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Cutting & Joining

By DarthVader

Date: 2022-07-09

Topic: 144 see comments

Post views: 1047


Cutting & Joining

Cutting

  • Cutting enables creating much more precise and accurate shapes than casting or forming.
  • Cutting is often required for further processing of cast or formed shapes.
  • Cutting also refers to abrasive wheels or paper such as those used for sanding down surfaces to achieve better surface finishes.
  • Almost any shape can be achieved by the use of cutting tools.
  • ‘Machining’ refers to the use of cutting machines such as; lathes, cnc machines etc.

Cutting tools need to be harder than the workpiece they will be cutting and ceramics are generally harder than metals, which are in turn harder than polymers. Material hardness is measures in ‘Vickers hardness’ with diamond being the hardest material with vk of 10,000

Materials used for cutting need to withstand high temperatures and wear.

There are basically 3 types of cutting-tool materials:

  • Tool steels
  • Composite materials based on metal carbides
  • Ceramics, including diamond-like materials

carbide = carbon, metal composite


Joining

Types of joining:

Mechanical joining

(Nails, screws, bolts, clips, rivets etc.)

  • Fasteners join at discrete points and do not by themselves seal the joint against liguids and gas, rubber gaskets or silicone etc. are required for sealing the fastened joint.
  • The hole that the fastener goes through in a mechanical joint is a potential weak spot, and failure often occurs at these sections.

Adhesive joints - gluing

Two parts are joined by placing a liquid between them which then solidifies. Soldering is an example of this where solder alloy is melted between two parts and then solidifies joining them together. Although the priciples of soldering are shared with all gluing processes, the word adhesive is usually taken to mean a type of polymer glue.

Advantages:

  • Almost all materials and combinations of materials can be joined
  • Curing temperatures are low
  • Many adhesives provide adequate strength
  • Heat-sensitive materials can be joined
  • No holes have to be made (unlike for rivets or bolts)
  • More contact area means more joint strength
  • Adhesives will fill surface imperfections

Disadvantages:

  • Most adhesives are  not stable above 180 °C
  • Surface prep and curing are critical for good and consistant results
  • The life expectancy of the joint is hard to predict
  • Depending on curing time, assembly may take longer than other methods
  • Some adhesives contain toxic chemicals and solvents

Welding:

Video: https://youtu.be/aq-rIpiY-y0

Solid-state welding

Processes that join without melting the material are called solid-state welding.

In highly deformable materials such as metals and thermoplastics, solid-state welding can be used. By forcing the 2 hot surfaces together so that plastic deformation makes their shapes conform to one another while breaking up the surface layers, allowing the materials to fuse without melting.

Fusion welding

In fusion welding the parts to be joined are brought togther, melted and fused to each other.

Fusion welding creates large differences in tmperature between the weld and the rest of the material. This leads to various amounts of thermal expansion and contraction which can cause distortion and, in extreme cases, the formation of cracks.

Welds are in fact small castings and contain the same microstructure and porosity found in cast materials.

Types of welding processes:

Similarities:

In all the methods an electric arc is created when an electrical discharge occurs between an electrode and the material.

A spark is formed, which causes the surrounding gas to be ionised.

A current will flow, creating the arc.

The arc generates heat, which melts (fuses) the base metal. The heat input to the fusion zone depends on the current, voltage and the speed of travel of the electrode.

 

NameObservationsAdvantagesDisadvantages
MMA

Coated and consumable electrode.

The electrode and surface of the material are melted to create a weld pool.

oxy_insert_endAverage temperature 600 degrees C.

Gas and slag protects the weld pool from oxygen and nitrogen.

Coated and consumable electrode.

The electrode and surface of the material are melted to create a weld pool.

oxy_insert_endAverage temperature 600 degrees C.

Gas and slag protects the weld pool from oxygen and nitrogen.

For long welds several electrodes are required.

Requires well trained and highly skilled welders.

Suffers from slag inclusion.

Generates unwanted fumes.

TIG

Electrode is non-consumable, filler metal is added separately.

Inert gases are used as a shield to protect the weld.

Ideal for root runs and thin sheets.

No slagging inclusions.

Could be used with or without filler wire.

Produces accurately controlled welds.

Weld dissimilar materials.

Low deposition rates.

Not economical on thick sections.

Not easy to shield the weld in draughty conditions.

MIG

Consumable wire electrode.

A shielding gas protects the weld pool.

Three different types of metal transfer (dip, spray or pulsed).

Suitable for sheets and thick plates.

High productivity manually or mechanised.

Easily automated.

Improvement in wire electrodes results in the process becoming independent of shielding gas suitable for windy field applications

Requires shielding from wind.

Equipment is complex.

Produces high levels of unwanted gas and fume.

SAW

Not a hand held process.

Arc between a continuously fed bare wire and the material.

Powdered flux protects the weld from contamination.

No spatter or fume.

Weld is shielded from the atmosphere.

No ultraviolet and infrared radiation effects.

Deep weld penetration.

High deposition rates.

Certain welding positions only.

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