EG284-lecture-notes - DR AMIT DAS PDF

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EG-284 MANUFACTURING TECHNOLOGY 2 DR AMIT DAS

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CONTENTS. 1. JOINING. 1.1 General Methods 1.2 Strengths of Joints 1.3 Types of joints 1.4 Fasteners 1.5 Adhesive Bonding 1.6 Brazing and soldering 1.7 Fusion Welding 1.8 Solid State Welding

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2. NON DESTRUCTIVE TESTING 2.1 Visual Inspection 2.2 Liquid Penetrant Inspection 2.3 Magnetic Particle Inspection 2.4 Electrical Test Methods 2.5 Ultrasonic Inspection 2.6 Principles of Radiography 2.7 Acoustic Emission Inspection

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3. MICROSTRUCTURE CONTROL 3.1 Nucleation and Growth Processes 3.2 Grain Refinement 3.3 Directional Solidification 3.4 Single Crystal Blades 3.5 Thermo-mechanical Processing

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4. PRINCIPLES OF PROCESS MODELLING 4.1 Numerical Algorithms 4.2 Physical Constants 4.3 Metallurgical Structure

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1. 1.1

JOINING

GENERAL METHODS

Discussion of manufacturing processes usually focuses on ways of making individual components, normally using only one material. However, most products have many components and it is necessary to identify ways of assembly, noting that sometimes joints are required to endure over the component life while other joints need to be capable of being undone. Basically, there are three types of joining technique:   

Fastening relies on the elastic and/or frictional properties of a material to hold two components together physically (e.g. rivets, nuts and bolts, nails and screws, snap fits, etc). Welding involves heating and/or working to create a joint, which physically and chemically bonds the two surfaces (e.g. solid-state bonding, fusion welding, etc). Gluing introduces a layer of another material between the two surfaces, bonding physically and chemically to the surfaces (e.g. polymeric glues or metallic glues covering brazing and soldering).

The strength of a joint depends on the inherent strength of the materials involved (i.e. the inherent strengths), the strengths of any bonds formed across the interfaces (i.e. the adhesive strength) and on the geometry of the joint in relation to the loading system. For riveted, welded and glued joints, failure can occur in the rivets, welds or glues, along the interfaces or in the components, which have been joined. Fasteners must be used if joints need to be disassembled or may be used for permanent joining depending on cost and/or performance (e.g. nuts and bolts, nails, screws, etc. can fall into both categories). However, using metal fasteners to join metal components (when the materials are different) can lead to corrosion problems. Moreover, fasteners used at discrete intervals do not in themselves seal the joint against gases or fluids, unless gaskets or sealants are employed. Welding can be achieved by bring two surfaces sufficiently close together, so they can be joined by plastic deformation. Called ‘solid-state welding’, the two components being joined do not melt, but joining normally required clean surfaces and sufficient pressure (often applied at elevated temperatures). At temperatures above half the absolute melting point, diffusion across the interface can result in strong bonding without need for a high degree of working (often called diffusion bonding). Fusion welding means that heat is applied (externally or by friction) to melt the surfaces of the components being joined. Often, a ‘filler metal’ is used to fill the gap between the surfaces, which are being melted. A strong joint is then formed as the molten regions solidify. Gluing introduces a fluid between the two surfaces, which flows into surface crevices. On cooling below the melting point, in the case of metal fillers in brazing and soldering, a strong joint is formed without melting the surfaces being joined. Similarly, on cooling below the glass-transition temperature, animal glues and thermoplastic adhesives also form strong joints (called adhesive bonding). With polymeric adhesives, joints may also be formed by evaporation of a solvent or carrier 3

liquid (e.g. bicycle tyre repair kits) or by chemical reaction (usually polymerisation or cross linking, as with ‘two-pack’ adhesives which are mixed and then solidified). 1.2

STRENGTHS OF JOINTS.

The strength of a joint depends on the inherent strengths of the materials involved, the adhesive strength at the interfaces and the joint geometry in relation to the loading system. However, with fusion welding, the inherent strengths of the regions that solidify after fusion and the un-fused regions differ, with the volumes of non-molten material next to the fusion interface also having different microstructures and properties by being exposed to temperatures approaching the melting point (called the heat-affected zone or HAZ). Moreover, with fusion welding, voids and pores may arise during solidification (as in casting) or because of incomplete fusion. Similarly, air bubbles maybe entrapped when using polymeric adhesives. Such flaws result in ‘stress concentrations’, with the scale of the problem affected by the size and shape of the flaws. Consider a plate containing an elliptical void of length, c, with a radius of curvature, r, at the void tip, aligned perpendicular to the tensile stress,  . The maximum stress adjacent to the flaw ( m) is  m   1 2 c / r 





For a spherical flaw (r = c), m = 3. For large values of c/r, the ‘stress concentration factor’ approximates to 2 (c/r). Even structures designed to avoid excessive elastic or plastic deformation can fail catastrophically due to fracture when there are existing flaws. This is especially severe in welded (or adhesive bonded) joints due to existing voids or cracks. With an existing crack (of length a) fast fracture can occur at a critical stress (). Conversely, at a given applied stress fracture can occur at a critical crack length. This condition is expressed by a  E.G /  2 where E is the Youngs modulus and G is the toughness (energy absorbed per unit area of crack creation). (NB with brittle materials, G = 2 where  is the surface energy). Both E and G are material properties, and their combination is expressed as the Fracture Toughness (Kc) of a material K c  E.G Kc is a material property dictating the critical crack size or critical stress for fast fracture of a component according to, K c   a Hence, it is important to be able to detect flaws in joints. While compressive stresses are benign, tensile stresses tend to open cracks, with bending stresses tending to concentrate the tensile forces. For this reason, it is necessary to use joint geometries that minimize adverse loading systems. 4

1.3

TYPES OF JOINTS.

Some typical geometries for the joining of two plates are show in figure 1.1. Figure 1.a. is a simple butt joint and is prone to bending and distortion under load. The simple lap joint of figure 1.b. helps with bend distortion but the uneven adherence across the joint surface can lead to uneven distributions of forces. Figure 1.c. is a scarf joint and its greater length as compared to the thickness of the parts to be welded reduces the risk of bending under load. Figure 1.d is a stepped lap joint. Since it is combination of a butt joint and a lap joint, it combines the advantages of both. Finally figure 1.e shows how additional strengthening struts can be added during the manufacture of the joint. This strapped butt joint has the effect of almost eliminating bending distortions. a

b

c d

e Figure 1.1 Typical joint geometries. Similar considerations apply to joints where the two parts are not in the same plane. 1.4

FASTENERS.

Mechanical fasteners are used for a number of reasons such as: The possibility to disassemble and reassemble components easily when this is desirable during the lifetime of the component, even semi-permanent fasteners involving rivets can be drilled out. Ease of manufacturing and low manufacturing cost considering that a wide range and sizes of such fasteners are mass produced. Very little surface preparation and cleaning required compared to other joining methods. Components requiring moveable joints. Ability to join dissimilar material and components of wide variety of size shape and joint design. A wide range of fastening methods has been developed. Integral fasteners (Figure 1.2) are areas of the components that have been formed so that they interfere of interlock with other components.

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a) is a lanced tab that fastens wires or cables to sheet. b) and c) assemble components through folded tabs and slots for different loadings. d) uses a flattened embossed protrusion whiles e) is a single lock seam.

Figure 1.2. Several type of integral fasteners

Discrete fasteners are separate pieces used to join the primary components.

Solid rivet Tubular rivet type type Figure 1.3. Discrete fasteners.

Blind rivet type

Assembled bolted joint

The third type of fastener uses shrink and expansion. Here a dimensional change is introduced to one or both of the components by heating or cooling. When temperature uniformity is restored, an interference fit (which relies on friction for its strength) is produced. Press fit fasteners employ a similar principle but using mechanical force rather than temperature difference. Fasteners are a cheap method of producing satisfactory joints in both high and low technology systems. Many aircraft skins are fixed in this way but some precautions need to be observed. It is possible to over-tighten fasteners resulting in poor overall 6

mechanical properties. The holes required in some situations may reduce the load bearing area of the component or may give rise to fatigue crack initiation due to stress concentrations. Metallic fasteners provide electrical contact between the components but improper selection also makes the joint susceptible to galvanic corrosion. In addition, fasteners only join components in discrete points and do not seal the entire joint area to gas or liquid penetration leading to corrosion. 1.5

ADHESIVE BONDING.

The use of adhesive bonding as an alternative joining method to fastening and even welding has become more attractive due to improvements in their strength and reliability combined with their ease of use and relatively low cost. Even quality and durability conscious areas, such as the automotive and aerospace industries, make extensive use of adhesive bonding. Adhesives in the automotive industry have advanced from the attaching of exterior and interior trim to the joining of major components, such as door, bonnet and boot assemblies. Adhesives can be divided into 2 main categories:  

Natural adhesives Synthetic adhesives

The natural adhesives are vegetable or animal derivatives. Compared with the synthetic adhesives they are relatively weak but are non-toxic. They are suitable for low strength applications but are increasingly being supplemented by the high strength synthetic adhesives. The main types of structural adhesive fall in the following categories: Epoxies. These are one of the oldest thermosetting adhesives (i.e. they require heat to make them set) and most commonly used for structural applications. Most consist of 2 blend components (resin and curing agent) but may also involve single part adhesive requiring higher temperature to cure. The curing process causes chemical changes to take place within the adhesive and once set they cannot be softened again. This makes them less temperature sensitive than thermoplastic adhesives. The epoxies can be used to join most engineering materials including metal, glass and ceramic. They have good tensile and sheer strength, creep resistance and can be used over a range of temperatures. High temperature structural epoxies have been developed for aerospace applications. However, epoxies are sensitive to moisture, expensive, needs long cure time and have low flexibility (brittle). Cyanoacrylates. (The so-called ‘super-glues’). These single component liquid adhesives are cured by moisture almost instantaneously at room temperature. They offer fast bonding and excellent strength. Effective application requires good component fit, non-absorbent surface and their use is limited by their high cost and brittleness. Anaerobics are thermosetting polyester acrylics that remain liquid when exposed to air. When oxygen is shut off (as in confined space such as a joint to be bonded), the polymer becomes unstable and next to copper or iron, polymerizes into a bondingtype resin. The anaerobics are very versatile, can bond almost anything including oily surfaces and offer good sealing to moisture. However, they are brittle and limited to low service temperature.

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Acrylics are relatively new two-part curing polymers. They are very versatile in bonding various types of materials and offer excellent strength and water resistance at very moderate cost. They are becoming increasingly popular as general purpose adhesive. However, they lack high temperature strength. Urethanes — these are a large family of polymer adhesives that are generally limited to conditions...