Buğra Çetinkaya 030070094 3rd Week Definitions

Spheroidizing (previous answer) (Annealing Method)

It is lowest temperature range of annealing process in which iron base alloys are heated 20 to 40°C below the lower critical temperature, held therefore a considerable period of time e.g. for 2.5 cm diameter piece the time recommended is four-hours. It is then allowed to cool very slowly at room temperature in the furnace itself. During this process, the cementite of steel which is in the combined form of carbon becomes globular or spheroidal leaving ferrite in matrix, thus imparting softness to steel. After normalizing of steels, the hardness of the order of 229 BHN and as such machining becomes difficult and hence to improve machining, these are spheroidised first and then machined. This treatment is carried out on steels having 0.6 to 1.4% carbon.
(R. SINGH, Introduction to Basic Manufacturing Processes and Workshop Technology, pg.139)

Spheroidizing (new answer) (Annealing Method)

In hypereutectoid steels, the cementite network is hard and brittle and must be broken by the cutting tool during machining. Spheroidizing annealing is performed to produce a spheroidal or globular form of carbide and improse machinability. All spheroidizing treatments involve long times at elevated temperatures, to produce a structure of globular carbide in a ferritic matrix. Application of spheroidizing is discussed in the chapter about cast iron.

In general spheroidizing is accomplished by the following procedures: 

1) Heating to a temperature just below Ac: and holding at that temperature for a prolonged time. 

2) Alternate heating and cooling between temperature range of just above Ac, and just below Ar. 

3) Heating to a temperature above Ac, followed by either slow cooling in the furnace or holding at a temperature just above Ar. 

4) Cooling at a suitable rate from the minimum temperature at which car­bide is dissolved to prevent reformation of carbide network, followed by reheating in accordance with either method number one or two above. This method is applicable to steels that base carbide networks, such as hypereutectoid steels.

(Ramesh Singh, Applied Welding Engineering: Processes, Codes, and Standards, p.101)

Prealloyed Powders (previous answer) (Production Method)

Prealloying is a special powder metalurgy term for the production of alloyed powders,usually from a melt by water or gas atomisation,the melt typically eshibiting the same composition as the emerging powder particles.(with the notable exception of oxygen content)Prealloyed powders for structural component applications are produced by water atomisation.

(Machinability of Powder Metallurgy Steels,A.Salak,M.Selecta,H. Danninger)

Prealloyed Powders (new answer) (Production Method)

The use of this technology for the production of TiAl components requires the use of prealloyed "atomized" powder. Various atomization techniques including gas atomization, the rotating-electrode process (REP), the plasma rotating-electrode process (PREP), and rotating-disc atomization have been discussed in the literature. The quality of a powder is an important issue and is determined by a number of factors including the particle size, the particle-size distribution, the particle shape, and the degree of in pick-up during the atomization process. From this point of view the different powder-production techniques have certain advantages and disadvantages. Unlike conventional metallic powders, TiAl powder is generally too hard to be cold compacted. It is therefore usually processed by hot isostatic pressing (HIP) followed by some sort of hot-working procedure. However other techniques such as spray forming and metal injection molding (MINI) have also been investigated in the literature, as described in later sections of this chapter.

A brief description of the main methods employed in the production of prealloyed TiAI powders is given below. Generally, all of the methods are capable of delivering high quantities of spherical-shaped powder. Due to the very complicated nature of all the atomization processes only basic principles will be described. For more debited information the reader is directed to the relevant technical literature.

(Fritz Appel, Jonathan David Heaton Paul, Michael Oehring, Gamma Titanium Aluminide Alloys: Science and Technology, p.522)

Slip Casting (previous answer) (Forming Technique)

Forming a hollow ceramic part by introducing a pourable slurry into a mold. The water in the slurry is extracted into the porous mold, leaving behind a drier surface. Excess slurry can then be decanted.

(Askeland, D.R.,The Science and Engineering of Materials,3rd Edition,PWS Publisihing Company, pg.464)

edit:

Green compacts for tungsten, molybdenum, and other powders are sometimes made by slip casting. The powder, converted to slurry mixture, is first poured into a plaster of Paris mold. Because the mold is porous, the liquid gradually drains off into the plaster leaving a solid layer of material deposited on the surface of the mold. For hallow objects, after sufficient time has been allowed for a desired thickness to accumulate, the remaining slurry is poured out. Upon drying the green components are sintered in the usual manner. This procedure is simple and permits considerable variation in size and shape.

Increasing emphasis is being directed to this process because parts can be made that are too large or too complex to pres. An expensive pres is not required, and improved physical properties are additional advantages. The principal disadvantage is time lag in producing parts.

(Amstead, B.H., Ostwald P.F., Begeman, M.L., Manufacturing Processes, 8th Edition, John Wiley & SOns, Inc, pg.202)

Slip Casting (new answer) (Forming Technique)

Slip casting is a forming technique that has a long history and wide applications. This technique cast slurry into plaster molds that could absorb most water in slurry. After opening plaster molds, green bodies could be taken out and then sent for drying and sintering. Currently this technique is still widely applied to fabricate corundum crucibles and other products. However slip casting has a big problem, i.e. inner and outer layers of green body have different density since water in slurry is absorbed by layers. The inhomogeneous density would bring inhomogeneous contraction during drying and sintering, and resultantly cause internal stresses or even deformation and fracture after sintering.

In recent years, a novel slip casting technique has been developed, i.e. gel casting. Organic polymer monomers are introduced into slurry. After casting for a certain period of time, monomers would be cross linked and polymerized under the effects of catalyst and initiator to form eel structure of three-dimensional network, and the slurry is in-situ solidified. The obtained green body has uniform structures and high strength with the dimensions of final products. Besides the density of different part is almost the same. The technique had two problems, i.e. certain toxicity of organic monomers; and possible cracks on the surface of green body. The two problems have already been solved. For example, liquid dehydrator is used to dry green bodies so that surface cracks could be prevented. Researchers from Tsinghua University and Central China University of Science and Technology have achieved good results in this respect. More recently, a forming technique called thermo-reversible gel casting (Montgomery, Prjal, Shull. 2002) takes use of physical process, i.e. reversible polymer gel, instead of chemical process for casting. Due to outstanding advantages, gel casting has attracted broad attention. Reference (Janney, 1998) lists some technical parameters of several casting processes.

(Qingrui Yin, Binghe Zhu,Huarong Zeng,  Microstructure, Property and Processing of Functional Ceramics, p.313)

Electroplating (previous answer) (Coating Method)

Electroplating, also known as electrochemical plating, is an electrolytic process (section 4.5) in which metal ions in a electrolyte solution are deposited onto a cathode workpart. The setup is shown in Figure 29.1. The anode is generally made of the metal being plated and thus serves as the source of the plate metal. Direct current from an external power supply is passed between the anode and the cathode. The electrolyte is an aqueous solution of acids, bases, or salts; it conducts electric current by the movement of plate metal ions in solution. For optimum results, part must be chemically cleaned just prior to electroplating. (Groover M.P., Fundamentals of Modern Manufacturing: Materials, Processes, and Systems 3^rd Edition, page 670)

Electroplating (new answer) (Coating Method)

Electroplating is a very widely used and diverse technology. In principle, the choice of substrate and coating metals may encompass a large number of single metals and alloys; in fact, the substrate may also be a polymer, a ceramic or a composite. The coating may be a single metal, an alloy or, indeed, a metal-polymer or metal-ceramic composite. In practice, the choice is restricted severely by the technical constraints, the possibilities for electrode position and the desired deposit properties as well as economic factors. An obvious trend may be noted towards the use of relatively cheap substrates as bulk constructional materials. The more expensive metals are usually encountered as thin coatings. Intermediate metals, such as nickel, may be used either as substrates or coatings. It is noteworthy that aluminium-coated steel would offer a formidable combination of properties, but the development of electroplated aluminium surfaces is severely restricted by the need to utilize fused salts or non-aqueous solvents as electrolytes.

(Derek Pletcher, Frank Walsh, Industrial Electrochemistry, p.386)

Stick Slip (previous answer) (Motion type)
The phenemenon of stick slip occurs if there is an intermittent change between static and kinematic friction.It becomes possible,if the dynamical proporties of the ragarded system like elasticity,mass distrubition and damping allow this kind of motion.
(Ulbrich H.,Günthner W., IUTAM symposium on vibration control of nonlinear mechanism and structure,p.223)

Stick Slip (new answer) (Motion Type)

Sliding of one body over another under a steady pulling force sometimes occurs at velocities that fluctuate widely. If the friction force or sliding velocity varies as a function of distance or time and produces a form of oscillation, it is termed as stick-slip phenomenon. The term was coined earlier by Bowden and Leben (1939). Figure 5.7 shows a schematic stick-slip behavior. 

During the stick phase, the friction force increases up to a certain value and once a large enough force has been applied, slip occurs at the interface. During the stick-slip phase, usually a saw tooth pattern in the friction force time curve is seen. The stick friction is markedly greater than the coefficient of kinetic friction. Sometimes harmonic oscillations are observed at high velocities if the coefficient of kinetic friction varies with sliding velocity and the slope of Mk versus velocity curve is negative at a velocity. The stick-slip process generally results in squeal (- 0.6-2 kHz) and chatter (< 0.6 kHz) in bearings, jerking of brakes, the chatter of windshield wipers on a partly wet window glass, inaccuracies in machining and positioning, etc. Sometimes stick-slip may be a source of pleasure, as in the bowing of stringed musical instruments. The stick-slip process may occur as a result of one or both of the following two features:

(a) The coefficient of static friction (Ms) is greater than the coefficient of kinetic friction (Mk)

(b) The rate of change of the coefficient of kinetic friction, as a function of velocity at the sliding velocity, is negative.

The stick-slip process may be prevented or minimized by the following means:

(i)                  The mechanical system may be so designed that amplitude of stick-slip oscillations would be small (reduce system compliance, increase system damping and inertia).

(ii)                The friction pair may be so selected that the difference between Ms and Mk is small (use of boundary lubricant film).

(iii)               A friction pair may be so selected that a positive Mk - V characteristic at the sliding velocity is exhibited (with soft metallic coating and polymers).

(Prasanta Sahoo , Engineering Tribology, pp.60-61)