final welding of ti alloys and mg alloys

8/6/2019 Final Welding of Ti Alloys and Mg Alloys

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WELDING OF Ti & its alloys

and

WELDING OF Mg & its alloys

Submitted by:

Arjyajyoti Goswami

Shantilal Meena

Rachna Chawla

Submitted to:

Dr. Reeta Wattal

Department of MECHANICAL ENGINEERING


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WELDING OF

TITANIUM & its

ALLOYS


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CHARACTERISTICS OF TITANIUM & its

ALLOY

Silver colored material , which is 45% lighter than steel having

almost same mechanical properties as steel

Some alloys of Ti may have strength up to 925 to 1080 N/mm2

Ti alloys are difficult to machine having a pronounced tendency to

get welded on the tool tip

The higher melting point of Ti makes it relatively difficult to cast, sowelding is a important process in utilization of Ti and its alloys.

Due to its strong affinity towards O2 Ti forms a stable oxide layer

on its surface even at room temperature


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Ti alloys Classification

Classified on the basis of the microstructure, because mechanical

properties depends on the phase ratios in the microstructure.

1. Commercially pure Ti : having 98 99.5% Ti, strengthened by

small amounts of O2. C,N,Fe may be present

2. Alpha and near alpha alloys : single-phase alloys containingup to 7% aluminium and a small amount (< 0.3%) of oxygen,

nitrogen and carbon

3. Alpha-beta alloys : two-phase microstructure formed by the

addition of up to 6% Aluminium and varying amounts of betaforming constituents - V, Cr and Mo

4. Beta alloys : contains a high percentage of phase stabilising

elements, but are not truly single phase.


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WELDING OF Ti & ITS ALLOYS

COMMERCIALLY PURE TITANIUM :

Has moderate strength but good ductility. Main reason for using is

high corrosion resistance, formability and weldability.

Should use filler material with low iron content. They can be

readily fusion welded

All sources of iron contamination must be avoided.

ALPHA ALLOYS :

Has good strength, toughness and weldability. The alloys arefusion welded in the annealed condition.

Has higher strength at elevated temperatures than commercially

pure Ti, so residual stresses are high and suitable stress relieving

must be done


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NEAR ALPHA ALLOYS:

Excellent creep strength at elevated temperatures. Good

weldability but high residual stresses can be a problemStress relieving is highly recommended.

Iron contamination degrades creep strength so it should be

avoided

BETA ALLOYS:

Weldable either in annealed condition or heat treated condition

Weld joints have good ductility but relatively low strength

Few commonly used grades are Ti-13V-11Cr-3Al , Ti-8Mo-8V-3Al-

2Fe


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ALPHA-BETA ALLOYS:

These have a characteristic two-phase microstructure formed bythe addition of up to 6% Al varying amounts of beta forming

constituents - vanadium, chromium and molybdenum.

The alloys are readily welded in the annealed condition

Welding of such alloys may significantly change their strength,

ductility and toughness characteristics

Ti-6Al-4V has the best weldability in this group

Alpha-beta alloys having highly stabilized beta phase tends tocrack when welded under highly stressed conditions


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WELDABILITY RATINGRating is done on the basis of their ability to produce tough and

ductile welds in them

CLASSCLASS COMPOSITIONCOMPOSITION RATINGRATINGCOMMERCIALLY PURECOMMERCIALLY PURE

GRADEGRADE

Ti AA

ALPHA ALLOYSALPHA ALLOYS TiTi--5Al5Al--2.5Sn2.5Sn BB

TiTi--0.2Pd0.2Pd AA

NEAR ALPHA ALLOYSNEAR ALPHA ALLOYS TiTi--8Al8Al--11--MoMo--VV AA

TiTi--6Al6Al--4Zr4Zr--2Mo2Mo--2Sn2Sn BB

ALPHA BETA ALLOYSALPHA BETA ALLOYS TiTi--6Al6Al--4V4V--ELIELI AA

TiTi--6Al6Al--4V4V BB

TiTi--7Al7Al--4Mo4Mo CC

TiTi--8Mn8Mn DD

BETA ALLOYSBETA ALLOYS TiTi--13 V13 V-- 11Cr11Cr--3Al3Al BB


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WELDING PROCESSES EMPLOYED:

GTAW, GMAW, PAW, RESISTANCE WELDING, FRICTION

WELDING, EBW, LBW

STEPS INVOLVED IN ARCWELDING OF Ti

AND ITS ALLOYS:

1. JOINT DESIGN

2. PRE CLEANING

3. SELECTION OF PRE HEAT AND INTER PASS

TEMPERATURE

4. PROTECTION DURINGWELDING

5. WELDING PROCESS SELECTION

6. PWHT


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JOINT DESIGN:

Same as that of steels. Performs the usual functions.

A typical weld joint for Ti has 700 groove angle, 0.5 mm root face,up to 0.25 mm root opening for plate thickness of 3mm

PRE CLEANING:

Cleaning must be done with a suitable inorganic solvent to avoid

contamination.

Light oxide coating may be removed by aqueous solution of HF

acid or nitric acid.Scales formed at temperatures above 600C should be removed

by mechanical methods such as vapor blasting and grit blasting

To control porosity often the edges of the joints are given special

treatment like wire brushing


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PRE HEAT AND INTERPASS TEMPERATURE:

Should be less than 120C otherwise surface oxidation may take

place and the oxides will dissolve in the weld metal and cause

brittleness.

PROTECTION DURING JOINING:

Ti is very sensitive to embrittlement, so any part which is being

heated above 260C must be shielded from atmospheric

contamination, by the use of a high purity shielding gas


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WELDING PROCESS SELECTION: 3 most commonly used

processes.

GTAW-

Maybe done in open atmosphere or in a chamber. DCEN is often

used. Chamber welding yields better results.

GMAW-

Has higher deposition rates than GTAW, but more susceptible to

contamination since the temperature during metal transfer is

higher. Ti filler material is used

PAW-

Welding is done with a transferred arc using DCEN. Can be done

by melt-in technique or keyhole technique. The latter provides

better joint penetration

Has higher welding speeds and joint penetration


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PWHT:

Serves the usual purposes.

The minimum temperature range is 540C to 700C for short timeoperations

Selection of time and temperature depends on mechanical

properties required, since grain size changes may take place

during the PW

HT.Before the PWHT all the contaminants must be removed to avid

stress corrosion cracking.


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PROBLEMS ASSOCIATED WITHWELDING OF

Ti & ITS ALLOYS

POROSITY is the major problem associated with welding of Ti. Itmay be due to improper welding technique, improper surface

preparation, improper shielding

Amongst other sources, the sheared edges of the workpiece is a

source of contamination. The precaution to be taken is removeall the burrs, and shear the metal only a few hours before welding

CONTAMINATION CRACKING is caused due to presence of

contaminants in the weld region

To avoid it, welding of Ti should be done in a specially dedicated

chamber


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WEDLING OF Mg &WEDLING OF Mg &

ITS ALLOYSITS ALLOYS


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Magnesium & Its AlloysMagnesium & Its Alloys

Magnesium is a silvery white metal and has the lowestMagnesium is a silvery white metal and has the lowestdensity of the common structural materials.density of the common structural materials.

Very Light: 1738 kgmVery Light: 1738 kgm--33

Magnesium has a melting point of 650Magnesium has a melting point of 650C.C. Modulus low: E = 44.7 GPaModulus low: E = 44.7 GPa

Crystal structure is fccCrystal structure is fcc

Alloying is for precipitation hardening and (wroughtAlloying is for precipitation hardening and (wroughtalloys) grain refinement.alloys) grain refinement.

Excellent machining propertiesExcellent machining properties

Alloys are weldable (inert gas)Alloys are weldable (inert gas)


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Two main classes of alloys:Two main classes of alloys:

Impact strengths lowImpact strengths low

High damping capacity (useful in machine casings)High damping capacity (useful in machine casings) Corrosion resistance very poor. Can coatCorrosion resistance very poor. Can coat chromate,chromate,

anodise, epoxy resin.anodise, epoxy resin.

alloyed with elements such as Al, Zn, Mn, Zr, etc.alloyed with elements such as Al, Zn, Mn, Zr, etc.

Alloying increases strength and corrosion resistanceAlloying increases strength and corrosion resistance


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ALLOYING ADDITIONSALLOYING ADDITIONS

Alloying:Alloying:

up to 1.25% Mn or 3.5% Mg for solid solutionup to 1.25% Mn or 3.5% Mg for solid solution

strengthening,strengthening,

up to 4.5% Cu, 7% Zn or (3% Mg + 1% Si) forup to 4.5% Cu, 7% Zn or (3% Mg + 1% Si) for

precipitation hardening,precipitation hardening,

up to 0.5% Cr for grain refinement,up to 0.5% Cr for grain refinement,

up to 17% Si, 7% Cu, 10% Mg for castingup to 17% Si, 7% Cu, 10% Mg for castingalloysalloys


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Continue..Continue..

Al, Zn, ThAl, Zn, Th Produce precipitation hardening very complexProduce precipitation hardening very complex

series of metastable precipitates, depending on alloyseries of metastable precipitates, depending on alloy

compositioncomposition ThTh very stable precipitates, good for creep resistancevery stable precipitates, good for creep resistance

MnMn corrosion resistance (ties up Fe and othercorrosion resistance (ties up Fe and other

impurities)impurities)

ZrZr strong grain refinement, reacts with Al and Mn, Mgstrong grain refinement, reacts with Al and Mn, Mg--ZrZralloys must be Al, Mn free.alloys must be Al, Mn free.


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Magnesium alloys possessMagnesium alloys possess

High strength to weight ratioHigh strength to weight ratio

Good fatigue strength.Good fatigue strength.

Good dimensional stability in service.Good dimensional stability in service.

Good damping capacity.Good damping capacity. High thermal conductivity.High thermal conductivity.

Relatively high electrical conductivity.Relatively high electrical conductivity.


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Welding Characteristics of MagnesiumWelding Characteristics of Magnesium

Most magnesium alloys are readily weldable.Most magnesium alloys are readily weldable.

Weld strengths above 90% are possible when the fillerWeld strengths above 90% are possible when the fillermetal usedmetal used

The weldability of magnesium alloys may be affectedThe weldability of magnesium alloys may be affectedbyby ::

1. Oxidation.1. Oxidation.2. Thermal expansion.2. Thermal expansion.3. Susceptibility to hot cracking (hot shortness).3. Susceptibility to hot cracking (hot shortness).

4. Grain growth and aging effects4. Grain growth and aging effects


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Base Metal Surface PreparationBase Metal Surface Preparation

usually supplied with an oil coating, an acid pickledusually supplied with an oil coating, an acid pickled

surface or a chromate coated surfacesurface or a chromate coated surface

Mechanical cleaningMechanical cleaning

Chemical cleaningChemical cleaning This cleaning solution is kept in a tank made up ofThis cleaning solution is kept in a tank made up of

ceramic or stainless steelceramic or stainless steel

The job is dipped in the bath, which is kept at 22 to 32The job is dipped in the bath, which is kept at 22 to 32C,C,

for about 3 minutesfor about 3 minutes Then is rinsed thoroughly in hot water and dried in air.Then is rinsed thoroughly in hot water and dried in air.


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Welding ProcessesWelding Processes

The various processes employed for welding magnesiumThe various processes employed for welding magnesiumalloys arealloys are

1. Gas Tungsten ArcWelding (TIG or GTAW)1. Gas Tungsten ArcWelding (TIG or GTAW)2. Gas Metal ArcWelding (MIG or GMAW).2. Gas Metal ArcWelding (MIG or GMAW).

3. Resistance (Spot) Welding.3. Resistance (Spot) Welding.4. GasWelding.4. GasWelding.5. Forge welding.5. Forge welding.6. Other processes6. Other processes (PrWelding, Brazing, EB welding,(PrWelding, Brazing, EB welding,TIG spot welding, Stud Welding).TIG spot welding, Stud Welding).


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GTAW for Mg AlloysGTAW for Mg Alloys

Most popular process for welding Mg alloysMost popular process for welding Mg alloys

DC straight or reverse polarity and AC withDC straight or reverse polarity and AC with

superimposed are commonly used.superimposed are commonly used.

Both manual and automatic methods are suitable.Both manual and automatic methods are suitable. On materials over 4.5 mm thick, A.C. is preferred,On materials over 4.5 mm thick, A.C. is preferred,

because it provides deeper penetrationbecause it provides deeper penetration

DCRP is preferred since DCSP is difficult to handleDCRP is preferred since DCSP is difficult to handle

manually and its arc lacks cleaning actionmanually and its arc lacks cleaning action


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Welding techniqueWelding technique for GTAWfor GTAW::

(i)(i) Arc length maintained should be about 0.8 mmArc length maintained should be about 0.8 mm

(ii)(ii) Forehand welding is preferredForehand welding is preferred

(iii)(iii) Weaving should be used only for fillet welds or largeWeaving should be used only for fillet welds or large

corner jointscorner joints

(iv)(iv) Minimize the number of stops during welding. After aMinimize the number of stops during welding. After a

stop, the weld should be restarted on weld metal aboutstop, the weld should be restarted on weld metal about

12 mm from the end of the previous weld12 mm from the end of the previous weld

(v)(v) To prevent weld cracking :To prevent weld cracking :

-- Make use of starting and run off plates (or tabs) toMake use of starting and run off plates (or tabs) to

start and end the weldstart and end the weld

-- Weld from middle of the job towards the endsWeld from middle of the job towards the ends


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GMAW for Mg AlloysGMAW for Mg Alloys

welded using DCRP. Argon shielding is most satisfactory.welded using DCRP. Argon shielding is most satisfactory.

yields welding speeds 2 to 4 times faster than TIG welding.yields welding speeds 2 to 4 times faster than TIG welding.

Speeds range from 60 to 150 cm per minute and even higher.Speeds range from 60 to 150 cm per minute and even higher.

Increased welding speeds result in reduced distortion.Increased welding speeds result in reduced distortion. Involves high metal deposition ratesInvolves high metal deposition rates

limited to flat, horizontal and vertical down welding positions.limited to flat, horizontal and vertical down welding positions.

MIG welding are equal to TIG welds in strength andMIG welding are equal to TIG welds in strength and

soundness.soundness.

constant voltage power supply must be used with shortconstant voltage power supply must be used with short--

circuiting type metal transfercircuiting type metal transfer


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ResistanceWelding for Mg AlloysResistanceWelding for Mg Alloys

Either AC or DC used to join Mg alloys.Either AC or DC used to join Mg alloys.

Can be welded as spot, seam and flash welding.Can be welded as spot, seam and flash welding.

In Spot welding thicknesses up to about 4.5 mmIn Spot welding thicknesses up to about 4.5 mm

Spot weld penetration in Mg should not be less thanSpot weld penetration in Mg should not be less than20%, nor more than 80%, into each of the parts being20%, nor more than 80%, into each of the parts being

joinedjoined

To prevent corrosion, the spot welded joints should beTo prevent corrosion, the spot welded joints should be

wire brushed and chemically treatedwire brushed and chemically treated


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GasWelding for Mg AlloysGasWelding for Mg Alloys

Gas welding is generally restricted to pure Mg and toGas welding is generally restricted to pure Mg and toMnMn--Mg alloys, and limited to groove weldsMg alloys, and limited to groove welds

OxyOxy--acetylene, oxyacetylene, oxy--hydrogen and oxyhydrogen and oxy--carbon hydrogencarbon hydrogengases may be employed.gases may be employed.

OxyOxy--acetylene is preferred for welding heavier gauges.acetylene is preferred for welding heavier gauges.

The filler rod should be either of the same compositionThe filler rod should be either of the same compositionas the BM or have a lower melting point than BMas the BM or have a lower melting point than BM

Flux ( KCl, NaCl ) applications to both sides of theFlux ( KCl, NaCl ) applications to both sides of the

groove and on to the rod are importantgroove and on to the rod are important


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ForgeWelding for Mg AlloysForgeWelding for Mg Alloys

Developed to join high strength Mg alloys.Developed to join high strength Mg alloys.

Joint efficiencies as high as 95Joint efficiencies as high as 95--100% (Mg100% (Mg--AlAl--Zn)Zn)

Upset pressures of 35 tonnes with a total upset of 50Upset pressures of 35 tonnes with a total upset of 50

mmmm are used to weld 19 mm diameter rod.are used to weld 19 mm diameter rod. Heating and welding times of less than 30 seconds atHeating and welding times of less than 30 seconds at

temperatures between 280 to 316temperatures between 280 to 316C are used for MgC are used for Mg--3%3%

AlAl--l % Zn alloy.l % Zn alloy.


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PROBLEMS ASSOCIATED WITH WELDING OF MgAND ITS ALLOYS

SOLIDIFICATION CRACKING:Solidification cracking or hot cracking, consists offractures at the weld metal boundaries in the solidificationprocess, during which the liquid phase of the mushy meltbecomes rich in impurities, mainly S and P

This tendency is augmented by addition of Zn and Ca to Mgand its alloys

Al, Mn and Zr have little or no effect on this tendency whileThorium inhibits solidification cracking

The most high strength high alloying types of Mg alloys arethe most prone to such cracking


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STRESSCO

RROSIONCRAC

KING

:Al bearing Mg alloys are susceptible to such type of cracking.To avoid this, the weld must be heat treated to 250C torelieve the stresses.

Zr and Th inhibits this type of cracking tendency

During joining of Mg foils, through arc welding or resistance

welding, there is a risk of catching fire.So precautionsmust be taken to avoid any fatalities


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final welding of ti alloys and mg alloys

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