CHAPTER 1 INTRODUCTION

Transcription

1 1 CHAPTER 1 INTRODUCTION Metal forming processes are classified into bulk forming processes and sheet metal forming processes. In both types of process, the surface of the deforming metal and tools in contact and friction between them may have major influences on material flow. The billet, rod or slab form are input materials for bulk forming and the surface area to the volume ratio is increased in the formed part due to the action of compressive loading. The bulk forming processes are rolling, forging, wire drawing and extrusion. In this forming processes ratio of volume to surface area and ratio of volume to thickness are high. One of the manufacturing process is the sheet metal forming process which is used for producing a high variety of products. In this process piece of sheet metal is plastically deformed by tensile load into three dimensional shapes, without any significant changes in thickness of sheet or characteristics of surface. The sheet metal process are bending, deep drawing, spinning, coining and embossing. The sheet metal operations are shearing, blanking, piercing, notching, trimming and nibbling. 1.1 DEEP DRAWING PROCESS Deep drawing is the sheet metal forming process which is used to produce containers from flat circular blanks. The central portion of sheet of blank is subjected to pressure applied by punch into a die opening to get a sheet metal of required shape without folding the

2 2 corners. This generally requires the use of presses having a double action for blank holding force and punch force. The process is used to forming shapes of circular, such as cooking pans, box shapes, or shell-like containers. There are no wrinkles produced inside wall of cup due to the clearance between the punch and die which is controlled to minimize the free spam. The blank holding force is applied on the sheet metal blank to prevent wrinkling and control the flange area and also to keep it in contact with the upper surface of the die Principle of Deep Drawing A flat blank of sheet metal is formed into a cylindrical cup by forcing a punch against the center portion of a blank that rests on the die ring. The different stages of drawing process for metal flow for making a cup from a flat circular blank as shown in fig.1.1. During the flat stage, the blank is contacted by the punch is shown in fig.1.1(a) and the fig.1.1(b) shows the sheet metal section 1 is bent and wrapped around the nose of the punch. The simultaneously and in sequence, the outer sections of the sheet metal blank 2 and 3, in fig.1.1 are moved radially towards the blank center until the remainder of the blank has to bent around the nose of the punch and a straight wall of the cup is produced as shown in fig.1.1(c) and 1.1(d). During drawing, the blank area under the punch is unchanged as it formed the cup bottom. When the blank is drawn over the die radius the areas that become the cup side wall as sections 1,2 and 3 in fig.1.1 are to change from annular shape parts to parallel side

3 3 cylindrical elements. The metal flow can occur until the metal has been drawn over the die radius, or a flange can be retained. In this process the flange undergoes radial tension as the blank is drawn radially inwards and compression is occurred in circumferential [1]. If the draw ratio is large or ratio of cup diameter to thickness is large then wrinkles are produced in the flange. For avoiding the wrinkles in the flange, a sufficient pressure is applied by the blank holder on the flange [2]. Fig.a Fig.b Fig.c Fig.d Fig. 1.1 Progression of metal flow in drawing a cup from a flat blanks

4 4 Due to punch force the radial tensile stress on the flange being drawn is produced by the tension on the wall of the cup. The high radial tensions are created on the flange and higher tensile stress is required on the wall of the cup for obtained cups at higher drawing ratios. The tensile stress on the cup wall helps in bending and unbending over the die radius. As The tensile stress which the wall of the cup can withstand is limited to the ultimate tensile strength of material. A very high blank holder pressure produces the higher frictional forces at contact surfaces of blank and blank holder, which may lead the metal to be restricted and may result in tearing on the cup wall. A very low blank holder pressure produces wrinkles on the wall of the cup as large amount of metal flows into die cavity very easily. Hence sufficient blank holding pressure is applied on the blank surface for preventing the wrinkles phenomenon in drawing process. It varies from very little to a maximum of about one third of the drawing load. The corner radius on the draw die helps to allow full freedom of flow of metal as it passes over this radius. If corner radius on die is too small, the fracture is developed in the cup at die corner. If corner radius on die is too high, soon the excess material is released by blank holder and wrinkles are formed on cup. So sufficient corner radius is provided on draw die. When the corner radius on the punch is concerned, a sharp radius will require higher forces when metal is folded around the punch nose and may result in excessive thinning or tearing at the bottom of the

5 5 cup. If corner radius on punch is too small, the fracture is developed in the cup at punch corner. If corner radius on punch is too high, the excess material is released by blank holder very soon and wrinkles are formed on cup. Hence sufficient corner radius is provided on punch. The clearance between the punch and the die is provided in the process. The blank may be simply sheared or pierced by the punch due to very small clearance. If the clearance is too high, the uneven thickness is developed in the cup wall and also wrinkles are formed. The clearance is the function of the thickness of blank. Drawing speed is the velocity at which the punch penetrates, the work piece. It often has a definite effect on drawing operation. The drawing speed should be reduced when cracking or excessive thinning occurs. In general the speed of drawing should be more in ductile and less for harder materials. 1.2 HYDROFORMING DEEP DRAWING PROCESS The hydroforming deep drawing process is a sheet metal forming process. In this process the pressurized fluid is used as a medium. This pressurized fluid is used to form component shapes. The process allows to manufacture lighter complex shapes more with increased strength at lower cost compared to more traditional techniques such as stamping, forging, casting or welding. The hydro formed components are used in the aerospace, automotive and other industries. The different types of fluid forming are Hydroforming process [3-7], hydromechanical deep drawing process[8-10], Aquadraw process [11],

6 6 hydraulic counter pressure process [12-14].These processes have some differences and some features are common. These principles are utilized for improvement in production of drawing cups with help of hydraulic pressure through conventional methods. These processes differ with conventional deep drawing process such as tools, producing process tribology, product quality, technology of production, stability and quality of the process, worn-out and existence of tool, geometry of product, stand at the contact surface and stress-strain and distortion force of process. In the present work an attempt has been made to evaluate the radial stresses, hoop stresses and drawing stresses of magnesium alloys and studied the influence of viscosity, blank radius, blank thickness, fluid pressure and punch speed on these stresses through fluid assisted deep drawing process. The fluid assisted deep drawing process as shown and explained in chapter 3. In this process the blank is subjected to fluid pressure on its periphery to get high forming limits and also preventing the failure. So there is improvement of deep drawing process for making the cups with utilization of fluid pressure. The contribution of hydraulic pressure to the deep drawing process is positively in several ways. The frictional resistance reduces in the flange due to lubrication of flange and die radius. In this analysis three magnesium alloys are AZ31B-0, HK31A-H24 and AZ61A-F and three different fluids are olive oil, castor oil and heavy machine oil are used.

7 7 1.3 MAGNESIUM AND MAGNESIUM ALLOYS Magnesium and magnesium alloys are used in a huge variety of applications of structural and nonstructural. The magnesium alloys are classified into two categories such as wrought alloys and casting alloys. The casting alloys of magnesium can be divided into sand, permanent mould, and die casting alloys, and wrought alloys of magnesium into sheet, plate, extrusions, and forgings. Magnesium is the largest of the commercially important metals, having a density 1.74 gm/cm 3.It is 30% lighter than aluminum alloys and 75% lighter than steel. The melting point of magnesium is C. In the pure state like aluminum, magnesium is relatively weak and for purposes of engineering is almost always used as an alloy. Its elastic modulus is between one fourth and one fifth that of steel and also even less than that of aluminum. Thick sections are needed to provide adequate stiffness, but the alloy is so light that it is often possible to use thicker sections for the required rigidity and still have a lighter structure than can be obtained with any other metal. The use of thick sections is generally not prohibitive due to less in cost per unit volume. For applications of engineering, magnesium is alloyed mainly with aluminum, zinc, manganese, rare earth metals, thorium and zirconium to produce alloys with high strength to weight ratios. The magnesium alloys applications include use in aircraft, missiles, industrial machinery, tools, and material handling equipment, automobiles, photoengraving and high speed computer parts. On the

8 8 other positive side, magnesium alloys have a relatively high strength to weight ratio with some commercial alloys attaining strengths as high as 380 MN/m 2. The vibration and good damping of noise develops in high energy absorption. While many magnesium alloys require enamel or lacquer finishes to impart adequate connection resistance, this property has been improved markedly with the development of high purity alloys. In this present work the magnesium alloys are AZ31B-0, HK31A- H24 and AZ61A-F are selected and determination of radial, hoop and drawing stresses for these magnesium alloys using with castor oil, olive oil and heavy machine oil medium from fluid assisted deep drawing process. And also studied on these stresses with parameters of the process. 1.4 FINITE ELEMENT ANALYSIS The idea of finite element analysis is to find the solution of complicated problem in relatively easy way. The finite element analysis has been a powerful tool for the numerical solution of a wide range of engineering problem. Applications range from deformation and stress analysis of automotive, aircraft, building, defence, missile and bridge structures to the field of analysis of dynamics, stability, fracture mechanics, heat flux, fluid flow, magnetic flux, seepage, and other flow problems. With the advances in computer technology and CAD systems, complex problems can be modeled with relative case. Several alternate configurations can be tried out on a computer before the first prototype is built.

9 9 The basics in engineering field are must to idealize the given structure for the required behaviour. The proven knowledge in the typical problem area, modeling techniques, data transfer and integration, computational aspects of the finite element method is essential. In the finite element method, the solution region is considered as built up of many small, interconnected sub regions called finite elements. The need of finite element method to predict the behaviour of structure the designer adopts three tools such as analytical, experimental and numerical methods. The analytical method is used for the regular sections of known geometric entries or primitives where the component geometry is expressed mathematically. The solution obtained through analytical method is exact and takes less time. This method cannot be used for irregular sections and the shapes which require very complex mathematical equations. On the other hand the experimental method is used for finding the unknown parameters of interest. But the experimentation requires testing equipment and a specimen for each behaviour of requirement. This in turn, requires a high initial investment to procure the equipment and to prepare the specimens. The solution obtained is exact by the time consumed to find the results and during preparation of specimens also more. There are many numerical schemes such as finite difference methods, finite element method, boundary element and volume method, finite strip and volume method and boundary integral

10 10 methods etc., used to estimate the approximate solutions of acceptably tolerance. The finite element method is so popular because of it s favourably use of digital computers. The finite element method predicts the component behaviour at desired accuracy of any complex and irregular geometry at least price. In Non linear finite element analysis and applications as in structural mechanics, a problem is nonlinear if the stiffness matrix or the load vector depends on the displacement. Non linearity in structures can be classed as material non linearity or as a geometric non linearity. A change in configuration may cause loads to alter their distribution and magnitude or cause gaps to open or close. Mating parts may stick or slip forming and extrusion processes must be analyzed in an attempt to reduce the production costs. The various material non linearities are plasticity, creep, other complex constitutive relations, in flow type situations the dependence of viscosity on velocity distribution. These material non linear problems can often be simply dealt without reformulation of the discretization proceeds indeed, if a solution to the linear problem can be arrived at by some trial and error process in which, at the final stage. The material constants are so adjusted that the appropriate new constitutive law is satisfied, that a solution is achieved. One important point needs, to be mentioned that in linear problems the solution was always unique, this no longer is the case in many non linear situations.

11 OBJECTIVES AND SCOPE OF WORK The Objectives and Scope of present work as follows The mathematical formulations are developed for radial stresses, hoop stresses and drawing stresses of magnesium alloys through fluid assisted deep drawing process. In this process three different fluids used such as olive oil, heavy machine oil and castor oil. Estimation of fluids pressure by using Flotran CFD analysis software. Evaluation of radial, hoop and drawing stresses for magnesium alloys by varying the blank radius and blank thickness for different punch radius using three different fluids through the mathematical modeling and Ansys - LS Dyna finite element analysis simulation software. Study on the effect of viscosity, fluid pressure, punch speed, blank thickness, blank radius and various parameters of the process on radial stresses, hoop stresses and drawing stresses and also comparison study on these stresses with viscosity, fluid pressures and blank geometry. The results with theoretical analysis are correlated with that of finite element analysis results. These stresses are used to study on drawing ratio, drawing time and limiting drawing ratios. There is no pump required for supplying the pressurized liquid to this process.

12 12 There is no effect of contact friction between the blank and both blank holder and die surface This thesis is divided into eight chapters. The chapter 1 describes the introduction to the conventional deep drawing process, fluid assisted deep drawing process, magnesium and its alloys and finite element analysis. Chapter 2 is the description on the literature survey of conventional and unconventional deep drawing processes, Finite element analysis and forming of magnesium alloys. In chapter 3 the theoretical analysis for the mathematical formulation is developed for radial stresses, hoop stresses, drawing stresses, drawing forces, radial and hoop stresses at die corner. These parameters are in terms of shear stresses and after shown in terms of viscosity, punch speed, height of gap, blank radius, blank thickness, clearance, yield stress and radial distance from job axis. The Chapter 4 deals the evaluation of fluid pressure with different punch radius at constant punch speed for olive oil, machine oil and castor oil medium. The punch radius ( r p ) 10mm to 50mm, radius of die opening ( r d ) is 15mm to 55mm and punch speed ( u ) is 10mm/sec. Also the studies on fluid pressure variation with different punch speed at constant punch radius for olive oil, machine oil and castor oil in fluid assisted deep drawing process. The punch speed ( u ) is 5mm/sec to 30mm/sec, punch radius ( r p ) is 40mm and radius of die opening( r d ) is 45mm.The evaluation of fluid pressure through Ansys - Flotran CFD analysis software and also Finite element analysis of deep drawing process is

13 13 studied. Chapter 5 gives the evaluation of radial stresses of magnesium alloys blanks such as AZ31B-0, HK31A-H24 and AZ61A-F at a radial distance from job axis ( r ) = 45mm to 85mm for different radius of blanks ( r j ) = 90mm,95mm,100mm at constant thickness (t )= 1.5mm with in the castor oil medium. The studies on radial stress distribution in these magnesium alloys. Also the comparison studies on the radial stresses of these magnesium alloys with olive oil, heavy machine oil and castor oil medium for constant radius of blank ( r j ) = 90mm and thickness of blank (t ) = 1.5mm.The punch speed ( u ) in the both cases is 10mm/sec. Chapter 6 deals the determination of hoop stresses of magnesium alloys blanks such as AZ31B-0, HK31A-H24 and AZ61A-F at a radial distance from job axis ( r ) = 60mm to 90mm for different radius of blanks ( r j ) = 95mm,100mm and 105mm, at constant thickness (t ) = 3mm with in the heavy machine oil medium. The punch speed ( u ) is 10mm/sec. The study on hoop stresses distribution in these magnesium alloys. Chapter 7 is describing the evaluation of drawing stresses of magnesium alloys blanks such as AZ31B-0, HK31A-H24 and AZ61A-F for different radius of blanks ( r j ) = 80mm,85mm,90mm,95mm and100mm at different thickness of blanks (t ) = 1.5mm,2.5mm and 3.5mm with in the heavy machine oil medium using radius of punch ( r p )= 35mm,radius of die opening ( r d ) = 40mm. In fluid assisted deep drawing process, the drawing stress in the cup wall at die entrance radius along the drawing direction is equal to radial stress occurred at beginning of die corner in radial

14 14 direction. Also the comparison studies on the drawing stresses of these magnesium alloys with, olive oil,heavy machine oil and castor oil medium for different radius of blank ( r j ) = 90mm,95mm,100mm and 105mm and constant thickness of blank (t ) = 3mm with radius of punch ( r p ) = 40mm,radius of die opening ( r d ) = 45mm. The punch speed ( u ) in the both cases 10mm/sec. Further the conclusion and future scope of work is reported in chapter 8.