INTRODUCTION
OF WELDING PROCESSES
DEFINITION OF WELDING:
A
weld is made when separate pieces of material to be joined combine and form one
piece when heated to a temperature high enough to cause softening or melting.
Filler material is typically added to strengthen the joint.
Ø Welding
is a dependable, efficient and economic method for permanently joining similar
metals. In other words, you can weld steel to steel or aluminum to aluminum,
but you cannot weld steel to aluminum using traditional welding processes.
Ø Welding
is used extensively in all sectors or manufacturing, from earth moving
equipment to the aerospace industry.
WELDING
PROCESSES:
The number of different welding processes
has grown in recent years. These processes differ greatly in the manner in
which heat and pressure (when used) are applied, and in the type of equipment
used. There are currently over 50 different types of welding processes; we’ll
focus on 3 examples of electric arc welding, which is the most common form of
welding.
Ø The most popular processes are shielded metal
arc welding (SMAW), gas metal arc welding (GMAW) and gas tungsten arc welding
(GTAW).
Ø All
of these methods employ an electric power supply to create an arc which melts
the base metal(s) to form a molten pool. The filler wire is then either added
automatically (GMAW) or manually (SMAW & GTAW) and the molten pool is
allowed to cool.
Ø Finally,
all of these methods use some type of flux or gas to create an inert
environment in which the molten pool can solidify without oxidizing.
SHIELDED METAL ARC WELDING (SMAW) :
SMAW
is a welding process that uses a flux covered metal electrode to carry an
electrical current. The current forms an arc that jumps a gap from the end of
the electrode to the work. The electric arc creates enough heat to melt both
the electrode and the base material(s). Molten metal from the electrode travels
across the arc to the molten pool of base metal where they mix together. As the
arc moves away, the mixture of molten metal solidifies and becomes one piece.
The molten pool of metal is surrounded and protected by a fume cloud and a
covering of slag produced as the coating of the electrode burns or vaporizes.
Due to the appearance of the electrodes, SMAW is commonly known as ‘stick’
welding.
Figure:
shielded metal arc welding equipment
SMAW
is one of the oldest and most popular methods of joining metal. Moderate
quality welds can be made at low speed with good uniformity. SMAW is used
primarily because of its low cost, flexibility, portability and versatility.
Both the equipment and electrodes are low in cost and very simple. SMAW is very
flexible in terms of the material thicknesses that can be welded (materials
from 1/16” thick to several inches thick can be welded with the same machine
and different settings). It is a very portable process because all that’s
required is a portable power supply (i.e. generator). Finally, it’s quite
versatile because it can weld many different types of metals, including cast
iron, steel, nickel & aluminum.
Some of the biggest
drawbacks to SMAW are
(1)That it produces a lot of smoke
& sparks.
(2)There is a lot of post-weld
cleanup needed if the welded areas are to look presentable. (3)It is a fairly
slow welding process and
(4)It requires a lot of operator skill to
produce consistent quality welds.
GAS METAL ARC WELDING (GMAW) :
In
the GMAW process, an arc is established between a continuous wire electrode
(which is always being consumed) and the base metal. Under the correct
conditions, the wire is fed at a constant rate to the arc, matching the rate at
which the arc melts it. The filler metal is the thin wire that’s fed
automatically into the pool where it melts. Since molten metal is sensitive to
oxygen in the air, good shielding with oxygen-free gases is required. This
shielding gas provides a stable, inert environment to protect the weld pool as
it solidifies. Consequently, GMAW is commonly known as MIG (metal inert gas)
welding. Since fluxes are not used (like SMAW), the welds produced are sound,
free of contaminants, and as corrosion-resistant as the parent metal. The
filler material is usually the same composition (or alloy) as the base metal.
Ø Availability
of equipment
Figure:
Gas metal arc welding equipment
GMAW is extremely fast and economical. This process
is easily used for welding on thin-gauge metal as well as on heavy plate. It is
most commonly performed on steel (and its alloys), aluminum and magnesium, but
can be used with other metals as well. It also requires a lower level of operator
skill than the other two methods of electric arc welding discussed in these
notes. The high welding rate and reduced post-weld cleanup are making GMAW the fastest
growing welding process
Figure
: Gas shielded metal arc welding (GMAW)
GAS TUNGSTEN ARC WELDING (GTAW) :
In the GTAW process, an arc is
established between a tungsten electrode and the base metal(s). Under the
correct conditions, the electrode does not melt, although the work does at the
point where the arc contacts and produces a weld pool. The filler metal is thin
wire that’s fed manually into the pool where it melts. Since tungsten is
sensitive to oxygen in the air, good shielding with oxygen-free gas is
required. The same inert gas provides a stable, inert environment to protect
the weld pool as it solidifies. Consequently, GTAW is commonly known as
TIG (tungsten inert gas) welding. Because fluxes are not used (like
SMAW), the welds produced are sound, free of contaminants and slags, and as
corrosion-resistant as the parent metal. Tungsten’s extremely high melting
temperature and good electrical conductivity make it the best choice for a
non-consumable electrode. The arc temperature is typically around 11,000° F.
Typical shielding gasses are Ar, He, N, or a mixture of the two. As with GMAW,
the filler material usually is the same composition as the base metal.
Figure : Gas tungsten arc welding equipment
GTAW is easily performed on a variety of materials,
from steel and its alloys to aluminum, magnesium, copper, brass, nickel,
titanium, etc. Virtually any metal that is conductive lends itself to being
welded using GTAW. Its clean, high-quality welds often require little or no
post-weld finishing. This method produces the finest, strongest welds out
of all the welding processes. However, it’s also one of the slower
methods of arc welding.
SELECTION OF THE WELDING PROCESS:
The selection of the joining process
for a particular job depends upon many factors. There is no one specific rule
governing the type of welding process to be selected for a certain job. A few
of the factors that must be considered when choosing a welding process are:
Ø Repetitiveness
of the operation
Ø Quality
requirements (base metal penetration, consistency, etc.)
Ø Location
of work
Ø Materials
to be joined
Ø Appearance
of the finished product
Ø Size
of the parts to be joined
Ø Time
available for work
Ø Skill
experience of workers
Ø Cost
of materials
Ø Code
or specification requirements
GENERAL GUIDELINES FOR SELECTING ONE
PROCESS OVER ANOTHER :
When selecting one process over the others, it is
often useful to examine the Principles Processes of each type of welding
covered in this table :
EXAMPLES OF WELDS :
WELDING DEFECTS :
Lack of
fusion is defined as
unfused area between weld metal and base material or
previously welded layer. This
happens when the base metal or
the previous layer are
not completely or
insufficiently molten.
Table
: welding defects
The
influence of welding parameters
on the development of lack of fusion. In the upper
part, arc characteristic lines of MAG welding
are shown using
CO2 and mixed
gas.. With present tension, the
welding current is
fixed by the
wire feed speed (thus
also melting rate)
Figure
: Defect Class: Lack of Fusion, Insufficient
Through-Weld
No comments:
Post a Comment