doesn't need additional nucleation time, so it happens faster, especially when the forging's temperature remains high after deformation.
Sub-dynamic recrystallization usually occurs in forgings where recrystallization has already started during hot forging. In real-world production, due to limitations in process control, the structure formed by dynamic recrystallization is often not fully preserved, so sub-dynamic recrystallization plays an important role.
In the forging process, the thermoplastic deformation and softening mechanisms of the forging are key factors that determine the performance of metal materials. Mechanisms like intragranular slip, twinning, grain boundary sliding, and diffusion creep have different effects under various deformation conditions. These mechanisms are closely linked to the softening processes of the forging, such as dynamic recovery, dynamic recrystallization, and static recovery. By controlling parameters like forging temperature, strain rate, deformation level, and the metal properties, it's possible to adjust the microstructure to optimize the material's mechanical properties. Understanding these deformation and softening processes is essential for improving the forging process and the final quality of the forgings. In practical production, controlling these mechanisms can not only improve performance but also ensure the excellent quality of the final product.
