2020 Volume 61 Issue 2 Pages 239-243
In microscale due to the increase in the ratio of open to close lubrication pockets and escalation in coefficient of friction, unlike in macroscale, selection of proper lubrication condition has become challenging. In alignment with the aim of microforming, by the meaning of making light-weight energy effective micro-parts, in this study, the behaviour of a novel superlight magnesium–lithium (Mg–Li) alloy LZ91 is investigated during micro deep drawing under some lubrication conditions. Some heat treatments are undertaken to study the deformation behaviour of the Mg–Li alloy. To mitigate the effects of increasing in open to closed lubrication pocket ratio size effect, an innovative TiO2 oil-based nano-additive lubricant is applied, and its performance is determined in regard with the commonly used condition, dry condition. The mass fraction of nano-particles in the lubricant is a critical parameter which in this study 0.5 wt% and 1 wt% are investigated. The results show the drawing force reduces significantly by utilising the 1 wt% TiO2 oil-based nano-additive lubricant in comparison with 0.5 wt% of nano-particle lubricant and dry condition. The unique mechanism of the nano-particles is capable of retaining the lubricant inside surface asperities and hold it during the deformation process.
Fig. 5 Surface characteristics measurement after MDD process under 1 wt% TiO2 oil-based nano-additive lubricant (a) formed cup made of as-received blank, (b) 3D surface diagram of formed cup wall, (c) formed cup made of heat treated at 150°C for 30 min blank, and (d) 3D surface diagram of formed cup wall.
Targeting on production of multifunctional smart devices besides declining pollution, cost, weight and volume within medical, telecommunication and automotive sectors have progressively promoted the miniaturisation trend.1) Correspondingly, microforming as a part of micromanufacturing has served breathtaking technologies with a vast range of opportunities for the production of Micro Electro Mechanical System (MEMS). Micro Deep Drawing (MDD) process is the fundamental process to form cuplike or hollow parts to be used in smart devices such as mobile phones, digital cameras, and sensors. However, unlike macroscale forming processes, microforming is associated with obstacles such as size effects on its way toward advancement.2) The upturn in surface to volume and thickness to grain size ratios, and effects of surface characteristics have affected the deformation behaviour in which some neglectable forming factors become unneglectable as their effects are significant.3) Correspondingly, in microscale, surface asperities are counted as the thickness of a part while, in macroscale forming processes, they are reckoned as surface characteristics which is related to the product quality. In order to define deformation behaviour in microforming processes, conventional models such as surface layer and Hall-Petch models have been modified. However, the insofar developments are yet faraway from its advanced stage.
By down scaling, the open to closed lubrication pocket ratio increases and leads to increase in the coefficient of friction.4,5) By increasing the coefficient of friction utilising lubrication conditions have become a sever concern, however, the upsurge in open lubrication pockets (OLPs) in regard with closed lubrication pockets (CLPs) suppresses the performance of commonly used lubricants. To address the mentioned obstacle, several studies have been dedicated to propose proper lubrication conditions in MDD process. Gong et al.6) studied the effects of various lubrication conditions including soybean oil, polyethylene film and castor oil in the MDD process, and it was concluded that the polyethylene film illustrated distinct performance in comparison with the other conditions by declining the maximum drawing force, increasing surface quality and drawability. Hu et al.7) advocate the reported results and state the treated die by DLC film induces low coefficient of friction and consequently low maximum drawing force. Gong and Guo8) studied the performance of titanium nitride (TiN) and molybdenum disulphide (MoS2) films in comparison with the DLC film and found the DLC film performs distinctly in MDD process. Shimizu et al.9) processed the MDD die by the High Power Impulse Magnetron Sputtering (HIPIMS) deposition technique to obtain a very smooth surface and it was found that the evenness along three axes reduces the coefficient of friction and consequently drawing force. In this while, Sato et al.10) decreased the coefficient of friction by applying fluid pressure in Micro Hydromechanical Deep Drawing (MHDD) which it was concluded that the reduction in the coefficient of friction was due to the retained fluid pressure inside the OLPs region.
In this study, due to the remarkable growing application of cuplike microparts, MDD process is investigated deeply for light-weight LZ91 Mg–Li alloy with consideration of proper lubrication conditions. An innovative TiO2 oil-based nano-additive lubrication condition is developed in which various mass fractions of TiO2 nano-particles including 0.5 wt% and 1 wt% are examined. To judge the performance of the nano-particle lubricant, dry condition is employed as the criteria because it is the most commonly used condition in MDD process. The 50 µm in thickness Mg–Li alloy blank is drawn into the die under the mentioned lubrication conditions. Due to the novelty of the Mg–Li alloy in microforming process, heat treatment applied to study its deformation behaviour.
The LZ91 Mg–Li alloy (Mg–9%Li–1%Zn) was processed by hot rolling process and the thickness was reduced down to 50 ± 2 µm. In addition to the as-received type, to study deformation behaviour of Mg–Li alloy due to its novelty in MDD process, heat treatment was performed at 150°C for 30 min to alter mechanical properties and release the blank’s residual stress retained during the previous hot-rolling process. Due to the high flammability and surface activity of Mg–Li alloy, heat treatment was performed by KTL tube furnace under argon (Ar) gas protection. To ensure protection from surface oxidation, the heated blanks were cooled in furnace under Ar shield.
Figure 1 shows surface morphologies of the as-received and heat treated at 150°C for 30 min observed by a 3D laser scanning microscope. Based on the coloured-height map of the blanks’ surface, the as-received blank undergone hot-rolling process illustrates more even surface in which the distribution of asperities is more uniform, while in the case of heat treated at 150°C for 30 min Mg–Li alloy, uneven distribution of the surface asperities beside rougher surface are the dominant surface characteristics. Moreover, the laser intensity images indicate uniform surface of the as-received blank. In the surface 3D scanning images the extreme peaks and valleys of the heat treated blank are more visible.
Surface morphologies of (a) as-received blank height-coloured map, (b) heat treated at 150°C for 30 min blank height-coloured map, (c) as-received blank laser intensity image, (d) heat treated at 150°C for 30 min blank laser intensity image, (e) as-received blank surface 3D scanning, and (f) heat treated at 150°C for 30 min blank surface 3D scanning.
Micro tensile tests were performed to obtain mechanical properties of the as-received and heat treated at 150°C for 30 min blanks in order to have a prejudge on the blanks’ drawing force during MDD process. The thickness of the micro tensile sample was chosen as 50 µm to reflect the size effects occurred during the forming process. In micro tensile test a high-precision strain measurement system is required. Consequently, an innovative non-contact high-precision strain was developed based on MATLAB image processing module. The designed program is capable of conducting pixel-based analysis on high-resolution video frames captured during the micro tensile test. Accordingly, elongation can be measured precisely and a force sensor records the tension during the tests.
Figure 2 shows the average true stress-strain values for the as-received and heat treated at 150°C for 30 min tensile samples. Due to the relatively low annealing temperature, the elongated fine grains generated during the rolling pre-processing stage remained as the dominant microstructural characteristic. However, by the applied heat treatment, the sample illustrates slightly higher strength. The young’s modulus in true stress-strain curve is equivalent to flexural modulus or bending resistant and dominant a major portion of maximum drawing force in MDD process.
True stress-strain curve for as-received and heat treated samples.
The MDD system was integrated with blanking process in order to facilitate the material handling handicap in which the complicated design of the die is capable of cutting the initial sheet of Mg–Li alloy into a round blank suitable for deep drawing process. During the process, die remains static and punch moves downward with velocity of 0.1 mm/s, the process key dimensions are listed in Table 1. The MDD process were performed under three conditions, including dry, 0.5 wt% and 1 wt% TiO2 oil-based nano-additive lubrication conditions for the as-received and heat treated at 150°C for 30 min blanks.
As the friction between blank and die dominants the total friction force and consequently drawing force, the lubricants are added into the clearance between blank and die. The TiO2 oil-based nano-additive lubricants contain various mass fractions of titanium dioxide were developed.11) The approximately 20 nm diameter of the titanium dioxide nano-particles solved in the oil as the dispersive solution. The density of the oil is 0.9 g/cm3 in ambience, and the velocity at 23°C is 1.20 Pa·s and at 80°C is 0.18 Pa·s. Ultrasonic stirring was employed to collapse persisted agglomeration and the resultant nano-particle lubricant illustrates high equilibrium without sedimentation.
The performance of the lubrication conditions were studied with consideration on the maximum drawing force and the micro-cup quality. Basically, by penetrating the blank into the die, bending resistance of the blank is the dominant force which causes the drawing force increases with relatively low gradient. Correspondingly, for each as-received and heat treated at 150°C for 30 min blanks the increasing slope is the same under various lubrication conditions. However, after a while, by increasing the contact area between the blank and the die, friction force increases up to a peak, the maximum drawing force. Although the as-received Mg–Li alloy illustrated higher young’s modulus in comparison with heat treated type, shown in Fig. 2, the maximum drawing force of the heat treated type under dry condition is higher than the as-received form. The fact that in microforming surface asperities are part of blank geometry play a crucial role. Surface morphology illustrates in Fig. 1 and the following part will discuss the reason of higher drawing force of heat treated type despite its lower flexural modulus as a result bending resistant and drawing force.
Figure 3 shows the maximum and last stroke drawing force for the as-received and heat treated at 150°C for 30 min blank under dry, 0.5 wt% and 1 wt% TiO2 oil-based nano-additive lubrication conditions. Due to the similarity of the bending resistance for each blank under various conditions, the lubrication conditions affect the friction force and as a result the maximum drawing force. In order to study effect of the lubrication conditions, the last stroke drawing force was investigated. The last stroke drawing force is governed by the friction force due to the significant contact between the die and the blank. By utilising the 1 wt% oil-based nano-particle lubricant, the maximum drawing force is reduced by more than 14.14% while the reduction of drawing force in the case of 0.5 wt% mass fraction is up to 9.6% in comparison with the dry condition. The reduction in last stroke drawing force is more significant by 43.38% by utilising the 1 wt% nano-particle lubricant. The real contact area between the blank, die and punch affects by their topology. Due to the fact that surface asperities are part of blank geometry in microscale, tribological characteristics of blanks can considerably affect drawing force and deformation behaviour. As it was mentioned, the poor blank heterogeneity in the heat treated at 150°C for 30 min blank influences the maximum drawing force substantially. Surface characteristics including mean roughness (Ra), skewness (Rsk), and the mean of maximum height of peaks and valleys (Rz) are investigated for the blanks and formed cups’ wall, as shown in Fig. 4. After MDD, Ra increases while a reduction happens by utilising the 0.5 wt% and 1 wt% TiO2 oil-based nano-additive lubricants. The formed cups and 3D surface diagram of the wall under 1 wt% TiO2 oil-based nano-additive lubrication condition for as-received and heat treated at 150°C for 30 min are shown in Fig. 5. The Ra reduces up to 18.18% by utilising 1 wt% nano-particle lubricant. The negative Rsk values in the dry condition indicate the cup wall is formed of valley more although by exploiting the 1 wt% nano-particle lubricants the Rsk values tend to get close to zero which means the surface of the final micro-cup is smoother and distribution of peaks and valleys is more even.
Maximum and last stroke drawing force for as-received and heat treated at 150°C for 30 min blank under various lubrication conditions.
Surface characteristics including Ra and Rsk for the as-received and heat treated at 150°C for 30 min blanks under dry, 0.5 wt% and 1 wt% TiO2 oil-based nano-additive lubrication condition.
Surface characteristics measurement after MDD process under 1 wt% TiO2 oil-based nano-additive lubricant (a) formed cup made of as-received blank, (b) 3D surface diagram of formed cup wall, (c) formed cup made of heat treated at 150°C for 30 min blank, and (d) 3D surface diagram of formed cup wall.
The unique mechanism of the nano-particle lubricant defines its outstanding performance in MDD process. The increase in OLPs to CLPs ratio causes the common lubricants drain and overflow from the surface during the microforming process. However, the reduction in the drawing force indicates the nano-particle lubricant is capable of retaining inside surface asperities during deformation. The 20 nm TiO2 nano-particles can penetrate into surface asperities and accordingly trapping oil inside lubrication pockets. The trapped oil by the nano-particles can remain on the surface during deformation process without huge drainage and overflow from surface. The low mass fraction of TiO2 nano-particles in the 0.5 wt% TiO2 oil-based nano-additive lubrication condition reduces the effectiveness of the lubricant, however the 1 wt% TiO2 oil based nano-additive lubricant illustrated outstanding performance during the process. Therefore, the proper mass fraction of the nano-particle lubricant can improve the MDD process by the meaning of reduction in the friction and drawing force, increase tools lifecycle, and improve surface quality of the formed products.
The as-received and heat treated at 150°C for 30 min blanks were drawn in MDD process under dry, 0.5 wt% and 1 wt% oil-based nano-additive lubrication conditions. The unique mechanism of the nano-particle in mitigating frictional size effect improves the MDD process efficiency in comparison with the dry condition. The maximum and last stroke drawing force are reduced up to 14.14% and 43.38% respectively under 1 wt% nano-particle lubrication condition. The formed cups quality under 1 wt% nano-particle lubrication condition were more distinct in comparison with the dry condition. To conclude, the 1 wt% nano-particle lubrication condition was the optimum mass fraction of the nano-particles, and it is crucial to control the mass fraction of nano-particle in the lubrication in order to prevent adverse influences of size effects.
This study is supported by Australian Research Council (ARC, Grant No. FT120100432) and National Natural Science Foundation of China (NSFC, Grant No. 51474127).
