practice 6: electrical arc welding · 2021. 1. 4. · monterrey campus laboratorio de procesos de...

Monterrey Campus

Laboratorio de Procesos de Fabricación 1/19

Engineering School

Mechanical Engineering Department

Fabrication Processes Laboratory

Practice 6: Electrical Arc Welding

THEORICAL FRAMEWORK

Welding is one of the most common joining processes. Used to unite two or more materials using different mechanisms, like piece heating and pressure.

There are more than 30 welding processes, classified according to their working energy. Filtered by this parameter they can be grouped in

a) Electrical Arc

b) Gas Combustion

c) Electrical Resistance

However, other classifications could be obtained by process nature, like

a) Material Base Fusion Welding

b) Filler Fusion Welding

c) Solid State Welding

Even though these types of processes are relevant in the industry, one of the most commonly used

is Electrical Arc Welding, for this reason this document will be centered in this process.

Electrical Arc Welding

This welding process, the required temperature to joint metallic parts is obtained by applying an electric arch that can achieve 3600ºC on the contact spot. This heat source allows the base material fusion and the filler to flow and form a solid mass.

The welding machine transforms electrical energy from the connected system to the right values (according the desired electrical arc process) and provides an specific current through a cable that ends in a torch. The circuit closes through a contact connector over the welded piece (called metal base). Schematic view is shown in figure 1.

1 Fuente: Kalpakjian, S. & Schmid, S.R.; Manufacturing Processes for Engineering Materials, 5

th edition,

Prentice Hall, 2007.

Monterrey Campus


Figure 1: Electrical Arch Welding components

There are consumable and non-consumable electrodes, with or without covering. In the case of permanent electrodes (non-consumable) the heat supplied by the electrical arc is used to cast one piece of the base material in the union to produce a continuous solid. In the case of the consumable electrode, it generates the base material fusion and also fills the joined materials. The process generates liquid metal at high temperatures, is required oxygen isolation from the atmosphere to avoid oxidation. This isolation is create a protecting atmosphere that can be obtained from external gas flow or a gas cloud that comes from the ceramic electrode coating. In the last case, has an advantage based in scrap generation from the material cast, that protect the piece in the cooling process. Factors that determine the way in which materials are transferred are the welding current, direct (direct or inverted polarity) or alternating, the electrode diameter, if it is consumable or not, arc distance (voltage), power source and the gas used in the process.

Process

SMAW: Shielded Metal Arc Welding

With this process metal fusion is obtained from the heat of the electrical arc between the coated electrode tip and the base metal surface to join. The electrode metallic nucleus conduces electricity and due to the generated heat, the metal is being filled in the union. The electrode coating, which contains various chemical ceramic and metallic compounds, function as when temperature rises.

a) Produces a shielding gas to prevent oxidation of the liquid metal.Deja una capa protectora (escoria) en la superficie soldada para proteger contra la oxidación y enfriamientos rápidos.

b) Leaves a protective layer (scrap) in the weld to protect against oxidation and rapid cooling surface.

c) Stabilizes the electrical arc using ionizable agents like potassium and lithium carbonate, thereby helping to drive the arc current.

Welding machine

Clamp

Electrode

Welding arc

Workpiece

Electrode cable

Work cable

Monterrey Campus


d) Adds alloying elements to the weld.

The process can be operated using 3 types of electrical current:

• Alternating Current

• Direct Current:

o Direct Polarity

o Inverted Polarity

In the case of alternating current has a balance in each cycle. However, in the direct current -

direct polarity, most energy is consumed to melt the electrode and the base material penetration is minimal. Using reversed polarity, the heat is greatest in the base metal and that maximum penetration is obtained. This arrangement with different polarities, we can see better in Figure 2.

Figure 3: Polarity Effects in SMAW Welding

This process advantages are that the used equipment is simple, versatile, portable and cheap. But, the deposition ratio is limited by the fact that the coating remains as brittle scrap and corrosive over the union which implies cleaning after the process. Also, the production rate is reduced, since the electrode must be changed when consumed.

GMAW: Gas Metal Arc Welding

Also known as MIG (Metal Inert Gas), this process consist in achieving base metal fusion and filling with the obtained heat with the electric arc that is maintained with the consumable electrode tip and the working piece (Figure 3). The zone to weld is protected by an inert gas, argon or helium, or by an active gas (process known as MAG). Commonly, this process uses voltage control.

Monterrey Campus


Figura : GMAW Process

In this welding type, arches with different characteristics have different appliances2:

Arc Type Image Permeation

Shorting arc Commonly used in thin materials and non-horizontal positions.

Sprayed Arc Commonly used in thick materials and horizontal position

Pulsed Arc

Reasonable permeation, fatigue resistant; Industrial use preferred.

2 http://www.weldreality.com/pulsed_welding_fundamentals.htm

See also FERRARESI, V. A.; FIGUEIREDO, K. M. and ONG, T. Hiap. Metal transfer in the aluminum gas metal arc welding. J. Braz. Soc. Mech. Sci. & Eng. [online]. 2003, vol.25, n.3, pp. 229-234. ISSN 1678-5878.

Control de Alimentación

Metal Base

Microalambre

Gas Protector

Maneral Voltaje

Alimentador

Retroalimentación Soldadora

http://www.weldreality.com/pulsed_welding_fundamentals.htm

Monterrey Campus


GTAW: Gas Tungsten Arc Welding

Formely known as TIG (Tungsten Inert Gas), achieves metals fusion with heat obtained with the electric arc that is maintained with the tip of the Tungsten electrode (non-consumable) and the working piece. The to be welded zone is protected with an inherent gas like Argon or Helium. (Figure 4)

This process can be connect with direct current and direct or inverted polarity, different to SMAW when using direct polarity, by not consuming the electrode, electrons are emitted which gain speed when travelling in the arc and attacking the zone causing a greater penetration. But in the other hand, when connecting the circuit with inverted polarity, the attacking effect will be over the Tungsten electrode instead of the working piece. For this reason direct polarity is used in this welding type.

Figure 4: GTAW Welding

SOME USEFUL QUANTITATIVE RELATIONSHIPS FOR GMAW

WELDING3

Voltage and Filler Metal Fusion

What causes the melting filler?

While the wire passes from the contact end of the torch to the start of arc is taking all the welding current and reaches high temperatures. Starts at room temperature and can exceed 800⁰C before forming the arc at the end depending on the length of the electrode, which is the distance between the torch and touch the tip of the wire (Electrode Extension sickout or "ESO").

3 Adapted from: Welding Math (and Physics) For Welders, Welding Students, Welding Instructors and Others Involved in Managing Welding Operations, Welding Accessories Technology, www.netwelding.com/Welding_Math.htm y Welding Design & Fabrication, http://weldingdesign.com/welding-qa/precalculating-wire-feed-speed-travel-voltage-7806/

Dirección del Proceso

Cable del Electrodo

Electrodo de Tungsteno Pasaje del

Gas Protector

Varilla de Aporte

Cordón de Soldadura

Metal Base y de Aporte Fundidos

http://www.netwelding.com/Welding_Math.htm

http://weldingdesign.com/welding-qa/precalculating-wire-feed-speed-travel-voltage-7806/

Monterrey Campus


Also, the current that leaves the wire or the one that is united to the surface, required a determined quantity of energy in order that electrons unite or leave that surface. This energy, generated on the surface, melts the wire. So, the electrode is positive, the anode potential is defined (called work function) and determined like working voltage. The consumed power is equal to the anode voltage multiplied by the current. So, material fusion rate can be defined as4:

Fusion rate lb = a * currentA+ b * ESOin* current2 A

2

where a and b are constants characteristic of the filler. For a carbon steel wire of 0.035" constants take the following values:

a = 0.0086 b = 0.000078

The first term of the equation represents the energy associated with the creation and conservation of the arc and the second that dissipated by the resistive heating.

A major implication of this relationship is that higher ESO (at a rate fixed wire feed) decrease in amperage. This has a significant effect on other parameters, like in weld penetration.

The interesting point is that the voltage that you measure in a GMAW welder is a combination of the following factors

a) Small Voltage decrease due to the resistive wire heating (I2R).

b) The cathode and anode power (required to take electrons from the wire to the liquid metal; ½ of the total measured)

c) Electric arc voltage falls.

The melting rate is directly proportional to the velocity with which wire is fed to the arc and its

diameter; and is not related to the voltage not with the forward speed of the torch. Whereas steel wires, the deposition rate can be calculated with the following expression:

fusionratelb = 13.1D2 *WireSpeed * Ef

hr

where D is the wire diameter [in], WireSpeed is the feed rate [in/min] and Ef is the packing factor [0.85] with internal flux and 1 to solid wire.

For example, if 0.045’’ solid wire is feed at 300 in/min fusion rate will be 7.96 lb/hr.

Torch Advance Speed

Once the fusion rate is known it is possible to calculate the torch advance speed. For example, a 3/8’’ Triangular plaque with 10% of penetration (0.413’’ by side) welded with solid wire of 0.045’’ of diameter with a feed rate of 300 in/min, the weight per foot can be calculated if the volume and the density are considered from the filler material. The following formulation calculates the example

Monterrey Campus


Material Vol in3

=

1 b * h *12in = 0.4125 * 0.4125 *12 = 1.02 in

3

ft 2 ft ft

Material Weight lb ft

= *Material Vol = 0.283*1.02 = 0.2887 lb ft

Torch Speedin = Fusion rate*# passedover

= 7.96 *1

= 5.52 in

min (60 12)* Materialweight 5 * 0.2887 min

To calculate the wire feed speed (WFS) it is required to consider the wire weight considering the geometry and density. Table 1 provides the information on solid steel weight per longitude foot in the next equation

WFS in Fusionrate 7.96 *1 295 in

min =

(60 12)* wireweight =

5* 0.0054 =

4 C. E. Jackson, The Science of Arc Welding, Welding Journal 39 (4), 1960, pp 129-s thru 230-s.

Table 1: Wire Weight Solid Steel

Dia. (in)

weight (lb/ft)

Dia. (in)

weight (lb/ft)

0.035 0.0033 3/32 0.023

0.040 0.0043 1/8 0.042

0.045 0.0054 5/32 0.065

0.052 0.0072 3/16 0.094

1/16 0.01 7/32 0.128

5/64 0.016

These calculations are simplified by using a factor conversión5 already considers the geometry of the cord and a given penetration (measured as a percentage of the size). Then equation reduces to:

Torchspeed in

min = Taza de Fusión lb * B

hr where "B" is precisely the Bartonian conversion factor for different types and sizes of cord. Tables 2, 3, 4, and 5 contain the factor "B" for strings to bone with 20% penetration, fillet seams with 10% penetration, bone beads with "v" with 10% penetration, and to drawstring top.

5 Bartonian Conversion Factor: retrived from Table 12-1 of “The Procedure Handbook of Arc Welding by the Lincoln Electric Co.” que muestra el peso de metal de aporte por pie de cordón para tipos comunes de cordón realizados con alambre de acero sólido.

Monterrey Campus


Table 2: Conversion Factor “B” for “Cordones a Hueso” with 20% Penetration

Thickness in the plate (in)

opening (in)

weight of the Metal (lb/ft)

1/16 1/8 3/16 1/4 3/8 1/2

1/8 0.027 0.053 0.080 0.106 0.159 0.213

3/16 0.040 0.080 0.120 0.156 0.239 0.319

1/4 0.053 0.106 0.159 0.213 0.319 0.425

5/16 0.066 0.133 0.199 0.266 0.398 0.531

3/8 0.080 0.159 0.239 0.319 0.478 0.638

7/16 0.093 0.186 0.279 0.372 0.558 0.744

1/2 0.106 0.213 0.319 0.425 0.638 0.850

3/4 0.159 0.319 0.478 0.638 0.969 1.28

1 0.213 0.425 0.638 0.850 1.280 1.700

1 1/2 0.319 0.638 0.956 1.280 1.910 2.550

2 0.425 0.850 1.280 1.700 2.550 3.400

Table 3: Conversion Factor “B” for “Cordones a Fillete” with 10% Penetration

Thickness of the plate (in)

Weight of the Metal (lb/ft)

Cordón Plano

Cordón Convexo

Cordón Cóncavo

1/8 0.032 0.041 0.036

3/16 0.072 0.093 0.081

1/4 0.129 0.165 0.145

5/16 0.201 0.258 0.226

3/8 0.289 0.371 0.325

7/16 0.394 0.505 0.443

1/2 0.514 0.6595 0.578

3/4 1.160 1.480 1.300

1 2.060 2.640 2.310

Monterrey Campus


Table 4: Conversion Factor “B” for “Cordones a Hueso en V” with 10% Penetration

Thickness of the plate

(in)

Angle of the “V”


14°

20°

30°

45°

60°

70°

75°

80°

90°

1/8 0.007 0.009 0.014 0.022 0.031 0.037 0.041 0.045 0.053

3/16 0.015 0.021 0.032 0.049 0.069 0.084 0.092 0.100 0.119

1/4 0.026 0.037 0.057 0.088 0.123 0.149 0.163 0.178 0.212

5/16 0.041 0.058 0.089 0.137 0.191 0.232 0.254 0.278 0.332

3/8 0.059 0.084 0.128 0.198 0.276 0.334 0.366 0.401 0.478

7/16 0.080 0.115 0.174 0.269 0.375 0.455 0.499 0.545 0.650

1/2 0.104 0.150 0.227 0.352 0.490 0.594 0.651 0.712 0.849

3/4 0.235 0.337 0.512 0.791 1.103 1.338 1.466 1.603 1.910

1 0.417 0.599 0.910 1.407 1.961 2.378 2.606 2.850 3.396

1 1/2 0.938 1.347 2.047 3.165 4.412 5.350 5.863 6.412 7.641

2 1.668 2.395 3.640 5.627 7.843 9.512 10.423 11.398 13.584

Table 5: Conversion Factor “B” for “Remate al Cordón” with 10% Penetration

Thickness of the Remate (in) )

High of the Remate (in)


1/16 1/8 3/16 1/4

3/8 0.027 0.053 0.080 0.106

1/2 0.040 0.080 0.120 0.1559

3/4 0.053 0.106 0.159 0.213

1 0.066 0.133 0.199 0.266

1 1/4 0.080 0.159 0.239 0.319

1 1/2 0.093 0.186 0.279 0.372

1 3/4 0.106 0.213 0.319 0.425

2 0.159 0.319 0.478 0.638

Monterrey Campus


Penetration

The weld penetration can be determined by a simple equation defined years6 ago. The

penetration distance is melted into the base material to make a weld plate measured in inches

and can be defined by the following simplified equation: 1

Pin = K

Corriente A4 3

Vel in min

Voltaje V 2

For steel wire solid carbon 0.035 "in diameter, the constant K = 0.0019. Using these equations we find that changing the ESO solid wire for wire 0.035 ", and maintaining an arc voltage of 24 V, welding speed of 5 in / min and a current producing 2.9 lb / hr variation fusion in penetration is as shown in Table 6.

Table 6: R e l a t i o n b e t w e e n t h e l o n g o f t h e e l e c t r o d e a n d p e n e t r a t i o n f o r G M A W w e l d i n g , w i t h w i r e o f 0 . 0 3 5 ” , p r o d u c i n g 2 . 9 lb/hr of melted metal @ 24 V y 5 in/min.

ESO (in) Corriente

(A) Penetración

(in) Pérdida de

Penetración

3/8 200 0.156 Caso Base

1/2 184 0.140 11%

5/8 172 0.128 18%

3/4 161 0.117 25%

7/8 152 0.108 31%

Note that with a feed rate of the wire and fixed ESO 3/8 "taper down from 200 A to 152 A the ESO was increased to 7/8". The resistive heating of the wire is a very efficient process. Therefore, the heat needed to melt the wire end when entering the arc is less because the wire is warmer with an ESO longer. Furthermore, the reduction in weld penetration is 25% when changing the ESO 3/8 "to 3/4". Therefore it is very important to maintain a constant ESO torch. For a constant wire feed speed, the shorter it is the better the penetration ESO because the current will be higher. When welding in short circuit mode is often desirable to use a long ESO even protrude from the gas shield cup. This helps aid visibility and welder can stay on the leading edge of the liquid metal (weld puddle).

6 C.E. Jackson, AWS Welding Handbook, Volume 1, 9

th Edition; pp 79.

Monterrey Campus

Fabrication Processes Laboratory 11/19

Shielding Gas Flow

The general rule (empiric) for the shielding gas flow in a SMAW Welder must be controlled between 30 and 40 ft3 / hr. This flow is achieved commonly with a 20-psi work pressure. But, depending in the welding type it can vary +/- 5 psi. For example, if you are working outdoors, pressure should rise to 25 or 30 psi due to the air currents that dissipate the protecting gas. On the other hand is you are working under controlled conditions, pressure could decrease to 15 or 12 psi.

Table 7 shows a guide to selecting the protecting gas base in the process time used and the material to unite.

Table 7: Protecting Gas Selection for GMAW7.

Metal Thickness in.

(mm) Recommended Shielding Gas

Advantages

MIG SHORT ARC

Carbon Steel Up to 14 gauge (0.1)

92% Argon / 8% CO2 Good burn through and distortion control Used also for spray arc welding.

14 gauge - 1/8 (3.2)

75% Argon / 25% CO2 88% Argon / 12% CO2

High welding speeds without burn through. Minimum distortion and spatter. Best puddle control for out of position welding. Provides best mechanical properties for any given wire.

Over 1/8 (3.2)

75% Argon / 25% CO2 88% Argon / 12% CO2

High welding speeds without burn through. Minimum distortion and spatter. Best puddle control for out of position welding. Provides best mechanical properties for any given wire.

50% Argon / 50% CO2 Deep penetration; spatter.

CO2 Deep penetration; faster welding speeds; high spatter.

Stainless Steel

Up to 14 gauge (0.1)

92% Argon / 8% CO2 Good burn through and distortion control. For use where corrosion resistance is not mandatory.

Over 14 gauge (0.1)

92% Argon / 8% CO2 Good burn through and distortion control. For use where corrosion resistance is not mandatory.

90% He 7.5% Ar 2.5% CO2

No effect of corrosion resistance. Small heat- affected zone. No undercutting, minimum distortion. Good bead shape and mechanical properties.

High Yield Strength Steels

Up to 14 gauge (0.1)

92% Argon / 8% CO2 Good burn through and distortion control. Used also for spray arc welding.

Over 14 gauge (0.1)

Argon - Hydrogen Excellent arc stability, welding characteristics bead contour, little spatter, high impacts.

7 Source: Weld Direct; http://www.weld-direct.com/gas.htm

http://www.weld-direct.com/gas.htm

Monterrey Campus


Table 7: Protecting Gas Selection for GMAW7.

Metal Thickness in.

(mm) Recommended Shielding Gas

Advantages

MIG SPRAY ARC

Carbon Steel All thicknesses 95% Argon / 5% O2 Improves droplet rate and arc stability.

92% Argon / 8% CO2

Produces a more fluid and controllable weld puddle; good coalescence and bead contour. Minimizes undercutting; permits high speeds.

Aluminum Up to 3/8 (12.7) Argon Best metal transfer, arc stability and plate cleaning. Little or no spatter.

Over 3/8 (12.7) Argon - Helium

Higher heat input. Produces more fluid puddle and flatter bead. Minimizes porosity.

Helium Highest heat input. Good for mechanized welding.

Low Alloy Steel

Up to 3/32 (2.4) 98% Argon / 2% O2 Reduces undercutting. Improves coalescence and bead contour. Good mechanical properties.

Over 3/32 (2.4) 92% Argon / 8% CO2 Excellent arc and weld characteristics.

Stainless Steel

All thicknesses

99% Argon / 1% O2

Good arc stability. Produces a fluid and controllable weld puddle; good coalescence and bead contour. Minimizes undercutting.

98% Argon / 2% O2

Can be used on more sluggish alloys to improve puddle fluidity, coalescence and bead contour.

Copper, Nickel & Copper- Nickel alloys

Up to 1/8 (3.2)

Argon

Good arc stability.

Over 1/8 (3.2) Argon - Helium

Higher heat input of helium mixture offsets high heat conductivity of heavier gauges.

Helium Higher heat input and improved penetration.

Magnesium Titanium

-- Argon Excellent cleaning action. Provides more stable arc than helium-rich mixtures

MIG CORED WIRE

Carbon Steel All thicknesses CO2 Deep penetration.

75% Argon / 25% CO2

Low smoke and spatter. Good puddle control. Bridges gaps

Stainless Steel

All thicknesses CO2 Deep penetration.

75% Argon / 25% CO2

Low smoke and spatter. Good puddle control. Bridges gaps

Monterrey Campus


OBJETIVES

1) The student will understand the security standards applicable to this practices

2) The student will know the operation principles of an electric arc welding system feed by micro wire.

3) The student will understand the function and correct usage of the safety equipment required.

4) The student will understand the basic operation between process parameters and machine controls (current, voltage, advance speed of filler material, welding speed)

5) The student will weld varying the process parameters. 6) The student will check the welded microstructures adequateness and defect rate and

associate them to the process parameters.

SECURITY

To use the welding station the following cautions are needed:

¡ ATENTION ! REASON

If the user wears contact lenses must

use glasses during the practice

Contact lenses during this practice is

prohibited. Contact lenses can adhere to the eye surface causing severe damage.

Make sure that electrode cables and connections are properly installed and isolated.

Cable and machine condition and/or gases are base for a secure process.

Disconnect the net current before cleaning and adjusting the welding machine.

It is important to deenergize the equipment to avoid closing circuits that could cause and accident.

Never change polarity or other adjustments when the machine is operating

Machine might damage from sudden changes

Keep the area dry and clean

Imperative to avoid accidents or contact with energized pieces or hot. Also, water can cause shorts and accidents.

Monterrey Campus


MATERIAL, TOOLING AND EQUIPMENT

1) Material

a) Process Electrodes (MIG, SMAW)

b) Steel Plaques to weld

2) Tooling and Safety Equipment.

a) Electrode Cables

b) Protecting Mask

c) Gloves

d) Shirtfront

3) Equipment

a) Welding Station

b) Welding machine

PROCEDURE

1) Print and read practices before attending the lab. Prepare Pre report.

2) Be 5 minutes before the starting hour with comfortable shoes and clothes, Without contact lenses with the pre report and printed practice (pg. 15 to 19)

3) Get equipment in the warehouse (material, tooling and accessories needed for the practice) 4) A quiz will be given at the start of the lab, which will evaluate the understanding of the

theoretical framework and will also use for attendance. 5) The instructor will explain the proper way to weld using the electric arc welder. 6) Every student will be allowed to weld steel plaques modifying the process parameters. 7) Each piece will be revised after welded.

Monterrey Campus


Engineering School Mechanical Engineering

Department Fabrication Processes

Laboratory

PRACTICE PRE REPORT

Receipt Acknowledgment

With my signature below, I accept standards and mandatory safety procedures for Practice 7: Arc Welding, Laboratory of Manufacturing Processes.

Name:

ID: Curse key: Group:

Signature: Date:

1) Investigate and explain at least 5 welding joints.

Practice 6


Student

ID

Group

Instructor

Monterrey Campus


2) Explain the nomenclature used to identify electrodes in SMAW

Bibliography

Bibliography

Monterrey Campus


3) Explain factors that influence transference type that will be obtained in GMAW welding type.

Bibliography:

Monterrey Campus


Engineering School Mechanical Engineering

Department Fabrication Processes

Laboratory

PRACTICE REPORT

1) Describe technical factors that must be taken to guarantee a good welding quality.

Practice 6


Student

ID

Group

Instructor

Monterrey Campus


2) Identify the welding type used. Describe the welding characteristics and explain why it generated that way. Illustrate the welded piece and the characteristics observed. (Direction, width, longitude, appearance, etc.)

DELIVER IN PRE REPORT AND PRACTICE REPORT

Make sure that the pre report and practice report has all data in the identification box and that all requested elements are answered and handle it to the instructor. After, return materials, tooling and accessories in the warehouse and make sure to leave the working area clean.

practice 6: electrical arc welding · 2021. 1. 4. · monterrey campus laboratorio de procesos de...

Documents