Joining and Machining

Question 1

Why is joining necessary?

Answer 1

Most complex shapes are multi cast pieces and joining type used is dependant on temp

Question 2

Rank joining techniques on temp uses

Answer 2

Power beans (highest T)
ARC processes 
Solid state processes 
Adhesive solders 
Fasteners (Lowest T)
1st 3 change microstructure & are permanent 
Last 2 are non-fusion techniques

Question 3

Describe rivets, their +ve and -ve and their failures modes

Answer 3

Hard to remove but can be drilled out, used for thin structures, have a good surface finish
Rivet passed through hole in two sheets, rivet stem deformed to bring together
Different materials can cause corrosion of eachother, if rivet isn’t tight movement can lead to failure

Question 4

Briefly describe fasteners

Answer 4

Rivets, bolts, screws

Little affect on structure but hole needed, strength based on residual stresses in fastener, can be removed

Question 5

Describe threaded fasteners, their +ve -ve and failure modes

Answer 5

Screw - requires sheets to have coherent threading = machinable materials
Bolts - passed through hole in sheet, washers and nuts used, Torque tightening causes fasteners to elastically unpaid and clamp sheets together
Failure modes = corrosion, movement causing failure

Question 6

How can you remove material from a component?

Answer 6

Mechanically
Chemically
Electrically (only for conductor)

Question 7

Describe single point machining, the +ve -ve and material requirements

Answer 7

Tool with cutting edge causes localised shear = material displaced as swarf
Tool can be worn by inclusions, high T can cause bonding between tool and swarf (inc with inc cut depth)
High conductivity tool = high speed & depth, FCC materials easiest to machine (hard material = more passes)

Question 8

Describe spark machining , their +ve -ve

Answer 8

EDM - conducting shapes die placed in dielectric field and moved close to workpiece = high localised field = sparks which vaporise workpiece removing material in shape of die 
Slow process (die moved slowly) and bad surface finish (high oxide content), only works on conductors

Question 9

Describe chemical machining

Answer 9

Can be used to roughen surfaces for adhesive bonding, very slow process with temp, concentration, pressure and voltage needing to be controlled

Question 10

Describe adhesive bonding, +ve -ve and failure modes

Answer 10

Applied as low viscosity liquid, solidifies over joint to form viscous bond (E.g. epoxy resin & hardener)
Strength low due (polymer), needs rough surface to increase joint strength (as inc SA) but too rough = no adhesive flow, surface roughening can remove oxides

Question 11

What is meant by wetting (in terms of adhesives)? And why is it and viscosity important?

Answer 11

Wetting means adhesive spreads over joint and doesn’t ball up
Metallic alloys have high surface energy = likely to wet, surface defects decrease wetting
Non-wetting or low viscosity = incomplete covering = gas trapping and weak bonding = weak join

Question 12

Why is curing adhesives important?

Answer 12

Curing causes crosslinking in polymer = higher strength bond
Curing temp/reaction is step that allows the chain mobility

Question 13

What is important about peeling and adhesive modifiers?

Answer 13

Peeling - forces lead to sheer stresses on joint, peel test can see if joint has undergone peeling
Adhesive modifier - put rubber in adhesive, causes deformation around rubber = increased joint strength

Question 14

Describe soldering and brazing

Answer 14

Sold - “metal glue”, diff metals can be used dependant on situation, wetting, surface finish & surface cleanliness important
Brazing - higher temp, strength inc with diffusion, clean surface important
Metal/glass brazing - requires braze with covalent/ionic&metallic bonding, glass can be layered onto joint for this

Question 15

Describe fusion welding,+ve -ve

Answer 15

Substrate edges form mixed weld pool, it cools and joins pieces (high energy to melt), either moving power source or moving material

Question 16

What is meant by ‘v prep’ prior to welding ? And how does solidification affect the weld?

Answer 16

Purposefully made v-shape in material, fluid flows in melt pool, solidifies to create columnar grains (heat flow into metals)
Weld pool absorbs atmos gas = gas trapping unless done in inert atmos or add metal to weld pool that forms oxides = precipitates/ rise to slag layer

Question 17

What are the power source requirements for welding?

Answer 17

Quick, easily controllable and precise - heat input governed by Pd and current, Thicker = hotter needed, hotter = less passes needed
When arc welding - electrode (consumable or non-consumable) and welding torch (other electrode)

Question 18

Describe spot welding ,+ve -ve

Answer 18

Sheets pressed together, current passed between them which melts to form meltpool, welding only occurs in sections that are pressed
Must be narrow sheets, depressions where electrodes meet sheets = low strength weld, quick cooling = radial grains = strength, good for low carbon/Ni based alloys

Question 19

Describe seam welding

Answer 19

2 wheels are electrodes that form a series of spot welds

Question 20

Describe manual metal arc welding ,+ve -ve

Answer 20

Consumable electrode(wrapped in protective flux) = tube filled with ceramic powder, flux breaks down in meltpool = electrode melts, the flux deoxidises meltpool forming an insulator slag layer

Question 21

Describe submerged arc welding

Answer 21

Bare metal consumable electrode = continuous stream = high production rate, with joint line covered in flux (allows more applied heat = thicker)
Loose powder readily trapped in melt pool = poor fracture toughness

Question 22

Describe metal inert gas welding

Answer 22

Inert shielding gas fed around consumable electrode = protective from atmosphere (conducts heat away from melt pool) but some contamination occurs
No slag layer = higher fracture toughness
Performance over production rate

Question 23

Describe tungsten inert gas welding

Answer 23

Non-consumable tungsten electrode surrounded by inert gas shield, heat input limit so tungsten doesn’t meant = slow process 
Produces cleanest (no slag layer) and highest toughness welds, filler rod can be added if filler needed in meltpool

Question 24

Describe general arc welding trends

Answer 24

Inc heat transfer to weldpool = inc efficiency, inc speed = decrease in quality and precision, thicker materials = higher heat/multiple passes, edges of weldpool have highest temp, metal around weldpool is heat affected, heat dissipated through base metal

Question 25

Describe what is meant by the heat affected zone

Answer 25

Heating causes annealing and grain growth, greater heat input = greater Zone, strength depends on peak temp and cooling rate (reduce cooling rate = reduced stresses&defects), comp and cooling rate can be chosen to dictate strength of weld compared to base metal

Question 26

What are the principle defects of welds?

Answer 26

Defects: porosity, centre line cracking (cooling rate too high), lamaella tearing (interface separation), hydrogen cracking (welding in damp environment, flux breaks down water), liquation cracking (happens during second pass)

Question 27

What are distortions and how can they be reduced?

Answer 27

Formed in tightly clamped welds as tensile stresses can be introduced to surface
Welding on both sides counteract distortions = reduced affect