Experimental and numerical investigations of deformation machining process

Abstract

Monolithic thin structure components with complex geometries are used in wide range of applications in aerospace, marine and automobile sectors. Such components are preferred over the assembled sheet metal components owing to enhanced strength, fatigue life and most importantly avoiding the failure of joints in critical applications where failure could lead to catastrophic loss. Fabrication of these monolithic structures with thin complicated geometries and profiles at reasonable manufacturing cost without compromising with the quality is a huge challenge for the researchers and engineers. Employing conventional manufacturing techniques and approaches for such applications result in process inflexibility, in addition to large tooling, equipment and inventory cost. A novel and hybrid manufacturing process- Deformation machining (DM), a combination of two emerging processes- thin structure machining and single point incremental forming/bending, aimed at solving most of the issues incurring in conventional manufacturing approaches, employing considerably simple tooling and equipment was proposed. In this process, firstly thin structures are machined in the desired orientation and size from the bulk and then incrementally bent or formed into the desired shape depending upon the requirement. This hybrid process could fabricate monolithic thin structures with novel and complex geometries in one setup. Incremental nature of operation provides flexibility in shape and size during fabrication. Therefore, this process could possibly enable overall cost reduction in equipment, fabrication, assembly, inventory stock and the weight of the components. First aspect of DM is the creation of thin structures out of the bulk raw material by thin structure machining. It requires different strategies and techniques than the conventional machining operations owing to the lack of stiffness in the machined structure. Machining vibrations in the thin sections induce chatter resulting in poor surface quality and tool contact with the already machined vibrating thin structure resulting in re-machining, affecting the dimensional accuracy. Various strategies or combination of strategies could be employed to overcome such issues like the use of long slender tooling along with high speed machining supporting sacrificial structural preforms. In present work, relieved shank tooling with multiple axial passes has been used in the machining of thin vertical geometries, so as to provide a relief for vibrating thin machined geometry coming in contact with the tool. In machining thin horizontal structures, plunge milling technique with suitable backing material has been employed. Second aspect of DM is the creation of complex shapes and profiles on the monolithic machined thin structure into the desired shape. For this single point incremental bending (SPIB) and single point incremental forming (SPIF) technique is used. SPIB and SPIF are die less forming process where a solid hemispherical shaped single point tool is used to deform the thin structure to a desired shape incrementally using a computer numeric controlled machine. In this process the thin structure or sheet metal is deformed locally into plastic stage, enabling creation of complex shapes according to the generated tool path. Incremental forming has enabled flexibility in creation of symmetric, asymmetric and random shapes with sufficient amount of accuracy. Moreover, incremental nature of deformation results in higher formability compared to conventional deforming methods like brake pressing, stretch forming and sheet metal spinning. This process can be broadly classified into two modes:– Deformation machining bending mode: In Deformation Machining bending mode the deformation is perpendicular to the tool axis resulting in bending of thin vertical structure. Firstly, thin vertical sections are machined from the bulk material and then bent incrementally using a single point tool to the desired shapes. Deformation machining stretching mode: In Deformation Machining Stretching Mode the deformation is along the tool axis resulting in stretching of thin horizontal structure. Firstly, thin horizontal sections are machined from the bulk material and then stretch formed using a single point tool to the desired shapes. Since, one of the salient features of deformation machining is the elimination of complex dies and tooling, dedicated machinery and equipment. Therefore, the sample preparation for experimentation and fabrication of components in deformation machining bending and stretching mode; was done on a conventional and general purpose 3-axis CNC vertical milling machine. A simple tooling and fixture was designed and fabricated for the same. The process simulation of deformation machining has been carried out using Abaqus 6.13, commercial simulation software based upon finite element methodology in parallel to the experimentation analysis. Deformation machining is a recently emerging concept, the state of art literature available is very limited. The primary focus of the current work is to develop the process as a potential replacement for conventional approaches employed in fabrication of thin structures with complex geometries. Aim is to experimentally and numerically determine the essential process parameters and evaluating their influence on the main input and output process dynamics. The material used in the present work is AA 6063-T6, a commonly used alloy in automobile, aviation and marine industry. Based on the current state of art literature on the two main aspects of DM: thin structure machining and incremental bending/forming, the following research objectives have been defined and achieved in the present work:  Surface quality and integrity in thin structure machining of monolithic ribs and webs  Comprehensive parametric evaluations of machining and deforming forces induced: determining the capacity of tooling and equipment required for the process  Comprehensive parametric evaluations of process repeatability and geometrical discrepancies: ensuring the overall quality and acceptability of the components  Evaluation of residual stresses induced in the process at surface and core of the component: influence on strength and applicability Monolithic thin structures, from the bulk, prior to deformation are generated by employing extensive machining. High quality machined surfaces are necessary for the overall acceptability of the components. Results from the experimental investigations on surface integrity in thin structure machining have been discussed in detail. This includes comparison of machined surfaces generated on the thin vertical structure by peripheral end milling process employing conventional and relieved shank tooling. Influence of machining parameters like cutting speed, feed rate and axial depth of cut on surface roughness in peripheral milling (for thin vertical structures) and face milling (for thin floors) has been included. Machining induced surface chatter, resulting in poor surface finish was profoundly evident on the surface generated using conventional tooling. No chatter and re-cutting was evident from the micro images of the thin machined surfaces generated using relieved shank tooling across all the selected conventional cutting speed and feeds. Use of relieved shank tooling resulted in radical improvement in quality of machined surfaces and dimensional accuracy of the thin machined vertical structures. The trends and magnitude of deforming forces and associated stresses induced in a sheet metal or thin structure during bending or forming operations affects the magnitude and direction of springback of the part after unloading. Moreover, the loading capacity of the equipment and machinery involved largely depends on the forces induced in the process. Therefore, knowing the amount of bending and forming forces, and important parameters influencing the forces is critical for the overall process definition. The trends and magnitude of forces induced in thin structure machining employing conventional parameters were evaluated. Deforming force comparisons of DM bending and stretching mode with conventional and incremental sheet metal bending and stretch forming revealed 33% decrease in bending forces and nearly 12 times reduction in stretch forming forces Comparison of stretch forming forces in DM stretching mode with conventional and incremental sheet metal forming. The results reveal 10 times reduction in average forces induced in incremental forming in comparison to conventional stretch forming. A detailed sensitivity analysis for the forces induced in DM bending and stretching was conducted. Investigation and prediction of geometrical discrepancies: elastic springback in bending, errors in formed geometries, influential parameters and its compensation is of utmost importance to ensure the overall quality of the manufactured components. A comparative analysis of average elastic springback in DM bending mode and average radial error in the profile of DM stretching mode with corresponding sheet metal components were conducted. The elastic springback in DM bending was found out to be considerably lower compared to sheet metal components. Prior anisotropy in bending also affects the elastic springback significantly. The radial error in DM stretching mode is prominent due to the bending effect associated with incremental forming. A detailed parametric sensitivity analysis for elastic spring back and radial error in DM bending and stretching mode, respectively was done. The results reveal that the maximum bent angle, feed rate during bending and dimensional attributes like wall thickness and height to length ratio have significant bearing on the elastic springback. Forming tool diameter, incremental depth and forming wall angle were significant parameters affecting the average radial error. Structural thinning in the stretch formed structres adversely affects the strength of the component. Considerable thinning in the formed section at varied forming angles along with highly non uniform thickness profiles was noticed in DM stretching mode components both experimentally and numerically. In the present work a theory is proposed backed by experiments and simulation regarding evolution of thinning in incremental forming. A compensation strategy for desired uniform thickness of the formed profile in DM stretching mode is proposed and realized by employing a varying thin section machining, prior to incremental forming. The magnitude and nature of residual stress distribution in deforming process affects the elastic recovery and strength of the component after unloading, especially in thin structure and sheet metal components. Stress distribution and elastic recovery in subsequent operations on the part is also largely influenced by the residual stresses left in the component by the previous operations. The present work includes parametric effects on surface residual stresses induced in thin structure machining. Highly compressive residual stresses were induced on the machined surfaces across all the tested parameters and levels. Results reveal a strong positive correlation between cutting forces and surface residual stresses induced. In addition parametric effects on surface residual stresses induced in DM bending and stretching have been explored. Incremental bending resulted in reduction of compressive residual stresses on the tensile face and increase in compressive residual stresses on the compressive face of the DM bending mode components. Reduction of compressive surface residual stresses was noticed at all the forming parameters. In comparison of DM with corresponding sheet metal processes, core residual stresses induced in DM bending are uniformly distributed across the section and comparable with conventional sheet metal bending. The magnitude and variation is significantly higher in incremental sheet metal bending. The magnitude and variation in core residual stresses induced across the section in DM stretching mode is least in comparison to incremental and conventionally stretch forming. The surface residual stresses in deformation machining components are highly compressive owing to prior machining, whereas tensile in corresponding sheet metal components.