Surface Finishing Processes -manufacturing Processes-ii 2o555b

Manufacturing Processes-II 5th sem Mechanical Section -II Sub Topic Chapter 7: Metal Finishing and Coating

Surface Finishing Processes

gradual improvement of surface roughness produced by various processes ranging from precision turning to superfinishing including lapping and honing.

Surface Finishing Processes • Improve appearance and sales value of product • Used to resist wear, electrolytic decomposition, and corrosive wear • Treatment process (chemical or electrical) produces oxide of original metal on surface • Common methods: burnishing, electropolishing, honing etc. 26-3

Processes • • • •

Honing Lapping Polishing and Buffing Super finishing

Honing

Honing • Honing is a finishing process, in which a tool called hone carries out a

combined rotary and reciprocating motion while the work piece does not perform any working motion. Most honing is done on internal cylindrical surface, such as automobile cylindrical walls.

• The honing stones are held against the work piece with controlled light pressure. The honing head is not guided externally but, instead, floats in the hole, being guided by the work surface

• It is desired that – honing stones should not leave the work surface – stroke length must cover the entire work length.

Honing Honing • Honing is a finishing process performed by a honing tool, which contains a set of three to a dozen and more bonded abrasive sticks. The sticks are equally spaced about the periphery of the honing tool. They are held against the work surface with controlled light pressure, usually exercised by small springs. • The honing tool is given a complex rotational and oscillatory axial motion, which combine to produce a crosshatched lay pattern of very low surface roughness

Honing tool

Honing  Abrasive process performed by a set of bonded abrasive sticks using a combination of rotational and oscillatory motions  Common application is to finish the bores of internal combustion engines  Grit number (grain size) range between 30 (medium) and 600 (very fine) (the smaller grain size, the larger grit number)  Surface finishes of 0.12 m (5 -in) or better  Creates a characteristic cross-hatched surface that retains lubrication

Honing

Figure 25.16 The honing process: (a) the honing tool used for internal bore surface, and (b) cross-hatched surface pattern created by the action of the honing tool.

Honing Tool

Figure : Schematic illustration of a honing tool used to improve the surface finish of bored or ground holes.

Honing Honing

Honing •

Stone – Al2O3 or SiC bonded abrasives

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The critical process parameters are: – Rotation speed – Oscillation speed – Length and position of the stroke – Honing stick pressure

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Parameters that affect material removal rate (MRR) and surface roughness (R) are: – Unit pressure, p – Peripheral honing speed, Vc – Honing time, T

Lapping

Lapping Lapping • In lapping, instead of a bonded abrasive tool, oil-based fluid suspension of very small free abrasive grains (aluminum oxide and silicon carbide, with typical grit sizes between 300 and 600) called a lapping compound is applied between the work piece and the lapping tool. • The lapping tool is called a lap, which is made of soft materials like copper, lead or wood. The lap has the reverse of the desired shape of the work part.

To accomplish the process, the lap is pressed against the work and moved back and forth over the surface. • Lapping is sometimes performed by hand, but lapping machines accomplish

the process with greater consistency and efficiency.

Lapping Lapping •

Lapping is regarded as the oldest method of obtaining a fine finish. Lapping is basically an abrasive process in which loose abrasives function as cutting points finding momentary from the laps. Material removal in lapping usually ranges from .003 to .03 mm but many reach 0.08 to 0.1mm in certain cases.

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The cutting mechanism in lapping is that the abrasives become embedded in the lap surface, and the cutting action is very similar to grinding, but a concurrent cutting action of the free abrasive particles in the fluid cannot be excluded.

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Lapping is used lo produce optical lenses, metallic bearing surfaces, gages, and other parts requiring very good finishes and extreme accuracy.

Lapping Characteristics of lapping process: •

Use of loose abrasive between lap and the work piece

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Usually lap and work piece are not positively driven but are guided in with each other

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Relative motion between the lap and the work should change continuously so that path of the abrasive grains of the lap is not repeated on the work piece.

Lapping Uses fluid suspension of very small abrasive particles between workpiece and lap (tool)  Lapping compound (fluid suspension) - fluid with abrasives, general appearance of a chalky paste  Typical grit sizes (Grit number) between 300 (medium) to 600 (very fine)  Applications: optical lenses, metallic bearing surfaces, gages  The same as polishing samples for microscopic or electronic microscopic tests

Lapping

Figure : The lapping process in lens-making.

Lapping

Figure : (a) Schematic illustration of the lapping process. (b) Production lapping on flat surfaces. (c) Production lapping on cylindrical surfaces.

Lapping Lapping

Schematics of lapping process showing the lap and the cutting action of suspended abrasive particles.

Lapping • Lapping

Figure (a) Schematic illustration of the lapping process. (b) Production lapping on flat surfaces.(c) Production lapping on cylindrical surfaces.

Lapping •

Abrasives of lapping – Al2O3 and SiC, grain size 5~100μm – Cr2O3, grain size 1~2 μm – B4C3, grain size 5-60 μm – Diamond, grain size 0.5~5 V

•

Lubricating materials of lapping

– Machine oil – Rape oil – grease •

Technical parameters affecting lapping processes are – unit pressure – the grain size of abrasive – concentration of abrasive in the vehicle

– lapping speed

Buffing

Buffing Polishing •

Polishing is a finishing operation to improve the surface finish by means of a

polishing wheel made of fabrics or leather and rotating at high speed. The abrasive grains are glued to the outside periphery of the polishing wheel. Polishing operations are often accomplished manually. Buffing •

Buffing is a finishing operation similar to polishing, in which abrasive grains are not glued to the wheel but are contained in a buffing compound that is pressed into the outside surface of the buffing wheel while it rotates. As in polishing, the abrasive

particles must be periodically replenished. •

As in polishing, buffing is usually done manually, although machines have been designed to perform the process automatically.

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Buffing wheels are made of discs of linen, cotton, broad cloth and canvas

Buffing Buffing

Super Finishing

Super Finishing Super finishing •

Super finishing is a micro finishing process that produces a controlled surface condition on parts which is not obtainable by any other method. The operation which is also called ‘micro stoning’ consist of scrubbing a stone against a surface to produce a fine quality metal finish.

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The process consists of removing chatter marks and fragmented or smear metal from the surface of dimensionally finished parts. As much as 0.03 to 0.05 mm of stock can be efficiently removed with some production applications, the process becomes most economical if the metal removal is limited to 0.005 mm

Super Finishing Similar to honing - uses bonded abrasive stick pressed against surface and reciprocating motion  Differences with honing:  Shorter strokes  Higher frequencies  Lower pressures between tool and surface  Smaller grit sizes

Super Finishing

Figure : Superfinishing on an external cylindrical surface.

Super Finishing

Figure : Schematic illustration of the superfinishing process for a cylindrical part. (a) Cylindrical microhoning. (b) Centerless microhoning.

Super Finishing Super finishing

Figure Schematic illustrations of the super finishing process for a cylindrical part. (a) Cylindrical mircohoning, (b) Centerless microhoning.

Surface Treatment

Introduction • Surface Treatments – Why? – Type

Why use a surface treatment? • Improves durability

• Controls Friction

• Reduces Adhesion

Why use surface treatment? • Improves Lubrication

• Rebuild Surfaces

• Aesthetics

Types of Treatments • • • • • •

Mechanical Surface Treatments Mechanical Plating & Cladding Case Hardening Thermal Spraying Vapor Deposition Laser Treatments

Mechanical Surface Treatments • Peening – – – –

Shot Peening Laser Shot Peening Water-jet Peening Ultrasonic Peening

• Roller Burnishing • Explosive Hardening

Mechanical Plating & Cladding • Mechanical Plating • Cladding – Laser Cladding

Case Hardening and Hard Facing • Case Hardening

• Hard Facing • Spark Hardening

Thermal Spraying • Combustion Spraying – Thermal Wire Spray – Thermal Metal-Powder Spray – Plasma Spray

Vapor Deposition • Physical Vapor Deposition • Vacuum Deposition • Sputtering

• Chemical Deposition • Ion Platting

Physical Vapor Deposition

Sputtering

Chemical Vapor Deposition • Thermochemical Process • Cutting Tools • Thicker • Tedious

Ion Implantation & Diffusion Coating • Particulates penetrate substrate • Modifies surface properties – Increases hardness – Improves durability

• Masking capability

Laser Treatments • • • •

Heating Melting Vaporization Peening

Electroplating, Electroless Plating, and Electroforming • Electroplating – Workpiece (cathode) is plated with other metal (anode) through a waterbased electrolytic solution – A SLOW Process!!! • 75 micrometers/hour – Solution must be replenished • Sacrificial anode • Additional salts of metal

Electroplating, Electroless Plating, and Electroforming • Operation Sequence – Chemical Cleaning – Acid Bath – Application of a Base Coat (Optional) – Final Electroplating – Rinse Tanks

• Common Plating Metals – Nickel – Cium, Copper – Tin, Zinc

Electroplating, Electroless Plating, and Electroforming • Electroless Plating – Chemical Reaction – More Expensive $$ – Uniform Thickness

• Electroforming – Metal-fabrication – Metal electrodeposited on a mandrel

Conversion Coatings • Anodizing– The workpiece is the anode in an electrolytic cell

• Coloring– Alters color of metals, alloys, and ceramics – Conversion of surfaces into chemical compounds: oxides, chromates, and phosphates

Hot Dipping • Workpiece is dipped into molten metal – Zinc- galvanized-steel sheet – Tin- food containers

• Hot-dipped Galvanizing line

Porcelain Enameling; Ceramic and Organic Coatings • Enamels- fuse a coating material by heating to 425 to 1000. • Ceramic coatings- Intense temp applied • Organic coatings- Wide range of properties: flexability, durability, color, texture…

Diamond Coating and Diamond-Like Carbon • Techniques – Chemical vapor deposition – Plasma-assisted vapor deposition – Ion-beam-enhanced deposition

• Diamond Properties – Hardness, wear resistance, thermal conductivity

Surface Texturing & Painting • Texturing Techniques – – – –

Etching Electric Arcs Lasers Atomic oxygen

• Paint Classification – Enamels – Lacquers – Water-based paints

What is Galvanizing? The process of galvanizing consists of coating metals, such as iron, with a thin protective layer of zinc.

Before & After Hot Dip Galvanizing

The zinc layer provides protection to the metal from corrosion.

Cathodic Protection

The steel is protected by the surrounding zinc even if it is scratched.

How Does Zinc Protect The Underlying Iron Surface? Zinc coatings prevent corrosion of the protected metal by forming a barrier, and by acting as a sacrificial anode if this barrier is damaged. When exposed to the atmosphere, zinc reacts with oxygen to form zinc oxide, which further reacts with water molecules in the air to form zinc hydroxide. Finally zinc hydroxide reacts with carbon dioxide in the atmosphere to yield a thin, impermeable, tenacious and quite insoluble dull gray layer of zinc carbonate which adheres extremely well to the underlying zinc, so protecting it from further corrosion.

The oxidation of zinc is more likely than the oxidation of iron.

* *

These potentials indicate the relative thermodynamic tendency for the indicated half-reaction to occur. Zn <--> Zn+2 + 2 e– E = –0.763 volts Fe <--> Fe+2 + 2 e– E = –0.409 volts

Anodizing

Purpose of Anodizing • Grow an aluminum oxide layer on the aluminum so it can be dyed • Corrosion and wear resistance • Hardening (Type III) • Color – cosmetic

Overview • • • • • •

Aluminum part immersed in acid electrolyte Apply electrical current, DC, ~12V The part is the anode (+) (thus the name) Electrolysis and chemical reaction occurs Porous aluminum oxide layer grows on the aluminum Up to 3000 times thicker than naturally occuring Al2O3 layer • Dye goes into pores, results in bright colors • Place in boiling water to seal pores

Electrochemistry • Electrolyte in Solution: Free ions ,conductive – Sulfuric, oxalic, or phosphoric acid typically used – 15% solution of sulfuric acid (H2SO4)

• Electrolysis: Extracts constituent elements from solution • Anode – Evolution of oxygen – 2Al + 3H20  Al203 + 6H+ + 6e-

• Cathode

– Evolution of hydrogen – 6H20 + 6e-  3H2 (g) + 6OH-

Pore growth • • • •

Acid electrolyte acts as solvent for oxide Dissolves portions of barrier oxide layer Oxide grows at metal/oxide interface Rate of growth dependent on current, concentration, temperature, voltage • Hexagonal shape

Anodizing Setup Materials • • • • • • • • •

Aluminum item (anode) Aluminum wire Aluminum sheet (cathode) Sulfuric Acid 15% Non-metal container Power supply Distilled/de-ionized water Dye (RIT clothes dye) Baking soda

Process

Anodizing Tank

Results

Anodizing in General • Other metals that can be anodized – Titanium, magnesium, niobium, tantalum, tungsten, zirconium – Ti utilizes interference property of oxide film instead of dye for color

• History – Anodizing developed around 1917 with first US patent in 1925 (*AAA)