The influence of aging treatment on the microstructural and mechanical behavior by ultra-micro hardness tester in Ni-rich NiTi alloy

The present study aims to assess the superelasticity behavior in Ni-rich NiTi alloy wire produced by rotary forging process. The thermomechanical process involved four steps of hot working at 800ºC, two steps of cold working with solution heat treatment at 800ºC between them, and subsequently a solution heat treatment (950ºC during 2 hours) followed by aging treatment at 350, 400 and 450 ºC during 30 minutes. X-ray diffraction (XRD) and instrumented ultra-micro hardness testers evaluated the present phase at each aged sample and were compared with their mechanical behavior. The results put in evidence the work-hardening effect on a forged condition associated with the final step of cold rotary forging. The solution treatment promotes stress relaxation and precipitate dissolution. The sample heat-treated shows the presence of the precipitated (Ni4Ti3) and R phase. The presence of these precipitates is beneficial because precipitation-hardening increases the yield strength of austenite, which in turn contributes to better functional stability.

A Novel Cold Rotary Forging Method of Producing Multiple Racks Using One Set of Punch

When producing racks by cold rotary forging, the top punch and the rack teeth definitely intervene and thus the top punch has to be amended, which makes the technical designing processes difficult and complex (Han et al., 2016, “Cold Orbital Forging of Gear Rack,” Int. J. Mech. Sci., 117(10), pp. 227–242). In this study, a novel cold rotary forging method of producing racks is put forward to avoid the interventions between the top punch and the racks. Thus, the top punch need not be amended and the technical designing processes correspondingly become simple. In light of this presented method, a novel idea for cold rotary forging of producing multiple racks using one set of punch is motivated. The concrete researches are as follows: First, the mathematical models are developed and three kinds of key forging conditions in cold rotary forging of racks are calculated to avoid the interventions between the top punch and the racks. The first one is the condition that the top punch and the rack teeth do not intervene. The second one is the condition that the top punch and cylindrical surfaces of racks do not intervene. The third one is the condition that the top punch can be successfully constructed. On the basis of these three kinds of key forging conditions, the workpiece is optimized and the cold rotary forging processes of racks with constant and variable transmission ratio are examined using finite element (FE) simulations. The experimental researches are also conducted. The results show that for both racks with constant and variable transmission ratio, the obtained key forging conditions are effective and the presented cold rotary forging principles of producing multiple racks using one set of punch are feasible.

Analysis of Innovative Incremental Cold Forming Process for the Manufacturing of Aerospace Rotating Parts

Cold rotary forging is an innovative incremental metal forming process whose main characteristic is that the workpiece is only partially in contact with a conical tool, reducing therefore the required forging loads. However, in spite of many benefits of such a process, wide industrial implementation of rotary forging is not possible without proper understanding of material behaviour. In the present work, the capability of rotary forging process was explored for the manufacturing of flared cylindrical parts by cold forming. Another main aim was to assess the cold formability of high-strength materials for aerospace applications (martensitic stainless steels) under incremental processes. In order to understand the impact of rotary forging on the final properties of formed components, microstructural and mechanical analysis were performed. Microstructural and hardness analysis were conducted on both axial and transverse sections along the cold formed flange in order to study the grain flow orientation and strain distribution. In a similar fashion, mechanical test specimens were machined from different positions and orientations along the rotary forged component. Further analysis was performed on the components in the as-treated condition in order to understand the response of cold-worked Jethete M152 components to subsequent heat treatments. Microstructural and hardness analysis clearly reveals a strong grain reorientation and strain localization around “pickup“ defects (material attached to the upper tool) observed on the flange top surface, close to the flange edge. These results suggest that an excessive deformation is localized during the early stages of the flange formation. Another characteristic feature found in the rotary forged parts is the presence of a buckling phenomenon which appears in later stages of the rotary forging process. Strain hardening along with the increasing flange length requires higher levels of forging loads to keep forming the flange. This results into a significant accumulation of compressive stresses in the transition region between the flange and the straight region. Gradually the resultant compressive force exceeds the critical buckling load, leading to the occurrence of the buckling phenomenon. This latter issue determines the limit of the cold flaring process. This can help to determine the maximum length of the flange part, achievable in this process, which is of great importance for the design of these manufacturing technologies. From the mechanical testing results, large differences were found as a function of both position and orientation (axial, transverse) throughout the rotary forged components (anisotropic properties). Concerning the impact of heat treatments on cold-worked components, no differences were found in the as-treated condition, in terms of microstructural and mechanical properties between regions with a large difference in strain distribution. These results denote the normalizing effect of conventional hardening treatments on cold-worked Jethete M152 components, restoring the homogenous and isotropic properties across the whole component.