CHAPTER 2 LITERATURE SURVEY

Transcription

1 34 CHAPTER 2 LITERATURE SURVEY 2.1 STEEL ALLOYS AND ITS APPLICATIONS Ferrous metals are iron base metals which include all varieties of pig iron, cast iron, wrought iron and steels. Steel is an alloy of iron and carbon with maximum carbon content up to 1.7% (Higgins 1998, Hajra et al 2012) and Plain or low carbon steel is the most commonly produced steel. It is an alloy of iron and carbon between 0.15% to 0.45% with less than 0.5% of silicon and 1.5% of manganese. It has good machineability and malleability. It is used for making angle, channels, case hardening steel, rods, tubes, valves, gears, crankshafts, connecting rods, railway axles, fish plates, small forgings, free cutting steel shaft and forged components etc. Rajender singh 2006). Low carbon steels are commonly used in many industrial applications including the fabrication of automobile frames and panels, gas storage cylinders springs, etc. (Zakaria Boumerzoug et al 2010). Deep Drawing steels are used in automotive applications involving simple to complex stampings which require very high formability, mainly stretchability and drawability. Exterior components such as starter end cover, petrol tanks etc. are made up of deep drawing quality steel (Magar et al 2013, Mayavan & Karthikeyan 2013). Galvanized steel is a material that combines good formability and high corrosion resistance properties. Such properties ensure capacity of use in

2 35 a wide range of commercial and automobile applications (José Daniel Culcasi et al 2009). Typical uses for galvanized steel include: cornices and other wall ornaments, door and window hoods, decorative formed shingles and pantiles designed to imitate other materials, roof ornaments such as crestings and finials, sheet metal for flashing, and gutters and downspouts and hot-dipped galvanized steel nails (Serope Kalpakjian 2007). Stainless steel is a family of iron-based alloys containing at least 11% Cr and other elements such as C, Mn, Si, Ni, Mo, Ti, and N. The high degree of chromium activity is actually the principal basis for utilizing it as an alloying element in corrosion resisting alloys (Umoru et al 2008). The corrosion resistant property of stainless steels combined with low maintenance needs and familiar lustre makes it an ideal material for many applications. The alloy can be shaped as coils, sheets, plates, bars, wire, and tubing. It can be used in cookware, cutlery, household hardware, surgical instruments, major appliances, industrial equipment (for example, in sugar refineries) and as an automotive and aerospace structural alloy and construction material in large buildings (Joseph Davis 1994). 2.2 INCREMENTAL FORMING OF ALLOYS Cold-formed steel is commonly applied in the fabrication of light steel structures including the frame systems of factory building, thin-walled steel trusses, grids and reticulated shell structures. With the increasing usage of cold-formed steel in the industrial and civil buildings it is important to study simple and economical ways of forming of steel alloys (Fengli Yang et al 2009). On account of the increasing trend in consumer demand towards stamped sheet metal products, study of flexible and incremental sheet metal forming and the small-scale production of stamped goods has received considerable attention (Iseki et al 2002).

3 36 Incremental sheet metal forming technologies have common advantages of simplified tooling and customized part production. Among these technologies, hand hammering technique offers high flexibility, while the CNC hammering enables more reliable production. Further, processes such as bullet jet forming or forming with blasting of glass beads are advances towards continuous forming when compared to the CNC and hand hammering. Certain non- hammering continuous forming techniques like the English wheel forming or Kraft forming, overcome some of the limitations of the hammering processes. Progress in the technology has enabled metal spinning in successful forming of rotational symmetrical parts. In recent times, the extension of the spinning limits led to the development of the Incremental sheet metal forming (ISMF) process, allowing for non-rotational shape production. Besides the production of non-rotational symmetrical shapes as well as deep drawn parts, most importantly, a shape realization directly from a CAD model has made the ISMF process highly recognized (Sanjay Jadhav 2004). Though metal spinning was developed in ancient Egypt (Wong et al 2003), the development of the ISMF is quite new. It originated in Japan (Matsubara 1994). Unlike metal spinning, the blank is fixed in ISMF enabling asymmetrical shapes production in the ISMF process. Matsubara's feasibility study concluded the realization of different truncated conical and pyramidal shapes. It also concluded that the taller products can be obtained by longer forming tools (Matsubara 1994). ISMF research can be classified into two main groups (i) Incremental stretch expanding and (ii) Incremental sheet metal forming. The first scientific work on the incremental stretch expanding was reported by Kitzawa et al (1994), where forming was carried out on a lathe machine by stretching the sheet. Further investigations on the CNC lathe machine

4 37 reported forming of shell components having sharp corners as well as hemispherical and cylindrical shapes using multistage approach (Kitazawa & Nakajima 1997, 1999, Kitazawa & Kobayashi 1999). Incremental Forming processes offer the possibility of a powerful alternative only if a few products (small lot) have to be produced. This possibility becomes a need in those applications in which it is clear that the product has to be unique. In fact, even if many products may be classified as similar ones, the natural differences in terms of anthropometrical items of each individual move the research interest through a high customization, in order to guarantee the best allowable performance of the product (Ambrogio 2005). Incremental sheet forming (ISF) is a flexible process in which a sheet of metal is formed by a progression of localized deformation. It is flexible because specialized tooling is not required; a simple tool moves over the surface of the sheet such that a highly localized plastic deformation is caused. Hence a wide range of 3D shapes can be formed by moving the tool along a correctly designed path. The principle goal which motivates the development of ISF is the possibility of forming sheet metal without the need to manufacture specialized dies. (Kathryn Jackson & Julian Allwood 2009, Amar Kumar et al 2013). When the ISMF process is conducted with flexible tooling, it can be realized economically as well. In this relevance, the economical viability for the industrial production was studied by Ambrigio et al (2003). The study concluded with a minimum quantity production for the process to be a substitute for the traditional punch and die forming approach. It also emphasizes the importance of smaller forming time in order to produce large quantity economically (Hirt et al 2004).

5 38 Malhotra et al (2011) reported that a challenge in Multi-Pass Single Point Incremental Forming (MSPIF) has been the geometry control of formed components, especially on the base of the component where multiple stepped features are formed unintentionally. They have attributed the step formation to the rigid body motion during the forming process and develops analytical formulations to predict such motion during each intermediate pass. Based on this model, a new tool path generation strategy was proposed to achieve a smoother component base by using a combination of in-to-out and out-to-in tool paths for each intermediate shape. (Malhotra, R et al 2011). Venkata Reddy and Jian Ciao reported that the major hurdles which have to be overcome before this process can be widely adopted are: a) the ability to design a desired tool path based on the required final configuration, b) the ability to produce features on both sides of the original sheet plane, which is particularly important for auto industry and c) the ability to produce parts with a desired surface finish (Venkata Reddy & Jian Cao 2011). 2.3 INCREMENTAL PROCESS PARAMETERS During the processing and forming of sheet materials, serious accuracy problems such as elastic spring-back effect may occur, especially if the gap between the tool and the support has been left large or forming without support is performed. This will appear after cut-out operations, when large relatively stiff edges are removed. In case of large parts, deviations will cumulate and may cause geometrical errors of several millimeters or more. These effects a need for through study of process parameters involved in incremental forming process (Meelis Pohlak 2007). Jeswiet et al (2002) stated that, while defining the incremental process parameters, it is vital to form the two shapes such as the pyramid shape and the cone shape. The pyramid shape produces primarily samples that

6 39 have flat surfaces after deformation had occurred. Cone shapes are produced to obtain curve shaped samples. Moreover the quality of the shape was defined as for the shapes produced, the walls could not have any tears and the surface roughness had to meet the requirements of the manufacturers. Ambrogio et al (2008) stated that to study the influence of process parameters effectively requires fixing the punch diameter (D p ) and sheet thickness(s). SPIF of AA1050-H111 sheets with thickness equal to and above 1.5mm thickness are known to experience surface quality problems due to severe damage characterized by pick-up of workpiece material on the tool and subsequent scoring of its surface usually involving the detachment of large fragments of material (a phenomenon that is usually designated as galling (Skjoedt et al 2007, Skjoedt et al 2009). Venkata Reddy & Jan Ciao (2011) reported that some major process variables influencing SPIF are table feed, tool rotational speed, step depth and forming angle. Hagan & Jeswiet (2003) stated that surface roughness is an important evaluation parameter. Variation in surface roughness between specimens was a concern when selecting the tool-path depth increment and the depth increment was studied and modified for each wall angle to create a similar surface finish on each specimen tested. As the deformation during SPIF is mainly plastic, a simple hardness test to provide an insight into the variation in mechanical properties is required. Hardness tests were performed by Hussain et al. (2009) with a Vicker s hardness tester to clarify the influence of metal hardness.

7 40 Process mechanics in SPIF is mainly characterized by stretching condition, resulting in the occurrence of a relevant sheet thinning, which penalizes the process suitability. Sheet thinning in the deformed zone needs to be identified as the final thickness relates to the slope of the formed surface (Ambrogio et al 2005). Incremental Forming processes were studied in recent years from different points of view due to interest induced in the industry and academic world. Geometric precision and cost saving are the main targets for industrial researchers. Among them material formability probably represents the most investigated aspect (Filice et al 2006). 2.4 EFFECTS OF PROCESS VARIABLES DURING INCREMENTAL FORMING One of the major process variables affecting SPIF process owing to the heating that causes is the tool rotational speed. The most obvious source of heating arising out of spindle speed is friction. As the tool travels over the surface of the work piece it is also spinning at a certain number of revolutions per minute. If the tool is stopped it will slide along the surface of the material, ploughing material ahead of the tool as shown in Figure 2.1. Heating will occur in all cases due to sliding friction. The spindle rotates so that the forming tool rolls over the sheet surface. Control of this variable, is necessary as heating of the sheet during its deformation possibly introduces further complication in analyzing the process. In single point forming, the forming tool has a hemispherical shape, which is pressed into the material causing deformation (Jeswiet et al 2005).

8 41 Figure 2.1 Single point incremental forming (Jeswiet et al 2005) Ham & Jeswiet (2006) studied incremental forming of aluminum alloy AA3003 sheets under varying feed rates, rotational spindle speeds, step sizes and forming angles. Faster spindle rotation speeds improved the sheet formability significantly. Tool diameter has negligible effect on the likelihood of forming a part. Thus smaller and shallower parts can be utilized which reduces machine time requirements. Durante et al (2009) performed temperature measurement and highlighted a proportional dependence with the speed of rotation, because of the relative motion between sheet and tool. Dependence on the direction of rotation, even if not particularly pronounced, is highlighted too. The other vital incremental forming process variable is the table feed. Even if the modern machines allow operating with high feed rates, incremental forming processes are very slow than the traditional stamping processes, resulting in a very low production rate. For this reasons, from an economical point of view, a study of tool feed is important for reaching the

9 42 breakeven point for small production quantities even if the equipment capital costs, as well as the tooling ones, are very low (Ambrogio et al 2004). It can be noticed that the highest feed rate assures the highest microhardness value of the formed part. This implies that the temperature increase due to the process deformation heating, which guarantees the formability increase, does not alter either the material microstructure with uneven modifications or the mechanical characteristics. The hardness increase at the highest speed can be probably ascribed to the spreading effect of the punch at the higher temperature, which applies higher deformations to the material along the surface. This phenomenon is relevant for the Ti6Al4V due to the higher power required for the process compared to the one for the Titanium grade 2 (Ambrogio et al 2013). Kim & Park (2002) studied the formability of sheet metal by analyzing the effect of process parameters tool type, tool size, feed rate, friction at the interface between tool and sheet, plane-anisotropy of sheet by experiments and FEM analyses. It was found that the formability is improved when a ball tool of a particular size is used with a small feed rate and a little friction. Attanasio et al (2006) shown that to achieve good results in terms of surface quality and geometric accuracy, it is important to use a tool path with a feed rate depending on the geometry of the part. Hussain et al (2008) in their experimental studies on tool and lubrication during negative incremental forming of titanium sheets have observed that high feed rate, increased temperature and friction at the tool blank can badly affect the surface quality of the component.

10 43 Petek (2009) has studied the deformation and force analysis of single point incremental sheet metal forming. The analysis explained the impact of the wall angle, tool rotation speed, vertical step size, tool diameter and lubrication on the magnitude of forming force and plastic logarithmic strain. Step depth was found to be an influential parameter having impact on the manufacturing time of the product. Step depth influences were studied during the experimental tests conducted by Attanasio et al. It was evident that the surface quality can be improved decreasing the values of step depth though this causes an increase in the manufacturing time. The big waves characterizing the worst profile were caused by the tool position. From the observations, it was evident that when using tool paths with high step depth the pocket geometry was not correctly reproduced (the pocket bottom was far from the ideal profile) (Attanasio et al 2006). The Forming Limit Diagram (FLD) for the ISMF was reported to be different from traditional forming process, like deep drawing. Negative slope was observed in the positive minor strain. It was also concluded that while forming, the crack occurs mostly at the corners, since the deformation at the corner is greater than that of along the sides (Shim & Park 2001, Filice et al 2002). Kopac & Kampus (2005) in their experimental studies on aluminum and steel sheets have determined that the formability of sheet metals enables larger deformations compared to conventional forming hence the minor axis stress is always a positive valve. Allwood et al (2007) conducted experiments to explain the higher forming limits observed in incremental forming. Their observations indicated that an appreciable amount of through thickness shear occurs during

11 44 incremental forming. It was reasoned that when through thickness shear is present, the tensile stresses responsible for fracture, are reduced resulting in greater forming limits. It was also observed that in a plane perpendicular to the tool path the deformation of the sheet is mainly by stretching and bending. Emmens et al (2007) made an attempt to understand the nature of deformation in incremental forming. Observations show that due to presence of bending in incremental forming, shear mode of deformation has to be present. Emmens & Boogaard (2008) proposed that bending under tension (simultaneous stretching and bending) as the main reason for the increase in formability during incremental forming. The findings are important to the understanding of failure mechanism in incremental forming. Kathryn et al (2009) studied the mechanics of incremental sheet forming on copper plates forming. The measurements show that the deformation mechanisms of SPIF are stretching and shear in the plane perpendicular to the tool direction, with shear in the plane parallel to the tool direction. Strain components increased on successive laps, and the most significant component of strain is shear parallel to the tool direction. Filice et al (2002 conducted analysis of material formability of AA 1050 O in incremental forming. The tests were aimed towards the achievement of different straining conditions and consequently to the determination of forming limit diagrams for progressive forming operations. The forming limit curve in incremental forming was quite different from the corresponding one in conventional forming. Much higher strains were achieved in incremental forming than in traditional processes. Such circumstances were justified by taking into account the peculiarity of the process mechanics. Plastic deformation induced by the small size punch is

12 45 strongly localized and confined to the close vicinity of the contact area; then it incrementally progresses as the tool moves along the assigned path. As a consequence higher strains can be attained in the material before that fracture occurs. Ham & Jeswiet (2007) determined the effect of material type, material thickness, shape, step depth and tool size on the maximum forming angle, effective strain, major and minor strains. Material type had the greatest effect on formability, followed by the shape; which was shown by the response curves in both maximum forming angle and effective strain. Tensile tests were used to determine whether a material will be formable, the lower the UTS the formability. Szekeres et al (2007), while measuring force in pyramid shaped parts with a spindle mounted force sensor, observed similar force trends while forming cone but not during forming the pyramid shape. Hence tool force measurements have been indicated not be very reliable during SPIF. Kim & Yang (2000) reported that the correct tool path generation has a direct impact on the dimensional accuracy, surface finish, formability, thickness variation and processing time. Tool path plays an important role in the final outcome and the geometric accuracy of the path generated. So tool path along with the features and interaction can be used for the improvement of the accuracy of features in the part produced by SPIF. Generally compensations are done in the CAD files of the part according to the predicted deviations of individual features. However in order to improve the accuracy of the entire part it is also necessary to take into account the behaviors of each individual feature and all feasible interaction between features (Petek et al 2009).

13 46 Kurra Suresh et al (2013) reported that the tool path has a significant effect on dimensional accuracy, surface roughness, processing time and thickness variation. Thus tool path generation is an important step in incremental sheet forming (ISF). Malhotra et al (2008) presented a generic methodology for tool path generation for an arbitrary component that can be formed by single point incremental forming (SPIF) to obtain required geometrical accuracy. Adaptive slicing concepts used in layered manufacturing have been modified and used for generating tool path for SPIF. Experiments and FEA have been carried out to study the effectiveness of the proposed methodology. Results indicate that the proposed methodology enhances the accuracy achievable in SPIF. (Rajiv Malhotra et al 2008) Ceretti et al (2003) emphasized the importance of the tool paths. Then work has revealed that the decision of tool path type is dependent on sheet material, thickness, surface finish, and the CNC machine. To know the effectiveness, the spiral and the profile tool paths were tested for a hemispherical dome forming. The profile tool path was found to be better to obtain good geometrical and thickness forming for this shape. Leach et al (2001) examined two different tool path strategies namely inside-out and outside-in by forming shallow rectangular pyramids. The investigations indicated that the vertical increment or pitch is important for the consistent thickness forming along with good surface finish. Jadhav (2004) has observed twist and dent in parts formed using helical tool path. To overcome this problem a bidirectional profile tool path was suggested. This tool path is similar to profile tool path except that in each subsequent cycle tool changes the direction of motion. This tool path

14 47 minimizes the twist and enhances the geometric accuracy of the formed component. The deviation of parts from the desired shape value can be corrected using correction strategies. For this purpose, Hirt et al (2003) shown a correction method, which proposes a new CAD model that is smaller by the unwanted deviation. Though the method shows good results for simple pyramidal geometries observations showed that the outcome for complex shapes is subjected to iterations (Hirt et al 2003). Amino et al (2002) reported the forming of complex real life shapes, including fender and bathtub to demonstrate the capabilities of its specially developed machine. The machine can form a maximum part size of 6 x 2.2 x 0.6 m. After more development work on the ISMF, Amino and Honda have formed automobile parts for commercial production using the Amino machine in However, the technical information about the process parameters was not fully known. The usefulness of lubrication was examined by Leach. The experiments proved that the use of the lubrication helps to achieve higher strain values (Leach et al 2001), whereas Kim mentioned the need of some optimum friction to enhance formability (Kim & Park 2002). 2.5 OPTIMIZATION OF INCREMENTAL FORMING PROCESS PARAMETERS Bonte et al (2008) reviewed the works carried out by most researchers towards optimization during metal forming processes. Observations show that most optimization results were very impressive, demonstrating the large potential of optimization techniques to optimize metal forming processes. However, with modeling an optimization problem well,

15 48 selecting a suitable optimization algorithm and proper application of this algorithm to a specific metal forming problem requires a lot of expertise on mathematical optimization. However it was established that a barrier exists for exploiting the full potential of optimizing metal forming processes. There is a need for a generally applicable optimization strategy for metal forming processes to overcome this barrier, a structured method that assists metal forming professionals in modeling and solving a variety of metal forming problems, regardless of the type of problem, product or process. Le et al (2008) studied Single Point Incremental Forming for thermoplastic materials. It was shown that all the analyzed parameters and their 2-way interactions were relevant to the process (very low P-values). The model had a fairly good fit and concluded that all forming parameters have to be taken into account to model the response surface. The correlation coefficient R 2 and adjusted R 2 for accuracy of the model was very satisfactory. Durante et al (2010), while comparing the analytical and experimental roughness values of components created by incremental forming, have stated that the possibility to predict the surface roughness values in incremental forming can result useful, in order to control this important target. The measurement of roughness was done on incrementally formed pyramid shaped AA7075- T0 sheets, by varying the tool radius, the step depth and the slope angle. The models were shown to give good qualitative and also quantitative correlation with the percentage differences less than 10%. Durante et al (2009) in an investigation on tool rotational speed during incremental forming have evaluated the surface roughness of the formed sheets. ANOVA tables were observed to be satisfactory to identify whether a process variable exerts significant influence on the process or not.

16 49 Capece Minutolo et al (2007) performed their experiments on the pyramid and cone for a single speed and feed for an increment of 5 degree of slope angle and the optimized value for the forming is not greater than 66 deg for cone and 63 deg for pyramid. Ham & Jeswiet (2006) executed an experimental investigation on the effects of process variables on formability of various aluminium alloys during incremental forming using two factorial designs of experiments. The objective of the experiment was to show the effect of material thickness, step depth and tool size on the forming angle. A thirty three full factorial design was prepared, but due to equipment and material availability this was later changed to two factors varied at three levels and one factor varied at two levels or a factorial design. The factors varied at three levels were step depth and material thickness, two levels was tool size. The experimented showed that step depth affects the maximum forming angle. Lin et al (2005) has used computational DOE to study the hemming process (a three-step sheet-folding process utilized in the production of automotive closure). The results provided the basis for process parameter selection to avoid hem surface cracking and particular insights for achieving acceptable formability. Damoulis & Gomes (2010) performed sheet metal forming analysis and optimization through the use of optical measurement technology to control spring back. The article described some industrial cases on how these new techniques can be applied to lay out industrial deep drawing processes. It could be accomplished by the use of optical measurement, improving design and process issues. Harshal & Deshmukh (2014), in a review on optimization techniques during sheet metal forming, have stated that one of the most

17 50 widely used methods is response surface methodology (RSM). In RSM the unknown mechanism can be approximated with an appropriate empirical model. RSM is a good way to describe the process and to find the optimum value of the considered response. With RSM it is possible to define the relationships between the responses and the main controllable input factors, as well. As it is a powerful technique, all independent variables can be measurable. A new response surface method involving moving least squares regression models and pattern search optimization was designed for optimization of parameters to avoid spring back during sheet metal forming. The resulting response surface algorithms involved iterative improvement of the objective and constraint functions employing locally supported nonlinear approximations. Response surfaces using a surrogate model was built based on a simplified one step Inverse Approach analysis. The resulting procedure was applied successfully for the design of tools geometry of ring benchmark (Chebbah & Naceur 2002). The approximate functions encountered in RSM rely mainly on multiple linear and second-order regression models. The regression coefficients are fitted by least squares (Myers & Montgomery 2002). Ambrogia et al (2007) conducted a case study on incremental forming of AA O blanks. The focus was on the investigation of the influence of the process parameters on accuracy through a reliable statistical analysis. A good prediction of the material spring-back was obtained, for the particular problem, using a statistical approach. Due to the problem complexity and the number of considered parameters, a response surface statistical model was implemented. The high values of R 2 and R 2 adjusted led to prefer the quadratic model to the linear and factorial ones. Two satisfactory

18 51 equations were found, which were able to describe the sheet behaviour with respect to the chosen output variables. 2.6 MICROSTRUCTURAL MODIFICATIONS DURING INCREMENTAL FORMING Changes in a the microstructure of a material induced in common steel materials by the incremental forming process have been investigated by Jeswiet et al (2005). The results of this investigation showed in both cases, the grains were significantly elongated due to the high strain. Neugebauer et al (2010) conducted a micro structural examination on single point incrementally formed magnesium aluminum composite. The interface demonstrated that a homogeneous interface was formed between magnesium and aluminum under the given test conditions. Shanmuganathan & Senthilkumar (2012) conducted profile forming of conical component using Al 3003(O) alloy. The SEM image of the full thickness of the ASTM grade 3003 Aluminum alloy sheet on the edges showed tool traverse marks. The inner edge showed the plastic deformation along the direction of the forming. The optical microscopic image showed fragmented particles of Mg2Si and some undissolved inter-metallic compounds of Al6 (Fe,Mn). The microstructure of the formed sheet in cross section displayed partial flow of grains along the direction of the forming. The grains were fragmented by the force of forming Huang et al (2011) evaluated the forming limit of textured AZ31B magnesium alloy sheet at different temperatures. Grains were observed to grow near the surface of the processed sheet, while those in the central region had little growth. The average grain sizes were 7.2 m and 7.5 m, respectively. The amount of deformation of the region near the surface was

19 52 higher than that in the center, which made it possible to produce the gradient microstructure with the coarse-grained surface layer and the fine-grained layer in the middle of the sheet. Neugebauer et al (2011), while investigating the temperature management for incremental forming operations, have found the blank temperature of C led to austenitic microstructure for two sheet alloys 22MnB5 (1.5 mm) and 10MnCrMo8-3-2 (1.6 mm). Between C and C a bainitic microstructure was obtained resulting in very good formability. However, 22MnB5 converted from the austenitic to the martensitic phase. Due to the fact that martensite has a hexagonal microstructure the formability is less than the microstructure of bainite. Generally the forming frontiers can be influenced positively by increasing the forming temperature. Krause et al (2006) conducted a microstructural examination of aluminum alloys subjected to incremental forming. The microstructure of aluminum sheets was examined through both optical and transmission electron microscopy for alloys subjected to a variety of temperatures and deformations. During incremental forming the sheet was subjected to a quick recovery heat treatment between each forming step, in order to partially undo the effects of work hardening. The parameters of the quick heat treatment in incremental forming must be chosen carefully to ensure that it is long enough to promote adequate recovery in the aluminum, but also short enough and of a low enough temperature to minimize artificial aging in 6xxx series alloys and excessive grain growth in 5xxx alloys. Hecker & Stout (1982) discussed the scattering effect of surface roughness on aluminum sheets. Grain occurrence was observed on free surfaces with very large plastic strains.

20 53 During incremental forming of free surface magnesium alloy under warm conditions the grains were found to be equi-axed but different in magnitude. Grains without deformation increased as temperature increased by grain growth. Grains with deformation increased, and then gradually decreased as recrystallization occurred. It seems that the abrupt decrease in grain size is related to the drastic increase in formability at this temperature (Ji & Park 2008). The influence of the initial grain size in micro-spif was proposed for thin sheets of copper. With the increase in the annealing temperature, increase of the grain size was observed. The effects of the specimen thickness t and the average grain sizes d are quantified by the ratio = t/d. The material was assimilated to an arrangement of single crystals with less stresses in the neighboring grains. Thus, only a small number of slip systems are activated to accomplish the deformation process, which may explain the low values of fracture strain. The influences of flow-forming parameters and the state of the microstructure on the quality and mechanical properties of D6ac steel were studied. The effect of feed rate was studied. Both optical and electron microscopy were used for the study of the microstructure. The appropriate heat-treatment leading to the best strength toughness combination was found with the results obtained interpreted within the framework of metal working theories and examined critically with existing models (Jahazi & Ebrahimi 2000). 2.7 FINITE ELEMENT ANALYSIS OF INCREMENTAL FORMING Carvalho et al (2008) observed that the simulation of metal forming processes using finite element method (FEM) technique to solve new and more complex problems has become an established technique.

21 54 Bonte et al (2008), while in devising an optimization strategy for industrial metal forming processes, have summarized that product improvement and cost reduction have always been important goals in the metal forming industry. The rise of finite element (FEM) simulations for processes has contributed to these goals in a major way. The choice for specific simulation software depends heavily on the process, the product and the preference of the user. The wide recognition of the ISMF as a die-less process is explicitly known through the growing scientific work and the outcomes of that indicate a substantial process potential. The die-less forming enables production of parts using a full or a partial support tool. In the former case, accuracy is good but in the latter case, a lot of work is needed to enhance the geometrical accuracy. Further, the realization from a CAD model directly by controlling the tool path is a key process characteristic. The potential, however, involves considerable work on the tool path analysis and generation. In addition, the development of the process simulation is needed in order to evaluate the process technical details without forming a part physically. Consequently, the modeling of the process to form a new shape will avoid the trial and error forming strategy. A clear insight in these areas and development of the process simulation can contribute to a reliable ISMF process development making it efficient for reliable industrial production (Sanjay Jadhav 2004). Arfa et al (2013) in their investigation on the tool forces required to deform plastically the sheet have concluded that the numerical simulation might be exploited for optimization of the incremental forming process of sheet metal. Hussain et al (2008) conducted an experimental study on formability evaluation methods in negative incremental forming.

22 55 Observations showed that in single point incremental forming (SPIF) the final thickness of a deformed sheet can be predicted. Further, the formability in SPIF can be expressed as the maximum deformation a sheet would endure without fracturing. Park & Kim (2003) carry out fundamental studies on the incremental sheet metal forming technique. Negative incremental sheet metal forming was compared with conventional forming techniques. Forming of rectangular cone, octagonal cone, bucket shape and stepped shape were done. Stretching and deep drawing process of the shape were simulated by PAM- STAMP. The forming limit curve appeared in a different pattern, revealing an enhanced formability. Sena et al (2011) conducted a detailed finite element study and validated the results with the available experimental results of SPIF processes. ABAQUS software was used for the numerical simulation of incremental forming process. Observations showed that, though solid elements provide more generality in analysis compared to shell elements, it has the disadvantage of having fixed number of integration points. The stress analysis carried out indicated a clear mechanism of bending and stretch, with residual stress levels in the normal direction. The results from the finite element formulation based on the enhanced assumed strain method and reduced inplane integration, for a variable number of integration points along the thickness direction (RESS finite element) were all convergent. Minoru Yamashita et al (2008) examined the applicability of numerical technique to incremental forming of sheet metal. A thin sheet of Young s modulus 110 GPa and Poisson s ratio 0.33 was incrementally formed into a square pyramid. The deformation behavior of sheet metal in an incremental forming process was numerically investigated using a dynamic

23 56 explicit finite element code DYNA3D. It was observed that finite element simulations can be successfully applied to model the incremental forming process. As for the formed product shape, a greater deviation from the goal shape was found when the height of the formed pyramid increases. It was concluded that the tendency may have come from the fact that spring-back is greater for a thinner walled product. Moreover it was observed that when the tool ravels in the horizontal direction both the vertical and horizontal components of the force acting on the traveling tool are almost constant. In order to know the process deformation behavior in advance, Sawada attempted stretch forming of sheet metal with CNC machine tools and the simulation by ABAQUS code. It was concluded that the process has axis symmetrical deformation (Sawada et al 1999, Batoz 1998). Another approach using the shell theory by incremental bulging for the deformation analysis was performed. It showed that the elements stretched were axis symmetrical, which confirmed the plane strain deformation model is effective (Hirt et al 2004). Recently, Junk et al (2003) reported modeling and simulation of the ISMF using an explicit FEM model. Results were proved to be in close agreement with the experiments carried out (Iseki 2001, Hirt et al 2002). Yongjun Wang et al (2009) presented a new rapid prototyping process of a thin sheet metal, Double Sided Incremental Forming of a cylindrical part without a die or a clamping device around the periphery of the sheet. The effects of process parameters on part shape, such as gap between two tool heads and feed rate, are examined experimentally with a special device mounted on a general lathe and numerically with a commercial finite element software package, Abaqus. A simple mathematic model to predict the

24 57 cone angle is proposed and compared well with experimental data. (Yongjun Wang et al 2009) He et al (2005) investigated both experimentally and numerically the process of single point incremental forming of an aluminum cone with 50-degree wall angle. Three-dimensional, elasto-plastic FE model was set up for the simulation of the SPIF process. The implicit FE package ABAQUS/Standard was used in this work. During the deformation, owing to the absence of real symmetry, a model for the whole geometry should be established, which is unfortunately very time consuming. Therefore, it was assumed that the center of the blank does not move horizontally during the process and a kind of axis-symmetric condition was maintained. As a result, only a 40-degree pie of the blank is considered in order to simplify the numerical model and reduce the computation time. In order to consider the neighboring material, proper boundary conditions were assigned to the external nodes. It was observed that one of the drawbacks of the SPIF process was the excessive thinning which limits the maximum wall angle of the deformed part. Therefore, it is important to predict the thickness distribution of the formed part correctly. A comparison of the sheet thickness between predicted and measured results showed quite good agreement with a maximum difference less than 0.1mm. It is shown that the thickness at the bottom of the cone remains almost unchanged, whereas the thickness in the wall region is reduced drastically from 1.2mm to 0.76mm. This was slightly lower than the thickness of 0.771mm derived from the so-called 'sine law' based on the plane strain condition of the SPIF process. Higher formability is of prime importance for realizing a shape by a new process. Therefore, much of the research has been conducted on the aluminum formability and pertaining details. The distinctive deformation

25 58 mechanism of the process was newly analyzed by Sawada et al. The two dimensional analysis carried out showed that the deformation is axis symmetrical (Sawada et al 1999). Aerens et al (2010) established practical formulae allowing to predict the forces occurring during the single point incremental forming process. This study has been based on a large set of systematic experiments on the one hand and on results of finite elements modeling simulations on the other. This led to analytical formulae allowing computing the three main components of the force for five selected materials as a function of the working conditions such as sheet thickness, wall angle, tool diameter and step down with a good precision. Moreover, a general model has been deduced to allow computing an approximate value for the force for any material, based on knowledge of the tensile strength only. 2.8 ECONOMIC ANALYSIS OF SPIF Due to high cost of die in sheet metal forming, conventional forming processes are suitable only for high-volume production. However, the pattern of demand for sheet metal product has undergone a change, which necessitates small-batch sizes. Single point incremental forming is a die-less forming process and can be employed for customizes sheet metal products made in small quantity (Schafer & Schraft 2004). During incremental forming, though the cost of the die is less, the cost of machine tool is high in this case. Apart from adapting the organization to low volume, the right forming process must be chosen. Break-even analysis is based on categorizing production costs between those, which are variable (costs that change when the production output changes) and those that are fixed (costs not directly related the volume of production). Total variable

26 59 and fixed costs are compared with sales revenue in order to determine the level of sales volume, sales value or production at which the business makes neither a profit nor loss i.e. the break even point (Syed Asad Raza Gardezi 2008). Production cycle of incremental forming is shorter than the conventional production. It was clear from the study that irrespective of the geometry of part produced incremental forming has much lower breakeven point. It shows that the breakeven point have slight dependence on the size of the part produced in the case of incremental forming. For relatively large parts; there is an increase in the breakeven point as compared to smaller part (Saad Arshad 2012). 2.9 NEED FOR THE PRESENT STUDY Formability is the ability of a given metal workpiece to undergo plastic deformation without damage. Knowledge of the material formability is very important to the layout and design of any industrial forming process. The plastic deformation capacity of metallic materials, however, is limited to a certain extent, at which point, the material could experience tearing or fracture (breakage). Forming processes are particular manufacturing processes which make use of suitable stresses (like compression, tension, shear or combined stresses) to cause plastic deformation of the materials to produce required shapes. Cold forming is a high speed manufacturing process whereby metal is shaped at room temperature often without the need for the removal of material. In this technique multiple dies and punches in succession are utilized to force metal beyond its yield (elastic) limit and to achieve complex shapes upon removal from the die.

27 60 Historically cold forming has been an experienced based technology, but this is changing as new computer based analytical tools are constantly being developed. Recently forming processes such as single point incremental forming (SPIF), Equal Channel Angular Pressing (ECAP), Accumulative Roll Bonding (ARB), High Pressure Torsion (HPT), and Multiaxial Alternative Forging (MAF) are evolving to obtain complex shapes with enhanced formability characteristics. Out of these, single point incremental forming is a flexible and innovative sheet metal forming process, which allows various shapes forming without dies by applying a series of local deformations using common tool materials in a CNC Machine. The absence of dies provides an attractive alternative to conventional metal forming processes which are not cost effective for small batch productions and prototyping of a component. Challenges exist in identifying practical ways of forming hard materials such as steel sheets into small batches of various shapes. SPIF provides a solution to this challenge. So a deep study on the influences of the incremental process parameters on the surface and formability characteristics of steel sheets is necessary OBJECTIVES AND SCOPE From the literature survey it was identified that the research on incremental forming processes have been carried out so far only on soft materials such as aluminum alloys, copper alloys, magnesium alloys etc. Though steel is commonly used in a number of applications, including automobile and aerospace industries, incremental forming of steel sheets has yet been negligible. Hence there is a scope for experimental and finite element studies on incremental forming of steel sheets. The significant contribution of this work is to study the mechanical, surface and formability characteristics of

28 61 incrementally formed steel sheet materials. Accordingly the following areas are focused and the objectives formulated. To develop an experimental setup consisting of a fixture and a forming tool for carrying out incremental forming process. To conduct the experimental work on IS 513 CR3 deep draw quality steel, AISI 304 stainless steel and IS: 277 low carbon galvanized sheets of constant thickness by varying the parameters like tool speed, table feed and step depth to form the pyramid and conical shapes. To investigate the effects of various process parameters such as tool speed, table feed and step depth on the surface roughness, formability, thickness variation and microstructure of the formed shapes. To optimize the input process parameters through response surface methodology and conduct confirmation experiments to verify the models. To perform a finite element analysis for predicting the formability characteristics and validating the same with experimental results. Figure 2.2 shows the methodology adopted in the study. The methodology was framed based on the identified objectives.

29 62 Figure 2.2 Methodology 2.11 SUMMARY The following salient points can be highlighted from this chapter. Steel materials IS: 513 CR3 deep drawing quality steel, IS: 277 galvanized steel and AISI 304 stainless steel find many applications as automobile, aerospace and structural materials. Incremental forming technique can be used successfully for sheet metal forming. However the process parameters of SPIF need to be optimized in order to produce defect free process zones. Faster spindle rotation speeds improved the sheet formability significantly. Formability increases as the feed rate decreases. Step depth to

30 63 be an influential parameter having impact on the manufacturing time of the product It is important to use a tool path with a feed rate depending on the geometry of the part. A bidirectional profile tool path minimizes the twist and enhances the geometric accuracy of the formed component. The development of mathematical models to predict the surface properties of SPIF sheets are important. The possibility to predict the surface roughness values in incremental forming can result useful. ANOVA tables can be satisfactorily used to identify whether a process variable exerts significant influence on the process or not. RSM is a good way to describe the process and to find the optimum value of the considered response. The choice of specific finite simulation software depends heavily on the process, the product and the preference of the user. Finite element analysis using ABAQUS is effective in predicting the behavior of SPIF alloys. The process is axis symmetrical. The process was found to be economical. The need for the study has been identified and provided. The objectives of the work are defined. The methodology adopted to achieve the objectives in the course of investigation is shown using a flow chart.