بیژن ملائی داریانی

Incremental forming of metal sheets is one of the new methods of forming, in which the local exertion of forming forces and the absence of a matrix enhances the forming limit of the sheet and extends the flexibility of process in producing of complex geometries. In this research, the forming limit of aluminum/copper bilayer sheets in the single-point incremental forming of different geometries was studied. Considering the instability mechanism of the sheet, the Xue-Wierzbicki damage criterion was used in the form of the VUMAT subroutine of Abaqus in the numerical prediction of the growth and damage initiation of bilayer sheets. Experimental tests showed that the type of geometry has influences on the forming height limit due to the different induced stress and strain states on the sheet. The prediction of the numerical model of the forming height limit on average for different geometries with difference of 8% compared to the experimental tests, which indicates the validity of the numerical model. Accordingly, using the numerical model, the effect of changes in equivalent plastic strain and triaxial stress as crucial variables on the distribution of surface strains and damage was analyzed. Also, cyclic and nonlinear loading in this process was shown by plotting the strain path for different geometries.

In this paper, the slider-on-flat-surface (SOFS) test was used to study the galling in automobile body drawing dies (made of GGG60 alloy). The SOFS tests were conducted on the DC06 (EN 10130) and DC04 (MSC standard) alloy sheet metals; according to the following two approaches: in the first approach, the slider was forced to move in a single linear path, repeatedly. While, in the second approach, the slider was moved on separate parallel paths. The second approach was performed using both flat and curved surfaces of the slider to simulate galling conditions on flat and curved surfaces of the dies, respectively. Moreover, the effect of hardness, surface roughness, and mechanical properties of the DC04 and DC06 sheet materials on die galling was investigated. The galling tests’ results were in agreement with the galling wear that occurred in automobile body drawing dies. The results showed that forming DC04 sheet metal will lead to more galling wear. Furthermore, a qualitative coincidence was observed between the results of the aforementioned approaches. Nevertheless, in the first approach, wear emerged in a shorter period; while, in the second approach, the slider was moved on a much longer path for creating observable wear.

Incremental forming of metal sheets is one of the new methods of metal forming with high flexibility in batch production of complex geometries. Due to the absence of a matrix and the gradual applying of forming forces, the forming limit in this process is increased compared to conventional ones. In this research, formability, forming, and finally fracture of aluminum/copper bilayer sheets produced by explosive welding method in the single point incremental forming process are studied. In the numerical prediction of growth and onset of fracture of sheets in this process, the Xue-Wierzbicki damage criterion was used as the VUMAT subroutine in Abaqus software. Using the numerical model, variations of the stress triaxiality and equivalent plastic strain as the variables affecting the damage growth in the incremental forming process were analyzed and explained, and the effect of cyclic and nonlinear loading in this process was shown. Experimental results show a different failure height of various geometries due to different loading conditions. Also, using the verified numerical model, in addition to predicting crack growth location, the fracture height in the formed geometries was predicted by 4.06% difference with respect to the experimental results.

In this study, aluminum sheets reinforced by carbon nanotubes were fabricated using the Accumulative Roll Bonding (ARB) method. The ARB process was chosen among the severe plastic deformation methods to strengthen metal sheets using carbon nanotubes owing to the enhanced microstructure and mechanical properties of final products. In order to evaluate the mechanical properties of the specimens, tensile tests were carried out and the strength of sheets made by ARB method was compared to single-layer pure aluminum and reinforced composite sheets. Microstructural changes of composite sheets were studied by optical microscopy after each cycle of rolling process. The results showed that spreading of (0. 5 to 1.5) wt% of CNTs increased the ultimate strength of the composite sheets while by aggregating the CNTs more than 1.5 wt% a decreasing trend of the ultimate strength was observed. Furthermore, the composites fabricated from 7 cycle of rolling process had a homogeneous distribution of particles and strong bonding between particles and matrix without having any porosity. Also it was found that the tensile strength of composite sheets also increased as the number of cycles increased.

Forming limit diagram (FLD) is one of the useful tools in the assessment of the sheet formability for designing industrial products. Experimental methods have been developed to determine FLDs. Costly and time-consuming experiments have led to several studies on the use of analytical methods and finite element softwares for predicting FLDs. In the present study, the necking and fracture forming limit curves of AA2024 aluminum alloy sheet were experimentally and numerically obtained through the hemispherical stretching test. Different geometries of the initial blank were considered to create different strain paths. The commercial finite element code Abaqus/Explicit was utilized to simulate experimental tests. Using theoretical equations and experimental results, fracture properties of the aluminum sheet in terms of the equivalent plastic strain at fracture, the stress triaxiality and the Lode angle parameter were captured and implemented in the Abaqus software. In order to capture necking forming limit strains, a numerical criterion based on the major strain variation in the necking zone has been considered. The comparison of the results shows that the numerical model can predict the forming and fracture limit strains with the maximum error of about 6%.

In order to increase the strength of vehicles body as well as reduce the fuel consumption, usage of Advanced High Strength Steels (AHSS) as raw materials for producing car body parts has been increased significantly in the automotive industries during the recent years. From the other aspect, higher strength of these steel types in comparison to common deep drawing steels leads to higher contact pressure on tools faces (dies) and work pieces (steel sheets). This issue has caused lower tool life. Galling, as a state of adhesion corrosion, is one of the main reasons that increases the die maintenance costs and tool scrap rate. In this study, the galling wear of Peugeot 405 door production die has been investigated in which DC04 steel sheets are formed. Considering the international standard tests for galling wear and professional metal forming software, galling resistance of common deep drawing steels are compared with the advanced high strength steels. The effects of different parameters including chemical composition of sheet, heat treatments and rolling process, blank holder pressure, and hardness and roughness of die on galling wear are determined. At last, the proper solutions for decreasing the wear of the die including changing the chemical composition of initial blank, coating the forming tools, and changing blank holder pressure are proposed.

In recent years several studies have been done in the field of laminar flame speed. One of the important characteristics of the combustion is the laminar flame speed. In this study, the effects of CO2 and N2 on the laminar flame speed of natural gas and gasoline fuel mixture have been investigated. Schlieren method has been used in this study. Experiments were carried out at pressure of 1 bar and 363 k in constant volume chamber. The mass fraction of CO2 and N2 varied from 0% to 12%.The result shows laminar burning velocity and the adiabatic flame temperature decrease with the adding CO2 and N2 Specific heat capacity of CO2 more than N2, for this reason the laminar burning velocity of N2 more than CO2. Also the result shows Addition of CO2 and N2 does not cause much change in stability.

The single point incremental forming, which is appropriate for low volume production, is one of the simplest varieties of incremental sheet metal forming process. One of the critical issues with single point incremental forming is excessive thinning which affects the strength of the part and confines the applicability of the process to produce only parts with small wall angle. In this paper, multistage single point incremental forming of a truncated cone with 70° wall angle made from an aluminum alloy sheet is studied to alleviate excessive thinning. By proposing a new two-stage forming strategy and obtaining the corresponding parameters using an appropriate algorithm, it is shown that thinning and forming time can be improved through a systematic design of multistage forming. The implementation of the designed two-stage single point incremental forming leads to less thinning in the part when compared to either the two or three-stage single point incremental forming based on a conventional strategy. The bulging at the bottom of the part, which is one of the drawbacks of multistage single point incremental forming, can also be controlled by using the proposed strategy.

In this research, formability of two layer sheet metals of Al1050 and St12 in single point incremental forming (SPIF) has investigated using numerical and experimental approaches. In order to study the sheet metal formability in this process, the tool paths defined in ABAQUS and CNC machine so that an increasing wall angle is created until the sheet metal reaches its maximum allowable angle and fracture is occurred. Since in this process, the tool exerts local stresses on the sheet metal, 3D simulation of the process is needed. In order to study the effect of process parameters, the analysis is done in three levels of tool radius and vertical step size. In order to derive fracture depth of sheet metal, the force diagram is considered in simulations. It is shown that the outer sheet subjected to higher plastic strains and therefore failure occurred initially at the outer layer. Results also showed that increasing the tool radius and vertical step size speed up process but they have inverse effect on the forming limit angle. For experimentally study and also to validation of simulation results, full factorial experiments with respect to forming speed up to three levels designed and carried out. The difference between FEM and experimental results is about %2.1 in forming limit angle.