Experimental investigation on properties of laminates produced by Large Strain Extrusion Machining

Abstract

Efficient manufacturing practices aim to minimize energy consumption and waste generation, promoting sustainability. This study focuses on fabricating titanium alloy laminates using the Large Strain Extrusion Machining (LSEM) process. Unlike conventional methods, LSEM allows for single-pass production of laminates with controlled microstructures. This approach contrasts with traditional rolling methods, which are complex for high-strength materials like titanium alloys due to the need for multiple stages and challenges in texture control. By employing LSEM, we aim to streamline production processes while ensuring precise microstructural control in a continuous operation, particularly beneficial for aerospace applications. An experimental test rig was fabricated and utilized alongside a lathe machine to facilitate the constrained production of laminates. In this study, three distinct materials were chosen: commercially pure titanium, Ti-6Al-4V, and Ti-6Al-7Nb, each intended for laminate fabrication with varying strain rates. Four different levels of strain rates, ranging between 0.25 s-1 to 2 s-1, were employed to produce four laminate samples for each material. Following production, the laminates underwent comprehensive mechanical and metallurgical characterization, including micro and nano hardness testing, surface roughness analysis, measurement of average crystallite size, and evaluation of microstrain. The results of each characterization parameters are presented and compared with it based own raw material of all the three selected materials. Overall, it is found from the study that the properties were improved from the bulk material i.e. commercially pure titanium, Ti-6Al-4V, and Ti-6Al-7Nb. Microhardness assessments of both the titanium alloy bulk materials and laminates consistently showed higher hardness values in the laminates compared to the parent materials. Specifically, the laminates exhibited improvements in microhardness (HV) ranging from 25% to 52% higher than CP Ti, 7% to 25% higher than Ti-6Al-4V, and 8% to 22% higher than Ti-6Al-7Nb. Furthermore, nano-hardness measurements via nano-indentation revealed significant enhancements in laminate hardness compared to the bulk materials, with improvements of approximately 60% for CP Ti, 41% for Ti-6Al-4V, and 8-10% for Ti-6Al-7Nb. These results highlight the potential for the laminates to possess superior mechanical properties and performance compared to their bulk counterparts. Based on the hardness testing results, it is evident that the processed materials for laminates exhibit significantly improved properties vi compared to the base materials across all three materials. Additionally, surface topography, a crucial parameter for laminates, was evaluated by measuring surface roughness using a surface roughness tester. The reported roughness values were compared and analysed to identify the best-performing laminate in terms of surface quality with different strain rate. X-ray diffraction (XRD) analysis of laminates produced via Large Strain Extrusion Machining (LSEM) reveals significant deformation and microstructural evolution. Line broadening analysis indicates increased microstrain and decreased crystallite size, reflecting the severity of the fabrication process. Grain refinement is observed, escalating with strain rates until a threshold, where temperature-induced recrystallization moderate deformation effects, elucidating complex microstructural dynamics. The results highlight the best-performing laminates identified through hardness testing and analysis. A notable decrease in crystalline size accompanied by an increase in microstrain is observed, indicative of enhanced hardness in the processed material. These findings underscore the effectiveness of the fabrication process in improving material properties. Further analysis was conducted to retrieve the machining parameter, namely the strain rate, required for achieving the desired mechanical and metallurgical properties. The inverse analysis was carried out by formulating an objective function using the Least Square Minimization Technique in conjunction with the Golden Section Search Method for single-parameter retrieval. The analysis was conducted on four independent parameters: hardness, surface roughness, average crystalline size, and microstrain, using retrieval studies to determine strain rate values. Subsequently, the obtained parameters were utilized to retrieve properties using empirical relations and experimental procedures for all four parameters. It was found that the maximum error in hardness retrieval was 1.2%, 7.2% for surface roughness, 32% for average crystalline size, and 3.6% for microstrain based on empirical relations. Similarly, retrieval studies were conducted using experiments by preparing samples at the obtained strain rate values from the inverse study. The overall error was found to be maximum in experimental testing, with 9% for hardness, 17% for roughness, and 23% and 9% for average crystalline size and microstrain, respectively. Overall, the inverse analysis is considered useful except for the Average Crystallite at 15 target value. Moreover, the error remains less than 10% for empirical relational and 17% for experimental testing for all the inverse retrieval parameters. vii Finally, tribological characterization was conducted using a tribometer at three different load (10N, 30N, 50N) and keeping the sliding velocity constant at 1m/s. The produced laminates undergone dry sliding wear tested in two directions: laminate formation and transverse direction. At 10 N it was observed that the wear rate of CP-Ti laminates improved by at least 42% for a sliding distance of 500 m compared to bulk CP-Ti. An overall enhancement ranging from approximately 51% to 85% was noted for the manufactured laminates up to a sliding distance of 2000 meters. When identical laminates were tested in the transverse direction, an overall wear rate enhancement were seen of approximately 47% to 75%. At a load of 30 N, CP Ti laminates demonstrated enhancements in the range of approximately 42% to 69% when tested along the formation direction. However, this enhancement decreased to around 41% to 67% when tested along the transverse direction. For the highest load of 50 N, this enhancement further decreased to approximately 25% to 66% when tested along the formation direction, while during transverse direction testing, it fell within the range of about 29% to 60%. Similar observations were made for the other two materials, Ti-6Al-4V and Ti-6Al-7Nb, with improvements in wear rate of 31% and 56%, respectively, for a sliding distance of 500 m. Overall, it is evident that the improved material properties primarily result from severe plastic deformation in the laminates. This is further supported by the microstructural refinement observed in the laminates produced using LSEM.