Occurrence Of Size Effect Research Articles

Investigation of Size Effects in Multi-Stage Cold Forming of Metallic Micro Parts from Sheet Metal.

Product miniaturisation and functional integration are currently global trends to save weight, space, materials and costs. This leads to an increasing demand for metallic micro components. Thus, the development of appropriate production technologies is in the focus of current research activities. Due to its efficiency, accuracy and short cycle times, microforming at room temperature offers the potential to meet the steadily increasing demand. During microforming, size effects occur which negatively affect the part quality, process stability, tool life and handling. Within this contribution, a multi-stage bulk microforming process from sheet metal is investigated for the materials Cu-OFE and AA6014 with regard to the basic feasibility and the occurrence of size effects. The results reveal that the process chain is basically suitable to produce metallic micro parts with a high repeatability. Size effects are identified during the process. Since several studies postulate that size effects can be minimised by scaling down the metallic grain structure, the grain size of the aluminium material AA6014-W is scaled down to less than one micrometre by using an accumulative roll bonding process (ARB). Subsequently, the effects of the ultrafine grain (UFG) structure on the forming process are analysed. It could be shown that a strengthened material state increases the material utilization. Furthermore, too soft materials can cause damage on the part during ejection. The occurring size effects cannot be eliminated by reducing the grain size.

Analysis of size dependent earing evolution in micro deep drawing of TWIP steel by using crystal plasticity modeling

As a promising microforming process, micro deep drawing has attracted great attention for manufacturing of microparts with unique cup-features. However, the occurrence of size effect (SE) and the intrinsic characteristics of crystallographic orientation can lead to an inevitable and uncertain plastic anisotropy during the micro deep drawing and further result in the formation of earing. In this research, a size dependent polycrystalline hardening model using crystal plasticity (CP) theory considering the interactions between slip and twinning was developed via incorporating the size scaling factor into the surface layer model to represent and model the SE and further analyze the entire micro deep drawing process of the TWIP steel. Then the micro deep drawing of polycrystalline TWIP steel with different grain sizes and thicknesses was conducted using the elaborately designed tooling sets. The size dependent crystal plasticity-based finite element modeling in conjunction with a virtual polycrystalline microstructure was employed to study the earing evolution induced by grain size, orientation and geometrical size. It is revealed that an obvious earing profile occurred in the workpiece with the larger size scaling factor. In addition, it is noted that the earing at micro scale is obviously higher than that at macro scale. The asymmetric distribution of earing profile is induced by the Goss and Brass orientations, while the Cubic type orientation leads to the symmetric distribution of earing profile. This issue is mainly attributed to the symmetric distribution of slip activities than that of twinning activities, since the activated quantity of twinning volume fraction is much smaller. Therefore, the proposed size dependent polycrystalline hardening modeling is efficient in predicting the earing formation at micro scale. The study thus provides an in-depth understanding of micro deep drawing of sheet metals and accurate prediction of shape microforming of microparts.

Size effect affected deformation characteristics in micro deep drawing of TWIP domed-bottom cups

Abstract The occurrence of size effect in micro-scale deep drawing leads to the geometrical uncertainties, which affects the deformation behavior, in process determination. Micro deep drawing of domed-bottom cups of Fe-30Mn-3Si-2Al-0.11C TWIP steel with different grain size and thickness size was investigated in this research to obtain the characteristics of micro-deformed sheet to study the size effect on deformation behavior, earing profile and defect formation of the micro-parts. Finite element simulation shows fracture in the place with the thinnest thickness of the micro-parts easily occurs around the punch corner. Deformation load decreases with the increasing grain size for each thickness size except the fracture case, while under the particular grain size the load increases with the increase of thickness. Earing easily occurs with the larger grain size and smaller thickness size in the well-formed domed-bottom cups. Inhomogeneous deformation leads to irregular geometry of the formed part. The research promotes the understanding of size effect on micro deep drawing and facilitates the development of micro forming process.

Size Effects on Formability of Copper Foil in Flexible Micro-Bending Process

The occurrence of size effects in the microforming leads to the uncertainties in process determination and quality control. In this research, a series of experiments were conducted in UTM4104 testing machine to investigate the grain size effect and feature size effect in micro-bending. Different grain size (d), thickness to grain size ratio () and micro-mold feature size (W) were prepared to explore size effects on formability of copper foil. The formability characterized by forming depth, deformation uniformity and surface integrity was discussed. It was found that the normalized forming depth presented a gradually rise and then declined markedly when N value further decreased to 0.79. The ductile fracture mode was observed for all grain-sized workpiece and the corresponding limit forming depth decreased with increasing grain size. Besides, the thickness thinning distribution and microhardness distribution showed the similar variation tendency like M. Both the standard deviation of thickness reduction and the roughed degree of surface topography indicated the worsening deformation uniformity of the foils with a larger grain size. The inhomogeneous plastic flow of material may be the reason to explain the depression near fracture location which is only observed in coarse-grained workpiece. Overall, it is concluded that the fine-grained copper exhibited better formability as the coarse-grained workpiece experienced severe strain incompatibility.

Plasticity and Size Effects in High Purity Cobalt: An Experimental Study

The occurrence of size effects in cobalt was examined by the analysis of mechanical properties of samples with thickness t, in a large range of grain size d giving a number of grains across the thickness t/d. On Hall-Petch plots, from the very beginning of plastic strain, two linear behaviors are notable: the polycrystalline one for higher t/d and the multicrystalline one for lower t/d in which the flow stress is strongly reduced. (t/d)c is the threshold value between the two behaviors taking a value of around 14. This high value is directly linked to the low stacking fault energy of cobalt. The microstructure of the polycrystalline samples exhibits a strong basal texture and a small proportion of a secondary face-centered cubic phase in a hexagonal close-packed main phase was evidenced. TEM analysis enables to characterize the dislocations and the stacking faults present in the two phases. To complete the analysis, two plasticity stages can be distinguished: stage A corresponding to dislocations gliding and stage B driven by twinning. Size effects in cobalt are found to occur during gliding process and could be related to surface effects as previously shown in face-centered cubic metals.

Scale effect on springback behavior of pure titanium foils in microbending at elevated temperature

The occurrence of size effects in microforming process influences the accuracy of the products significantly. To investigate the scale effect and the mechanism of springback behavior of thin pure titanium (Ti) foils bent at elevated temperatures, scaled microbending processes assisted by resistance heating are performed for the foils with different thickness of 0.02, 0.05, and 0.1mm. It is found that the springback angle increases with decreasing foil thickness at room temperature. However, after bending at elevated temperatures, the springback angle is found to decrease with decreasing foil thickness. Due to the lower flow stress of surface, the influence of surface area is thought to be the dominating factor that results in the less springback of thinner foils at elevated temperatures. To confirm this and to predict the springback angles at elevated temperatures, a new theoretical model of surface area increasing with increasing temperature is proposed. The springback angles are calculated by theoretical analysis using conventional plastic theory and the proposed model. As temperature increases, the large reduction of springback angles compared to 298K can be captured by using the proposed model successfully. The validity of the proposed model is confirmed.

Effect of heat on tensile properties of thin pure titanium foils

The occurrence of size effects in microforming process may result in nonhomogeneous material characteristics. Heat assisted microforming is an effective approach to reduce the influence of size effects. To improve the heating rate, resistance heating method is introduced to the microforming process. To investigate the size effect of heat on material deformation for thin foils in microforming, uniaxial tensile tests were performed for the foils with different grain sizes at different temperatures by tensile testing system incorporating with resistance heating method. The results show that the reduction of the stress at elevated temperatures compared to room temperature is higher for the foils with larger grain size than that for the foils with smaller grain size. The fracture strain decreases with increasing temperature when the temperature is below 300 °C, and then increases as the temperature increases when the temperature is above 300 °C. In addition, the work hardening behavior of the material is independent of the grain size. As temperature increases, the work hardening increases.

Experimental and simulation studies of micro blanking and deep drawing compound process using copper sheet

Microforming process is a promising micromanufacturing technology for producing microparts due to its high efficiency, low production cost and good product quality. The occurrence of size effect in the process, however, leads to the uncertainties in process determination, tooling design and product quality control and renders the design full of challenges. In-depth study of material deformation behavior in microforming process is thus crucial for development of quality microparts. In this research, the micro compound blanking and deep drawing of copper sheet is conducted. To investigate the size effect, the similarity theory is employed in die design and deformation process simulation. The grain size effect is studied by preparing the copper sheets with different grain sizes, while the feature size effect is also studied via using different punch radii. Based on the analysis of experimental results, it is found that the deformation load decreases with the increase of grain size, but this decrease is not significant when there are only a few grains in the cross section of the sheet metal. The deformation becomes inhomogeneous with the decrease of formed part size and the increase of grain size. This further leads to the irregular geometry and rough surface finish of the formed part. Furthermore, the simulation is carried out to reveal the entire deep drawing process. The deformation loads predicted by simulation have the same trend as the experimental ones, but the difference exists in-between. This indicates that the simulation of microforming process needs to consider the inhomogeneous deformation of material in the process.