Tensile properties and constitutive model of ultrathin pure titanium foils at elevated temperatures in microforming assisted by resistance heating method

Pure titanium (Ti) is often used for microparts in biomedical devices and implants. Microforming is a promising technology for the manufacture of microparts. Owing to the occurrence of size effects in microforming, the material flow is nonhomogeneous and the process parameters exhibit considerable scattering. Heat-assisted microforming is an effective process for solving these problems. To improve the heating rate, the resistance heating method has been introduced into the microforming process. To design an effective resistance-heating-assisted microforming process, the relationship between the electric current and the flow stress of the material should be determined.To achieve this, a tensile testing system incorporating the resistance heating method is developed in this study. The tensile properties of 0.05-mm-thick pure Ti foils are investigated by performing uniaxial tensile tests at elevated temperatures. The tensile tests are carried out at different angles (0°, 45°, and 90°) relative to the rolling direction, at various temperatures from room temperature (298 K) to 723 K, and under different strain rates from 10⁻⁴ to 10⁻¹ s⁻¹. To contribute to the design of the resistance-heating-assisted microforming process, the effect of the temperature and electrical current density on the material properties of ultrathin pure Ti foils is discussed. A constitutive model based on the Fields–Bachofen (FB) equation is derived to describe the flow stress of ultrathin pure Ti under different forming conditions. The effect of the electrical current density on the work hardening and strain rate sensitivity is included in the derived constitutive model. The good agreement between the calculated and experimental results confirms the feasibility of the proposed constitutive model for resistance-heating-assisted microforming.     

Plastic anisotropy of ultra-thin rolled phosphor bronze foils and its thickness strain evolution in micro-deep drawing

Metal foils are highly advantageous for producing microcomponents with high-aspect-ratio three-dimensional shapes by miniaturizing the process dimensions in sheet-metal-forming technologies. To characterize existing rolled metal foils at manufacturing sites and to clarify the impact of its strong anisotropic properties on micro-sheet formability, tensile tests and micro-deep drawing tests were performed on phosphor bronze foils with thicknesses of 20–300 μm. Focusing on the Lankford value (r-value) as a useful parameter for conventional sheet-metal-formability, the relation between the r-value of ultra-thin rolled foil and its applicability in micro-deep drawing is investigated. Ultra-thin rolled foil is characterized with a higher r-value due to the strong texture of {1 1 0} and {1 1 1} textures. Although the in-plane tendency of the r-value showed a strong correlation with the thickness distribution of micro-drawn cups, the obtained higher r-value for thinner foils does not correspond to the lower formability of thinner metal foils. As relevant parameter for indicating the forming limit for thin-rolled metal foils, the nonuniformity in thickness due to surface roughening is introduced. The importance of a geometrical anisotropy, such as orientation of surface topography and defects, for the unstable deformation of ultra-thin rolled metal foils is experimentally demonstrated.     

微型模具及微成形技术的发展

这里阐述了微型模具及微成形技术的基本概念和研究现状，介绍了微型模具制造的原理和技术以及微成形技术的特点，最后提出了该技术领域需要解决的一些关键技术问题和发展趋势。     

微机械加工技术─LIGA技术

简要介绍了LIGA（利嘎）工艺过程、特点和应用实例。     

On the influence of workpiece material on friction in microforming and lubricant effectiveness

The frictional behaviors between metal forming tool and three different metallic materials were evaluated using the microforming T-shape test. A mathematical function is proposed to describe the calibration curves for different friction coefficients. Round bars of copper, aluminum and silver of diameter 1 mm and length 5 mm were used as the workpieces to study the material influence on friction factor, m, during unlubricated microforming process through comparison between simulation and experimental results. Furthermore, various lubricants were used with the aluminum and copper to examine their performance in microforming. The results have shown that the workpiece materials not only determine the friction factor, m, during unlubricated microforming, but also influence the performance of lubricants. Lubricant can be completely ineffective and may not produce discernible friction reduction in microforming, unlike in conventional metal forming. By considering the influence of contact pressure on lubricant effectiveness, a novel pressure dependent frictional model and a lubricant evaluation method are proposed.     

Size effects on process performance and product quality in progressive microforming of shafted gears revealed by experiment and numerical modeling

As one of the indispensable actuating components in micro-systems, the shafted microgear is in great production demand. Microforming is a manufacturing process to produce microgears to meet the needs. Due to the small geometrical size, there are uncertain process performance and product quality issues in this production process. In this study, the shafted microgears were fabricated in two different scaling factors with four grain sizes using a progressively extrusion-blanking method. To explore the unknown of the process, grain-based modeling was proposed and employed to simulate the entire forming process. The results show that when the grains are large, the anisotropy of single grains has an obvious size effect on the forming behavior and process performance; and the produced geometries and surface quality are worsened; and the deformation load is decreased. Five deformation zones were identified in the microstructures with different hardness and distributions of stress and strain. The simulation by using the proposed model successfully predicted the formation of zones and revealed the inhomogeneous deformation in the forming process. The undesirable geometries of microgears including material unfilling, burr and inclination were observed on the shaft and teeth of gear, and the inclination size is increased obviously with grain size. To avoid the formation of inclination and material unfilling, the punch was redesigned, and a die insert was added to constraint the bottom surface of the gear teeth. The new products had then the better forming quality. The full text can be downloaded at https://link.springer.com/article/10.1007/s40436-022-00414-0