WELD DESIGN & WELDABILITY OF MATERIAL

 

WELD DESIGN & WELDABILITY OF MATERIAL

 

SUBJECTS OF INTEREST

·         Reviews of weld design and weld ability

OBJECTIVE

·         Students are required to understand the causes of residual stresses, distortion and their remedies.

·         Students are also required to differentiate weld defects that might occur during metal welding for example, solidification cracking, liquation cracking, distortion, weld embrittlement.

·         Students can suggest possible remedies associated with individual weld defects.

WELD DESIGN- JOINT TYPE

Welds are made at the junction of all the pieces that make up the weldment (assembled part).

There are eight types of welds

 

1.    Fillet weld: On the joint

2.    Groove weld: In the joint

3.    Back weld: Made on the back side of the weldment.

4.    Slot weld: Used with prepared holes.

5.    Spot weld: Weld at the interface of the members.

6.    Seam weld: Without prepared holes.

7.    Stud weld: Welding a metal stud.

8.    Surface weld: Weld beads deposited on the base metal or broken surface.

 

WELD DESIGN- FILLET WELD

 

1.     Base metal

Metal to be welded.

2.     Bond line

The junction of the weld metal and base metal.

3.     Depth of fusion

The distance that fusion extends into the base metal.

4.     Face to weld

The expose the surface of the weld on the side from which the welding was done.

5.     Leg of weld

The distance between root of the joint to toe of the weld.

6.     Root of the weld

The points as shown in cross section at which the bottom of the weld intersects the base metal surface.

7.     Throat of the weld

The shortest distance from the root of the fillet weld to its face.

8.     Toe of the weld

Junction between the face of weld and base metal.

WELD DESIGN- GROOVE WELD

 

1.     Bevel angle

The angle formed between prepared edge of member and plane perpendicular to the surface of member.

2.     Groove angle

The total included angle of the groove between parts to be joined by groove weld.

3.     Groove face

The surface of the member included in a groove.

4.     Root face

That portion of the root face adjacent to the root of the joint.

5.     Root opening

The separation between the members to be joined at the root of the joint.

TYPES OF GROOVE WELD

There are seven basic Groove weld: Square, V, Bevel, U, J, Flare V, Flare bevel.

 

WELDABILITY

·         The capability of the material to be welded under the imposed fabrication conditions into specific, suitable design structure and to perform satisfactorily in the intended service.

·         Weld ability depends on various factors such as, nature of metals, weld designs, welding techniques, skills, etc.

·         It has been stated that all metals are weldable but some are more difficult than another.

·         Steel is readily weldable (in many ways) than aluminium and copper.

·         Copper is not easily welded due to its high thermal conductivity which makes it difficult to raise the parent metal to its melting point. _require preheating ~300-400°C.

·         Some aluminium based die casting alloys give weld pool too large to control, and aluminium welds normally have oxide inclusions and porosity.

WELDABILITY: STEEL

·         Weldability of a steel is inversely proportional to the hardenability due to martensite formation during heat treatment.

·         There is a trade of between material strength and Weldability.

 

·         Austenitic stainless steels tend to be the most weldable but suffer from distortion due to high thermal expansion. _Cracking and reduced corrosion resistance.

·         Ferritic and martensitic stainless steels are not easily welded, often to be preheated and use special electrodes.

·         Ferritic steels is susceptible to hot cracking if the ferrite amount is not controlled.

WELDABILITY: ALUMINIUM AND ITS ALLOY

·         Weldability of aluminium depends on chemical composition of the alloy.

·         Aluminium alloys are susceptible to hot cracking, oxide inclusions, dross, porosity (hydrogen).

·         Most of wrought series, 1xxx, 3xxx, 5xxx, 6xxx, and medium strength 7xxx can be fusion welded by TIG, MIG while 2xxx and high strength 7xxx are not readily welded due to liquation and solidification cracking.

WELDABILITY: COPPER AND ITS ALLOY

Copper

·         High thermal conductivity so required preheating to counteract heat sink effect.

·         Can be TIG or MIG welded.

Brasses

·         Volatization of zinc (toxic) is the main problem, reducing Weldability.

·         Low zinc content brass can be TIG or MIG welded.

Bronzes

·         Most are weldable, except gun metal or phosphor bronze.

·         Required careful cleaning and proper deoxidization to avoid porosity.

Silicon improves Weldability due to its deoxidizing and fluxing actions.

Oxygen causes porosity and reduce strength of welds.

Tin increases hot-cracked susceptibility during welding.

Precipitation hardened alloys should be welded in the annealed condition, and then precipitation hardening treatment.

WELDABILITY: TITANIUM ALLOY

·         Weldability of titanium alloy is depends on the chemical composition of the alloy.

·         Titanium alloy with low alloying element can readily welded.

·         Highly stabilised beta titanium alloys are difficult to weld due to segregation.

·         Welding at above 500-550°C requires special precaution.

·         Fluxes are not normally used since they combine with titanium to cause brittleness.

·         Welding processes: TIG, MIG, PAW, LBW, EBW, FW, RW.

·         Shielding gases: Ar, He or the mixture of the two (avoid contact with oxygen).

·         Filler metal grades should match the alloys being welded, normally with lower yield strength to retain ductility. (Used unalloyed with lower ββββ content to avoid martensite transformation and with minimised O, N, H contents)

·         Thoriated tungsten electrodes (EWTh-1 or EWTh-2) are used for TIG welding.

WELDABILITY: MAGNESIUM ALLOY

·         Welding processes: Arc welding, RW as well as oxyacetylene welding, brazing. 

·         TIG and MIG are recommended.

·         Strength of the weld joint is lowered in the base metal, in the work hardened condition, due to recrystallization and grain growth in the HAZ.

·         Similar to welding of aluminium, magnesium has low melting point, high thermal conductivity, thermal expansion, oxide surface coating.

·         In Mg-Al-Zn alloys (AZxx), Al >10% improves Weldability by refining grain structure, while Zn > 1% increases hot shortness.

·         Filler metals are selected by the composition of the base metals.