Heat Input Effects In Welding

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Heat Input EffectsThermal Cycles

Fluid Flow in Weld Pools


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Heat Input Parameters

• Welding Current

• Arc Voltage

• Travel Speed


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Current• The current is the amount of electric charge that

flows through the welding cable in one second

• Current is measured in amperes (I)

• An increase in welding current increases heat input to the weld

• An increase in welding current also increases productivity by increasing:– melt-off rate of electrode– amount of deposited weld metal


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Voltage

• Voltage (V) is the force that causes the current to flow

• Higher welding voltage results in greater base metal penetration

• Lower welding voltage results in more shallow base metal penetration

• An increase in welding voltage increases heat input to the weld


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Travel Speed

• Travel speed (S) is rate with which a welding electrode traverses along a weld joint while welding

• Slow travel speeds make wide weld beads with deep base metal penetration

• Fast travel speeds make narrow weld beads and shallow base metal penetration

• A decrease in travel speed increases heat input to the weld


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Effects of Heat Input• The thermal cycle of a weld is the heat input-

time-temperature relationship

• Total heat input to a weld must be balanced to produce desired weld properties

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Thermal Efficiency:

GTAW: 60-65% SMAW: 75-80% GMAW: 80-85% FCAW: 85-90% SAW: 95-100%


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Effects of Heat Input• Excessive heat input can lead to

these problems in the weld:– Poor mechanical properties – distortion– cracks

Miller 1998


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Weaving

Weave: This is the transverse oscillation of an electrode or blowpipe nozzle during the deposition of weld metal and this technique is generally used in vertical-up welds.

Stringer bead:A run of weld metal made with little or no weaving motion.


Thermal Cycles

• 2-D & 3-D heat flow– Effect of heat input– Effect of preheat– Cooling rates


Isotherms in thick, thin and intermediate thickness plates. For steel, “thin” plates are about 4 mm in thickness; whereas, “thick” plates are generally larger than 100mm in thickness.

thick thin

intermediate

Thermal Cycles


Heat Flow Calculation in Welding Rosenthal Equation

Assumptions:

• Heat flow in a workspace of sufficient length is steady with respect to the moving heat source. In other word, the temperature distribution and the pool geometry do not change with time.

• No convection exists in the weld pool, no heat losses from the workpiece surface.

• The thermal properties are constant.


2-D Heat Flow Rosenthal equation:


Bassel Function


3-D Heat Flow

Rosenthal equation


Isotherms for steel


Thermal properties

Thermal conductivity (J/mm sec ˚C) k

Specific heat (J/g˚C) c

Density (g/mm3) r

Thermal diffusivity (mm2/sec) k

c


Typical thermal cycle in HAZ

Essentially the temperature goes up and then it goes down! How fast it goes up is not too important, How fast it goes down and what the peak temperature was are very important!


Effects of preheat and heat input

Preheat causes the curve to move upward. Heat input magnifies the value of each ordinate

T To Q f t, ,v, y, z


Typical thermal cycles in HAZ

Thermal histories at various distances from the axis of the weld


Effects of preheat and heat input


Mass quench effect

Cooling rates in thick (left) and thin (right) plates


Effect of thickness on thermal history

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Power Density of Heat Source


Power Density of Heat Source

• The power density of the heat source increases, the heat input to the workpiece that is required for welding decreases.

• Excessive heating can cause damage to the workpiece, including weakening and distortion.

• the advantages of increasing the power density of the heat source are deeper weld penetration, higher welding speeds, and better weld quality with less damage to the workpiece


Forces controlling droplets



• Buoyancy Force • Lorentz Force• Shear Stress Induced by Surface Tension

Gradient• Shear Stress Induced by Plasma Jet



1. Buoyancy Force: The density of the liquid metal (ρ) decreases with increasing temperature (T). Because the heat source is located above the center of the pool surface, the liquid metal is warmer at point a and cooler at point b. Point b is near the pool boundary, where the temperature is lowest at the melting point. Gravity causes the heavier liquid metal at point b to sink. Consequently, the liquid metal falls along the pool boundary and rises along the pool axis. The buoyancy force is negligible.


2. The driving force for fluid flow in the arc is the electromagnetic force or Lorentz force. The welding arc is an ionic gas, that is, a plasma, with an electric current passing through. This converging current field, together with the magnetic field it induces, causes a downward and inward Lorentz force. The liquid metal is pushed downward along the pool axis and rises along the pool boundary



3. Shear Stress Induced by Surface Tension Gradient: In the absence of a surface-active agent, the surface tension of the liquid metal decreases with increasing temperature. The warmer liquid metal with a lower surface tension at point a is pulled outward by the cooler liquid metal with a higher surface tension at point b causing the liquid metal to flow from the center of the pool surface to the edge and return below the pool surface. Surface-tension-driven convection is also called thermo-capillary convection or Marangoni convection.



Marangoni Flow

In the presence of a surface-active agent, on the other hand, the cooler liquid metal of lower surface tension at the edge of the pool surface is pulled inward by the warmer liquid metal of higher surface tension near the center of the pool surface. The flow pattern in Figure 4.16e favors convective heat transfer from the heat source to the pool bottom. In other words, the liquid metal carries heat from the heat source to the pool bottom more effectively, thus increasing the weld penetration.


Marangoni Flow



4. Shear Stress Induced by Plasma Jet: The plasma moving outward at high speeds along a pool surface can exert an outward shear stress at the pool surface. This shear stress causes the liquid metal to flow from the center of the pool surface to the pool edge and return below the pool surface.

Heat Input Effects In Welding

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