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Investigation of Hot Cracking Behavior in Transverse Mechanically Arc Oscillated Autogenous AA2014T6TIG Welds N.S.BIRADAR and R.RAMAN

Hot cracking studies on autogenous AA2014T6TIG welds were carried out.Signi?cant

cracking was observed during linear and circular welding test(CWT)on4-mm-thick plates.

Weld metal grain structure and amount of liquid distribution during the terminal stages of

solidi?cation were the key cause for hot cracking in aluminum welds.Square-wave AC TIG

welding with transverse mechanical arc oscillation(TMAO)was employed to study the cracking

behavior during linear and CWT.TMAO welds with amplitude=0.9mm and fre-

quency=0.5Hz showed signi?cant reduction in cracking tendency.The increase in cracking

resistance in the arc-oscillated weld was attributed to grain re?nement and improved weld bead

morphology,which improved the weld metal ductility and uniformity,respectively,of residual

tensile stresses that developed during welding.The obtained results were comparable to those of

reported favorable results of electromagnetic arc oscillation.

DOI:10.1007/s11661-012-1126-4

óThe Minerals,Metals&Materials Society and ASM International2012

I.INTRODUCTION

H OT cracking during welding was studied experi-mentally.[1–6]Di?erent manifestations of hot cracking during fusion welding are as follows:(1)solidi?cation cracking in the fusion zone(FZ),(2)liquation cracking in the partially melted zone/heat-a?ected zone(PMZ/ HAZ),and(3)a combination of the two.Of these various manifestations,aluminum alloys commonly experience both solidi?cation cracking in the FZ and liquation cracking in the PMZ,which are mainly intergranular.

Generally,solidi?cation cracking occurs during the terminal stage of solidi?cation,when the tensile stresses developed across the adjacent grains exceed the strength of the almost completely solidi?ed weld metal.[7,8] Liquation cracking is produced due to the combination of thermally induced strains arising during the welding process and formation of low melting liquid?lm along the grain boundary and to some extent at the grain interior.[9,10]Solidi?cation cracking has been known to be favored by the factors that decrease the solid-solid contact area during the terminal stages of solidi?cation. Two of the most important factors are the extent of formation of low-melting eutectics and grain size.Low-melting eutectics at the grain boundaries may exist as a liquid?lm to a temperature well below the equilibrium solidus and reduce the grain boundary contact area to a minimum.[3]Also,the coarser the grain structure,the less the grain boundary contact areas for a given amount of nonliquid.Hence,coarse-grained FZ structures are generally more prone to solidi?cation cracking than is ?ne-grained material.

Similarly,liquation cracking has been known to be favored by the following factors:(1)the extent of liquation,(2)the grain structure,(3)the hot ductility, and(4)weld metal contraction and the degree of external restraint.[9]Signi?cant e?ort was devoted to characterize the relative weldability of di?erent alloys, using a variety of weldability tests.[10–16]One test that is used currently to qualify the weldability of aluminum alloys is the circular patch test(CPT),[10,12,17]which is a representative test that attempts to reproduce the actual welding conditions in small test samples.

Hence,the formation of?ne-grained structure in the weld metal and reduced liquation in the PMZ is important in controlling hot cracking.Many methods were reported in the literature to control the grain structure in the weld metal.Grain re?nement techniques such as inoculation with heterogeneous nucleants,[18,19] arc pulsation by pulsing welding current,[20]double-sided arc welding,[21]weld pool stirring,[22]longitudinal oscillation of weld table,[23]arc oscillation by electro-magnetics,[24–29]and mechanical torch vibration[30] means were frequently used to re?ne the weld metal grain structure.

In the recent past,grain re?nement was produced through the electromagnetic arc oscillation technique and was the usual choice of many researchers;this technique involves de?ecting the arc column electro-magnetically either in the transverse or longitudinal direction.Kou and Le[24,25]achieved grain re?nement in AA6061and AA5052GTA welds by electromagnetic arc oscillation using multiple magnetic probes.Again, Kou and Le[26]reported the bene?cial e?ect of arc oscillation in reducing liquation cracking sensitivity in AA2014GTA welds,where cracking susceptibility was evaluated by the Houldcroft test.Sundaresan and Janakiram[27]achieved considerable re?nement of the

N.S.BIRADAR,Research Scholar,and R.RAMAN,Professor, are with the Welding and Equipment Design Laboratory,Department of Metallurgical Engineering and Materials Science,Indian Institute of Technology Bombay,Powai,Mumbai400076,India.Contact e-mail: biradarns@iitb.ac.in

Manuscript submitted October13,2011.

Article published online May26,2012

FZ grain structure in a -b titanium alloys welded using this technique.The preceding authors opined that grain re?nement and the extent of liquation were due to reduced net linear heat input as a result of arc oscillation.Advantages of arc oscillation frequently reported in the literature include grain re?nement in the FZ;reduction in PMZ/HAZ width;less distortion;reduction in segregation,leading to reduced hot crack-ing sensitivity;and reduced residual tensile stresses.[28,29]

The aim of the present investigation is to study an alternate method,i.e.,transverse mechanical arc oscil-lation (TMAO),on hot cracking tendency,which has not yet been reported in the open literature.The ranges of TMAO parameters,amplitude and frequency,studied were 0.9to 2.0mm and 0.3to 1.5Hz,respectively.The hot cracking susceptibility test was carried out employ-ing the circular weld test (CWT)along with TMAO using those parameters that showed signi?cant reduc-tion in cracking tendency during linear welding.The extent of weld metal grain re?nement,microstructural changes,extent of liquation in PMZ,and consequent changes in the hot cracking tendency of the welds with and without TMAO were studied.

II.

EXPERIMENTAL

A.Material

AA2014high strength aluminum alloy in the T6condition,4-mm thick was procured from the Depart-ment of Materials and Mechanical Entity (MME,VSSC,ISRO,Trivandrum).This alloy was selected due to its propensity to hot cracking during fusion welding both in the FZ and PMZ unless care was taken.Table I gives the chemical composition of the chosen alloy material.B.Experimental Setup

Figure 1shows the experimental setup specially designed and fabricated in the laboratory that was used in the present study.It mainly consists of a TIG welding machine (Fronius Magic Wave 2200,Austrian make),seam welding unit,circular turn table,mechanical arc oscillator with controller,and circular weld test ?xture.1.Square wave AC TIG welding machine

A Fronius MagicWave 2200(Austrian make)TIG welding machine with square wave alternating current (AC)was used to provide an optimum combination of current-carrying capacity,arc controllability,and arc cleaning action.The power source settings were di?erent from the conventional settings without which it was di?cult to obtain consistent arc because of physical disturbance during arc oscillation.These settings include

the following:wave balance,i.e.,electrode negative/electrode positive EN/EP:40/60,AC frequency:70Hz.Apart from a consistent arc,the square wave also ensures no arc extinction during the transfer of current from the positive to negative cycle.[31]Furthermore,it provides a much faster current transfer rate than the conventional sinusoidal wave form,which helps in penetration.A nonconsumable 2pct thoriated tungsten electrode of 2.4-mm diameter with hemispherical ball tip (approximately 2mm)was used.Argon was preferably used to protect the mechanically oscillated arc as it provides a uniform blanket over the arc because of its higher density compared to other shielding gas with a gas ?ow rate of 6to 8SLPM,as recommended for aluminum alloys.[32]

2.Mechanical arc oscillation

In the present study,transverse arc oscillation was performed using a mechanical oscillator MO-150,[33]as shown in Figure 2.The design of the MO-150provides a welding system,the ability to oscillate the arc reliably and consistently over a weld joint either in the transverse or longitudinal (with respect to welding)direction.Arc oscillation was obtained by the movement of the single axis slide,which is monitored with a 9200A oscillator control that includes circuitry to start and stop oscilla-tion,set oscillation speeds for each direction,and set dwell times at the end of each stroke.The circuitry allows the operator to adjust the oscillation width and adjust the center point of oscillation.Typically,obtain-able oscillation width ranges from 1.4to 100mm and frequency from 0.3to 10Hz.Oscillating frequency is limited to 10Hz,beyond which the inertia of the arc and welding torch causes overshoot and dynamic instability

Table I.Chemical Composition of AA2014T6

Mn Si Cr Cu Ti Fe Mg Al 0.66

0.90

0.016

4.49

0.016

0.22

0.68

remainder

Fig.1—Experimental setup to study hot cracking in aluminum welds.

of the system while welding.The main advantage of MO-150is its design to handle most of the welding processes,including TIG,plasma,and MIG and the feasibility of use in the manufacturing environment.This mechanical oscillator can also be used in weaving the arc for surfacing or stitching of plates.

Oscillation of the welding arc is a function of amplitude and frequency exerted on it by external means,i.e.,via a mechanical oscillator.In the case of arc oscillation,oscillation parameters (amplitude,fre-quency,and dwell time)were the most important parameters,which were reported in the published literature.[30–34]However,in the present case of TMAO only,amplitude and frequency were considered keeping zero dwell time.

3.Circular weld test

Figure 3shows a schematic design of the CWT along with standard specimen.The design of the test setup used in the present study was similar in design to the CPT used by Huang and Kou [10]and Nelson et al .[16]The design of the CWT depends upon various test conditions.The restraint condition during the test can be imposed by a number of variables such as specimen size,thickness,and depth of the groove (nonautogenous welds).However,in the present case,these variables are typically selected prior to testing,such that they represent the restraint conditions of interest and usually do not change.

During CWT,the specimen is highly restrained (by being bolted down to a thick stainless steel plate)to prevent it from contracting freely during welding.The specimen is sandwiched between a copper plate at the bottom and a copper ring at the top.The copper plate was 1529152919mm.The copper ring was 19-mm thick,with an 83-mm inside diameter,and had dimen-sions of 1529152mm on the outside.The specimen,together with the copper plate and the copper ring,was bolted down tightly to a 25.4-mm-thick stainless steel base plate 2039203mm.The bolts at the center and each corner of the specimen were tightened with a torque wrench to ensure uniform well-de?ned restraint

on the specimen.The specimen was actually separated from the copper plate and ring with washers;without the washers,it was di?cult to produce full penetration welds because of the heat-sink e?ect of copper.C.Experimental Procedure

Experiments were carried out in two stages:(1)auto-genous linear welding with and without TMAO to arrive at a favorable set of oscillation parameters and (2)eval-uation of hot cracking susceptibility by employing a standard CWT with and without TMAO on 4-mm-thick AA2014T6plates using the previously arrived arc oscillation conditions.

1.Autogenous linear welding

Linear weld prepared with the welding parameters given in Table II (a)produced full penetration weld with signi?cant cracking in the FZ.[35]Using these welding parameters,several welds approximately 100mm in length were prepared with di?erent combinations of arc oscillation parameters.The arc oscillation parameters used were in the amplitude range of 0.9to 2.0mm and frequency range of 0.3to 1.5Hz.

2.Circular weld test

Hot cracking susceptibility testing was carried out by employing CWT without and with TMAO,using those parameters that resulted in crack-free linear welds.This was done to verify whether favorable results obtained in linear welds could be maintained at higher restraint stress levels.In the present study,instead of using a circular patch (usually for nonautogenous welds),a circular weld of 50-mm diameter was prepared on a standard test specimen and will be referred to as the CWT in the remainder of

the

Fig.2—Typical mechanical oscillator (MO-150)used to produce TMAO.[33

]

Fig.3—Schematic of circular weld test:(a )standard test specimen and (b )CWT ?xture.[16]

text.Welding parameters used for CWT given in Table II(b) were di?erent in value as compared to linear welds as circular welding speed was adjusted to obtain full penetration to avoid excess melting.Weld prepared with smaller diameter(30mm)produced excess penetration and melting,while larger diameter weld of60mm showed less cracking tendency with reduced penetration.On the other hand,the weld prepared with50-mm diameter produced full penetration with signi?cant cracking in the FZ,and the same was used for the circular weld test with and without TMAO.

For CWT with TMAO parameters(0.9-mm ampli-tude and0.5-Hz frequency)was used speci?cally as these combination showed higher crack resistance during linear welding.The reason for the same is explained in the subsequent Section III–A.

Prior to welding,the standard test specimens were mechanically brushed with a wire brush to remove the tenacious oxide formed on the surface and further thor-oughly cleaned with acetone before?xing in the circular patch?xture.The?xture was mounted horizontally on a circular turntable to allow rotation under a stationary straight mount TIG torch attached to the mechanical oscillator.Appropriate external restraint stresses were applied with the help of a torque wrench(51N m, according to the design stress for the unwelded alloy based on the weld tensile strength per British standard code of practice C1181969).[28]Autogenous AA2014TIG circular welds of50-mm diameter were made in a fully automatic Square wave AC TIG welding machine with an attachment of commercially available mechanical arc oscillation equipment(mechanical oscillator MO-150).

Visual inspection was carried out to evaluate quanta-tively hot cracks initially with the help of high magnifying lens;each crack was characterized as(centerline or transverse)and is represented on the overview macro-graph,as shown in Figure5.Second,the length of the uncracked portion of the weld length was determined. The angular uncracked quantity refers to that portion of the weld in which no continuous surface breaking center-line cracking is present.Finally,crack length was measured by placing a protractor on the sample and measuring the length in degrees directly from the as-welded surface of the sample after thorough cleaning.The angular cracked region(cracked length)was used as the index in the assessment of cracking susceptibility in this study.Once cracking was characterized and recorded,the weld bead aspect penetration(D)and bead width(W)were measured and the cross-sectional area(CSA)of the weld was determined using a stereo-microscope.

Microstructural characterization was limited to CWT welds only.The resultant welds were sectioned(transverse) to the welding direction using an abrasive cutter to the required size and shape for metallographic examinations. Both transverse and planer sections of the weld surface were polished using a standard metallographic procedure, followed by polishing(on cloth using alumina slurry and diamond polished with1-l m diamond paste)and etching. Specimens were dip etched(20seconds)with Keller’s reagent(3pct HCL,5pct HNO3,2pct HF,and190mL of H2O)to reveal the microstructure.Microstructures of the specimens were studied under optical microscope (Olympus GX51,I R Technology Services Pvt.Ltd., Mahape,Navi Mumbai,India)and scanning electron microscope(Hitachi S-3400N,Forevision Instruments(I) Pvt.Ltd.,Hyderabad,Andhra Pradesh,India)at15kV. Image analysis of micrographs of the weld metal was carried out using image analyzing software(OLYSIA m3, Olympus,London,United Kingdom).

III.RESULTS AND DISCUSSION

A.Linear Welding

Figure4shows the e?ect of TMAO on the cracking tendency of AA2014autogenous linear welds prepared with and without TMAO.Solidi?cation cracking in the FZ and no visible liquation cracking in the PMZ were observed in the weld prepared without arc oscillation.It is seen that as compared to weld prepared without arc oscillation,all welds prepared with TMAO showed reduced solidi?cation cracking sensitivity.The reduc-tion,however,depended on the combination of fre-quency and amplitude of arc oscillation used.It is well reported that transverse arc oscillation leads to grain re?nement.[25–29]One of the reasons for the grain re?nement was attributed to an increase in cooling rate as a result of reduced heat input.[26,28]In the present case,e?ective net heat input was reduced due to an increase in e?ective welding speed as a result of TMAO. Accordingly,the e?ective welding speed for ampli-tude=0.9mm and frequency=0.5Hz)works out to be4.02mm/s,which is0.4mm/s higher than the actual

Table II.Welding and TMAO Parameters for Both Autogenous Linear and CWT of AA2014T6Alloy

Welding Parameters Mechanical Arc Oscillation Parameters Range

(TMAO)

Current(Amps)Speed(mm/s)Voltage

(V)Amplitude(mm)Frequency(Hz)

(a)Linear welding

165 3.613.4without oscillation

165 3.613.4(0.9to2.0)(0.3to1.5) (b)CWT

175 5.313.4without oscillation

175 5.313.40.90.5

linear welding speed (3.6mm/s).As a result,the cooling rate in TMAO welds was 20pct higher than that of unoscillated welds.The cooling rate was calculated using the empirical equation used for calculating weld thermal cycle parameters.[45]Since full penetration welds were prepared,the equation pertaining to a thermally thin body was used (the Appendix provides the calcu-lation).This,however,cannot be the only reason as the reduction in cracking sensitivity does not show mono-tonic dependence on the reduction in e?ective net heat input,which in?uences the weld cooling rate.Note that at a given frequency,the e?ective net heat input reduces with increasing amplitude and vice versa .

Garland,[30]by employing torch vibration parallel to welding direction (frequency of 10Hz and amplitude 1.2mm),found a reduction in crack length while TIG welding of 3.2-mm-thick Al-1.7-2.8pct Mg aluminum alloy.The reduction in crack length was attributed to extensive grain re?nement due to longitudinal torch vibration.He opined that grain re?nement was the result of back washing of the weld pool over the solid-liquid interface,which periodically fragmented and reoriented the substructure composing the trailing end of the weld pool,to yield in situ the necessary supply of nucleants,which act as grain re?ning agents.

Similarly,Kou [17]showed signi?cant reduction in crack-ing sensitivity when transverse electromagnetic arc oscil-lation (amplitude =1.9mm and frequency =1Hz)was employed during TIG welding of AA2014aluminum alloy.They argued that transverse arc oscillation frequency produced an alternating orientation of the columnar grains,leading to winding of the crack length and making it di?cult to propagate further.

In the present work,microstructural evidence,as shown in Figures 9and 10(for CWT welds as micro-structural characterization was limited to CWT only),supports the view that TMAO leads to breaking up of columnar structures to form equiaxed structures.Crack-ing sensitivity was least (restricted to crater)for welds prepared with amplitude of 0.9mm and frequency of 0.5Hz.This should be attributed to the fact that due to transverse arc oscillation,there is a constructive inter-ference in the solidi?cation process.This constructive interference is maximum at observed TMAO parameters (amplitude =0.9mm and frequency =0.5Hz).Such an

action is required mainly along the narrow region around the weld centerline.Transverse arc oscillation amplitude of 0.9mm roughly corresponds to 21pct of FZ width (8.7mm).Note that the ?rst approximation weld solid-i?cation rate (R )is given by (u cos h ),where u is the linear welding speed and h is the angle between the normal at the point on the interface in the welding direction.[17]That is why amplitudes greater than a particular value lead to reduction in the bene?cial e?ect of TMAO.

The useful amplitude,over which TMAO will be bene?cial,will be decided by both the weld pool size and shape of the trailing edge.These,in turn,will depend on the primary welding parameters,linear welding speed,and welding current.As far as arc oscillation frequency is concerned,it is likely that it will depend on the linear welding speed.In the case of full penetration welds with substrate plates of di?erent thicknesses,it is quite possible that the optimum combination of TMAO may be di?erent.This is because di?erent thicknesses require di?erent welding parameters,leading to di?er-ences in the weld pool size and shape.Thus,the observed reduction in cracking sensitivity can be attrib-uted to mechanical agitation and an increase in cooling rate,both of which lead to grain re?nement.B.Circular Welding Test

Figure 5shows the overview of the macrographs of CWT welds prepared with and without arc oscillation.For better visibility,the crack path observed was traced with a dark line.From the overview of the top surface,many transverse cracks before the centerline cracks were seen,which were very minute and di?cult to quantify.The circumferential length corresponding to the chosen diam-eter of the weld was 50mm;the total crack length was expressed in terms of angle covered remembering that 360deg approximately corresponds to a length of 160mm.As many as three centerline cracks at di?erent locations of the weld were observed;and these accumulated for a total crack length of 48.5mm i.e.,30%of cracking in weld prepared without oscillation.However,it should be noted that these centerline cracks originated from the PMZ region,indicating transverse liquation cracking,but there was no longitudinal PMZ cracking along the circumferen-tial length of the weld.It also should be noted that as compared to linear welds,which did not show any PMZ cracks with or without arc oscillation,CWT has shown indications of PMZ cracking in addition to FZ cracking in CWT welds prepared without arc oscillation.However,in arc oscillated welds,these cracks were also eliminated except for small FZ crater cracks,as shown in Figure 5(b).In the present case,for welds without oscillation,cracking was not very severe;this is mainly because the alloy contains a higher weight percent (4.45)of Cu,which lies in the less crack sensitive region of the curves of the hot cracking sensitivity of aluminum alloys.[36]However,alloy AA2014also contains a small amount of Mg,which depresses the solidus temperature,but it does not a?ect the coherence temperature;therefore,the coherence range extends and the hot cracking tendency increases.It is interesting to note that solidi?cation cracking and liquation cracking did not co-exist,i.e.

,

Fig.4—E?ect of TMAO parameters on AA2014autogenous linear welds:amplitude (0.9to 2.0mm)and frequency (0.3to 1.5Hz).

side by side,as seen from Figure 5(a).It is also evident that liquation cracking stopped at the point where solidi?cation cracking started.This is probably because solidi?cation cracking in the weld metal signi?cantly relaxed the residual tensile strains in the adjacent PMZ.Similar incidences of absence of co-existence of liqua-tion and solidi?cation cracking were reported when the AA2219alloy was welded with di?erent ?ller (1100,110A,and 2219)during TIG welding with CPT.[10]

CWT frequently causes cracking because of the design of the test and specimen as both the FZ and HAZ are subjected to higher tensile stresses.[37]The greater the radius of the circular weld,the lower the additional improved tensile stresses.On the other hand,a smaller weld size corresponds to a higher restraint.The cracking tendency is inversely related to the radius of the circular weld.[37]

Transverse liquation cracks were seen only along the outer edge of the weld,but there was no liquation cracking either transverse or longitudinal along the inner edge.In CWT,the specimen is held tightly against a strong back (the stainless steel base plate,as shown in Figure 3).This keeps the weld metal from contracting due to thermal contraction and solidi?cation shrinkage when it cools.[10]As a result,the outer edge of the weld experiences tensile stress because of the restrained stresses from all four corners,while the inner edge is subjected to compressive stress as opposed by the substrate from the weld center.This explains why transverse liquation cracks were observed only along the outer edge of the weld.

Figure 5(b)shows the overview macrograph of weld prepared with TMAO parameters:(ampli-tude =0.9mm and frequency =0.5Hz).No trace of any solidi?cation cracks,except a small crater crack at the end of the weld,was observed.The weld bead appeared to be wider (8.74mm)with reduced depth (4.01mm)as compared to without oscillation (width of 7.85mm and higher depth of 4.08mm),with smooth weld surface demonstrating the bene?cial e?ect of TMAO in reducing hot cracking tendency.The elimination of cracking was attributed to grain

re?nement and reduced amount of segregation along the interdendritic regions as a result of reduced linear e?ective net heat input,which is explained in Section III–C–1.

1.Hot cracking sensitivity

Generally,weld metal solidi?cation cracking in alu-minum alloys occurs when higher levels of thermal stresses and solidi?cation shrinkage are present during welding.[17]It is in?uenced by a combination of mechanical,thermal,and metallurgical factors.In practice,the solidi?cation cracking sensitivity of alumi-num alloy weldments is determined by the chemical composition and welding conditions.The usual methods of eliminating cracking in aluminum welds are to control the weld metal composition (using overalloyed ?ller,which facilitates back ?lling)and to use low heat input,such as use of arc oscillation techniques.[26–28]However,in the present case,weld metal composition control was beyond the scope due to autogenous welding,and the latter was obtained by employing TMAO.Figure 6illustrates the e?ect of TMAO on e?ective net heat input for a given amplitude of 0.9mm (the Appendix provides the calculation).The cracking sensitivity in aluminum welds is shown schematically and discussed.

Figure 7illustrates the weld top surface and trans-verse cross section of the welds prepared with and without TMAO,respectively.It is clear that the TMAO weld (Figures 7(b)and (d))has the higher cracking resistance (no cracks)as compared to that without oscillated weld (Figures 7(a)and (c)),which experienced cracking.Stress concentration in the welded joint of the aluminum alloy is induced in two ways:thermal stress,due to the high coe?cient of thermal expansion,and large solidi?cation shrinkage,almost twice that of steel.In normal welding processes,i.e.,without transverse arc oscillation,the FZ typically is V shaped,as shown in Figure 8(a).Shrinkage forces within the V-shaped zone cause the plate to have an angular distortion.

The

Fig.5—Overview of macrographs of CWT showing crack morphology:(a )without oscillation and (b )with TMAO (amplitude =0.9mm and frequency =0.5Hz).

shrinkage-induced stresses increase from the bottom to the top surface,[38]as shown in Figure8(a).If the plate is constrained during welding,the distortion will decrease; however,the residual stress in the weld zone will greatly increase.

On the contrary,TMAO weld showed improved bead morphology similar to the schematic shown in Figure 8(b)with increase in weld width;8.74mm at the top and 7.0mm at the root as compared to7.85mm and4.2mm width at the top and root of the weld,Figure7(c)and(d);consequently,increasing the weld metal cross-sectional area i.e.,28.00mm2as compared to 20.01mm2in case of weld prepared without oscillation. However,the weld bead morphologies depend on the oscillation parameters:amplitude and frequency.It is observed that,at lower frequency,the bead shape seems to be wide as compared to that at high frequency.[39] This means that there is a de?nite change in concentra-tion of shrinkage forces,which nearly approaches uniform at both the top and bottom of the weld zone during cooling,as seen in Figure8(b),and better

resists Fig.6—E?ect of TMAO parameters on e?ective net heat input

(amplitude=0.9

mm).

Fig.7—Weld bead morphologies of CWT autogenous AA2014welds:(a)and(c)without oscillation and(b)and(d)with TMAO(amplitude= 0.9mm and frequency=0.5

Hz).

Fig.8—Schematic of the shrinkage forces in aluminum autogenous

TIG welds:(a)without and(b)with TMAO.[21]

cracking.Thus,from the observed results and based on the earlier reported observation,[21]it can be said that TMAO helps reduce the cracking sensitivity of the alloy during welding.

C.Microstructure Analysis

1.Weld metal solidi?cation structure

The weld metal solidi?cation microstructure is another critical factor in?uencing cracking sensitivity in aluminum alloy weldments.[40,41]In autogenous welding,the grain structure is controlled by a combination of the thermal conditions that prevail at the solid-liquid interface and the crystal growth rate,which is directly related to the welding speed.[42]The thermal conditions are determined by the heat input and the weld speed for a given sheet thickness.Furthermore,the conditions vary considerably depending on the position at the solid-liquid interface along the trailing edge of the weld pool.[17]

Two di?erent types of grain structures were observed,as a function of the welding conditions,i.e.,with and without arc oscillation.Figures 9(a)and (c)represent the grain structure of weld without arc oscillation,and Figures 9(b)and (d)that of the weld prepared with TMAO (amplitude of 0.9mm and frequency of 0.5Hz).As shown in Figure 9(b),the arc oscillated weld

produced equiaxed grain with average grain diameter of 23.30l m with fragmented interdendritic arms at the weld centerline,as compared to coarse grains measuring 30.35l m and darkly etched dendritic structures with continuous interdendritic arms in weld prepared without TMAO (Figure 9(a)).On either side of the equiaxed grains,columnar-dendritic grains were found,which solidi?ed on the base metal epitaxially and grew toward the central axial region.The epitaxially grown dendrites were curved toward the heat source,as shown in Figure 9(c),so that the maximum thermal gradients,G ,present at the solid-liquid interface,are maintained as growth proceeds without oscillated welds.Furthermore,the lower welding speed (5.3mm/s)results in a lower solidi?cation rate,R .[26,42]With high-temperature gra-dient,G ,and small growth rate,R ,the alloy solidi?ca-tion microstructure consists of columnar dendritic grains,[17,43]as is observed in Figure 9(c).

On the contrary,the epitaxially grown dendrites were straight and smaller in size,as shown in Figure 9(d),in the weld prepared with arc oscillation.Furthermore,the higher e?ective welding speed (5.6mm/s)causes the e?ective net heat input to be small,[17]which results in smaller temperature gradient,G .Also,the increase in e?ective welding speed during arc oscillation leads to an increase in solidi?cation rate,R .[17]For

smaller

Fig.9—Comparison of autogenous AA2014T6CWT welds:(a )and (c )without (b )and (d )with TMAO (parameters:amplitude =0.9mm and frequency =0.5Hz).

temperature gradients,G ,and larger solidi?cation rates,R ,the alloy solidi?cation microstructure consists of equiaxed grains at the center and straight columnar grains toward the fusion boundary.[17,43]It is reported that transverse arc oscillation (electromagnetic)produces equiaxed grains in the weld metal.[26,27]Similar grain structure was observed,as illustrated in Figures 9(b)through (d).It is clear that grain re?nement is possible with TMAO;however,its extent will depend upon the oscillation parameters.2.Effect on subgrain structure

Figure 10shows the SEM microstructures taken at the weld center and near the fusion boundary for welds made with and without arc oscillation.It is seen that the subgrains’structure is signi?cantly modi?ed in the TMAO weld compared to that in the weld without arc oscillation.This is attributed to the increase in cooling rate as a result of transverse arc oscillation,as explained below.

According to the principle of solidi?cation process-ing,[36,44]the higher the cooling rate is during solidi?ca-tion,the ?ner the resulting subgrains’structure.This is because higher cooling rates allow less time for the coarsening of the subgrains to occur during solidi?cation.

Similar modi?ed microstructures were observed in welds prepared with transverse electromagnetic arc oscillation with amplitude of 1.9mm and frequency of 1Hz.[26]Subgrain re?nement in the case of transverse arc oscillation was explained by Kou.[17]According to Figure 11,in the case of weld with TMAO,the weld pool has two velocity components:one in the

welding

Fig.10—Comparison of subgrain structure between welds prepared with and without TMAO:(a )weld center and (b )near FZ boundary;at amplitude of 0.9mm and frequency of 0.5

Hz.

Fig.11—E?ect of arc oscillation on welding speed.[17]

direction(u)and the other transverse to the welding direction(v).The resultant velocity(w),of course,is greater than the velocity of the non-oscillated weld pool(u).

The welding speed for the weld without oscillation is u=5.3mm/s.On the other hand,for the oscillated weld with amplitude0.9mm and frequency0.5Hz, v=1.8mm/s,which means the arc travels (0.99490.5=1.8mm/s)transverse to the welding direction.Therefore,the resultant welding speed,w,is 5.6mm/s,which is0.3mm/s higher than the non-oscillated weld;consequently,it produces higher cooling rate during solidi?cation,resulting in?ner subgrain structure in the TMAO weld.

3.PMZ structure

PMZ is the region of weld located immediately adjacent to the fusion boundary.It is the area where the peak temperature during welding is below the liquidus temperature and above the solidus,and it is characterized by melting of the eutectic material, particularly along the grain boundaries.As shown in Figure12(b),the extent of grain boundary melting is less severe in the TMAO weld as compared to the weld without oscillation(Figure12(a)).Furthermore,a sig-ni?cant reduction in the partially melted region was also observed,as shown in Figure12(b).This could be attributed to an increase in cooling rate due to reduced e?ective heat input as a consequence of arc oscillation. The extent of melting in the adjacent PMZ and its size depend upon the kinetic strength of the weld thermal cycle.The thermal cycles that cause partial melting have peak temperatures lower than those that cause complete melting in the FZ,but above those that do not cause melting but a?ect the microstructure of the solid surrounding the FZ.The extent of liquation along the GBs appears to be higher in welds prepared without TMAO(Figure12(a)).

Figure13illustrates the extent of PMZ width formed in welds prepared with and without TMAO.PMZ width was measured from the FZ boundary to a distance where grain boundary(GB)liquation was observed clearly under optical microscope.Figure13(b)shows the average width of the PMZ in TMAO welds,which is relatively narrower, i.e.,1.88mm,and approximately25pct lesser than that

of Fig.12—Comparison of grain boundary melting in the PMZ between two welds made without and with TMAO at amplitude=0.9mm and 0.5

Hz.

Fig.13—E?ect of TMAO on PMZ width:(a)for welds prepared without oscillation and(b)with TMAO.

the weld prepared without oscillation,as shown in Figure13(a),measuring2.52mm.

Figure14(a)shows the PMZ microstructure of weld prepared without TMAO,where incidences of liquation cracks in the PMZ were observed as noticed from the overview macrograph(Figure5(a)).The formation of liquation cracks is due to the formation of thin liquid?lm along the GBs because of the melting of low melting elements.First,this liquid eutectic upon resolidi?cation forms solid eutectic,which is mechanically weak(a combination of hard and soft phases)and can cause severe loss in ductility during PMZ solidi?cation.Second,the formation of tensile strains due to external restraint stresses from all four corners and the center of the weld specimen.It should be noted that in the present case no continuous circumferential(longitudinal)liquation cracks were ob-served because of the relaxation in residual tensile stress due to solidi?cation cracking in the FZ.Such a crack in the adjacent PMZ observed is shown in Figure14(a).How-ever,in the case of TMAO weld,no such incipient cracks were seen in the PMZ,as shown in Figure14(b).

IV.CONCLUSIONS

The e?ect of TMAO on the hot cracking behavior of autogenous AA2014T6aluminum alloy TIG welds was investigated.

1.Hot cracking susceptibility with CWT demonstrated

that AA2014aluminum alloy in T6is prone to hot cracking,i.e.,solidification cracking in the weld me-tal and liquation cracking in the PMZ.

2.During normal welding(without arc oscillation),

solidi?cation cracking accounted for30pct of the total length of the weld;however,transverse liquation cracks were di?cult to quantify as they just initiated and terminated as solidi?cation cracks occurred.

3.TMAO weld showed higher hot cracking resistance.

Oscillation parameters,amplitude0.9mm and fre-quency0.5Hz,proved to be bene?cial in eliminat-ing solidi?cation cracks,except some crater cracks,and ensuring complete absence of transverse

liquation cracking.The preceding improvements were attributed to the following.

(a)An increase in solidi?cation cracking resistance

due to TMAO is a result of weld metal grain

re?nement with signi?cant modi?cations in the

subgrain structure.

(b)The complete elimination of liquation cracks in

the PMZ is due to reduced extent of grain bound-

ary liquation and consequent reduction in PMZ

width.

(c)Change in bead morphology resulting in unifor-

mity of residual tensile strains also helped in

reducing cracking tendency.

4.The preceding obtained results were comparable

with those obtained for transverse electromagnetic arc oscillation used in autogenous welds of AA2014 alloy reported in the literature.[26]

5.TMAO obviously appears to be a potential alterna-

tive method of arc oscillation to mitigate hot crack-ing in fusion welds.

ACKNOWLEDGMENTS

The authors thank the Indian Space Research Organi-zation(ISRO,Trivandrum,India)for providing the mate-rial AA2014aluminum alloy in the T6condition necessary for carrying out the experiments in the research study.

APPENDIX:CALCULATION OF EFFECTIVE

WELDING SPEED(W)

w?u2tv2

àá1=2

?3:62t1:82

àá1=2

?4:02mm=s linear welding

eT

?5:32t1:82

àá1=2

?5:6mm=s CWT welding

eT

where u=linear welding speed(3.6mm/s for linear welding and 5.3mm/s for CWT welding)

and Fig.14—Liquation cracking in the PMZ:(a)without oscillation and(b)with TMAO(amplitude=0.9mm and frequency=0.5Hz).

v=transverse welding speed of the torch in mm/s. E?ective welding speed for the TMAO parameter (amplitude=0.9mm and frequency=0.5Hz): v?0:9?4?0:5

eTfor one cycle of oscillation

?1:8mm=s

HEAT INPUT CALCULATIONS(H NET)USED FOR TUNGSTEN INERT GAS WELDING WITH AND WITHOUT TMAO(AMPLITUDE=0.9MM AND

FREQUENCY=0.5HZ)

For linear TIG welding:

H Net without TMAO

eT

?V?I?g=u

?165?13:4?0:40=3:6

?245:6J=mm

H Net with TMAO

eT

?V?I?g=w

?165?13:4?0:40=4:02

?220J=mm

For CWT TIG welding:

H Net without TMAO

eT

?V?I?g=u

?175?13:4?0:40=5:3

?176:9J=mm

H Net with TMAO

eT

?V?I?g=w

?175?13:4?0:40=5:6

?167:5J=mm

where V=voltage in volts,I=current in Amps, g=arc e?ciency(assumed as0.40for TIG),[32] u=welding speed in mm/s(without arc oscillation),and w=e?ective welding speed due to TMAO in mm/s.

COOLING RATE CALCULATIONS,R,FOR TIG WELDS WITHOUT AND WITH TMAO

(AMPLITUDE=0.9MM AND

FREQUENCY=0.5HZ)

For Linear TIG welding:

R=2p k q C(s/H Net)2(T c–T0)3for thin plates at full penetration were produced.[45]

R without TMAO

eT

?2p?0:23?0:00274=245:6

eT2?911:15à303:15

eT3?505:75K232:6 C=s

eT

R with TMAO

eT

?2p?0:23?0:00274=220

eT2?911:15à303:15

eT3?563:05K289:9 C=s

eT

For CWT TIG welds:

R without TMAO

eT

?2p?0:23?0:00274=176:9

eT2?911:15à303:15

eT3?721:45K448:3 C=s

eT

R with TMAO

eT

?2p?0:23?0:00274=167:69

eT2?911:15à303:15

eT3?771:35K498:2 C=s

eT

where k=thermal conductivity(J/mm2K),q C=ther-mal capacity(J/mm3K),s=thickness of the plate(mm), H Net=e?ective heat input(J/mm),T c=liquidus tem-perature(K),and T0=initial temperature(K).

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01-文献综述范文-翻译-当前零翻译研究问题与对策

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焊接方法与几种焊材的中英文对照

焊接方法与几种焊材的中英文对照 自己在工作中总结出了几种焊接方法和焊材的英文名称，看大家是否用得上 MIG --------------Metal Inert Gas arc welding ----------熔化极惰性气体保护电弧焊MAG--------------Metal Active Gas arc welding----------熔化极活性气体气体保护电弧焊TIG --------------Tungsten Inert Gas Welding--------------钨极惰性气体保护焊SAW——Submerged Arc Welding——埋弧焊 其它: FCAW——flux cored arc welding——药芯焊丝电弧焊 FCW-G——gas-shielded flux cored arc welding——气保护药芯焊丝电弧焊 FCW-S——self-shielded flux cored arc welding——自保护药芯焊丝电弧焊GMAW——gas metal arc welding——熔化极气体保护电弧焊 GTAW——gas tungsten arc welding——钨极气体保护电弧焊 焊材与焊接常用词汇: 焊丝welding wire. Welding rod 实心焊丝solid wire 镀铜焊丝copper-plating welding wire 或copper-coating welding wire 药芯焊丝flux-cored wire 填充焊丝filler wire 焊条electrode/ covered electrode 酸性焊条acid electrode 高钛型焊条high titania (type) electrode 钛钙型焊条lime titania type electrode 钛铁矿形焊条ilmenite type electrode 氧化铁型焊条iron oxide type electrode/ high iron oxide type electrode 高纤维素型焊条high cellulose (type) electrode 石墨型焊条graphite type electrode 碱性焊条basic electrode/ lime type covered electrode 低氢型焊条low hydrogen type electrode 高韧性超低氢焊条high toughness super low hydrogen electrode 奥氏体焊条austenitic electrode

焊接 翻译

Welding There are a number of methods of joining meta l articales together,dep ending on the type of meta l and the strength of the joint which is reqired. Soldering gives a satisfactory joint for light articles of steel,copper or brass. but the strength of a soldeing is rather less than a joint which is brazed,ri veted or welded.These methods of jointing meta l are normally adopted for srtong permanent joint. The simplest method of welding two pieces of meta l toghter is known as pressure welding.The ends of meta l are heated to a white heat-for iorn, the welding temperature should be about 1300℃-in a flame.At this tempera ture the meta l becomes plastic.The ends are then pressed or hammered tog ether,and the joint is stoothed off.Care must be taken to ensure that the su rface are thoroughly clean first,for dirt will weaken the weld.Moreover, the heating of iron or steel to a high temperature causes oxidation,and a firm of oxide is formed on the heated surfaces.For this reason,a flux is applied to the heated meta l.At welding heat,the flux meta ls,and oxide particles are dissolved in it together with any other imputities,and the flux is squeezed out from the center of the weld.A number of different types of weld may be used ,but for fairly thick bars of meta l,a vee-shaped weld should norm ally be employed.It is rather stonger than the ordinary butt weld. The heat of fusion welding is generated in several ways,depending on the sort of meta l which is being welded and on its shape.An extremely h ot flame can be produced from an oxyacetylene torch. For certain welds a n electric arc is used. In this method,an electric current is passed across t wo electrodes,and the meta l surface are placed between them.The electrodes are sometimes made of carbon,but more frequently they are meta lllic. The work itself constitutes one of them and the other is an insulated filler rod. An arc is stuck between the two ,and the heat which is generated melts t he meta l at the weld. A different method, known as spot welding, is usual

学术英语理工类文献综述英文

学术英语 学院名称：材料科学与工程学院学生姓名：张庆飞 学号：7301013016 专业班级：新能源材料与器件131 2015年 6月16 日

The current development of genetically modified crops of China and its safety issues Zhang Qingfei (College of materials science and engineering, Nanchang University, 10001) Abstracts: GM technology as a new, highly efficient genetically modified technology, already widely used in the cultivation of new varieties of field crops. China's transgenic technology started earlier, China's current GM technology level in the forefront of the world, especially made great achievements in the cultivation of new varieties of crops. In this paper, the status quo using gene transfer on Chinese agriculture and safety of transgenic technology are introduced, and the resolve of GM safety issues some thought. On this basis, I made some new ideas of transgenic technology and application of transgenic technology development prospects were discussed. Keywords: GM technology; GM crops; GM safety issues; China Introduction: As the core of the biotechnology giant leap .Transgenic technology is known as the second "green revolution." In 1983, the world's first strain of transgenic tobacco plants marks the arrival of the era of gene transfer plants. In the ensuing decades, genetically modified crops developed rapidly. In 2014, there are 28 countries in the world planted GM crops, the planting area has reached more than 180 million hectares, Chinese genetically modified crops planting area is 3.6 million hectares. With the widespread planting of genetically modified crops, the impact of transgenic technology in agriculture is growing. As the research of GM crops, China has made great achievements in the research of rice and cotton. The development of anything will not be smooth. With the development of GM technology, its security deposit issues also be exposed. The safety of genetically modified crops has been disputed by people, opponents argue that GM crops have great potential risk, it should be resisted. What is more, some people exaggerate the risk of genetically modified crops. Aroused people's fear of genetically modified. In this paper, the people of GM crops concerns, the status of the development of GM crops were elaborated. Besides, security problems of genetically modified crops and their solutions will be discussed. The current development of genetically modified crops in china GM refers to the Technology that use molecular biology method to transfer the artificial separation and some modified biological gene to other species and change the genetic characteristics of the species .Plant transgenic technology is the genetic transformation of plants, making plants to meet human needs in aspects of shape, nutrition and consumer quality. China's biotech crop improvement research began in the 1980s, during the past 30 years, Chinese agricultural and biological high-tech has been developing rapidly. At present, China's hybrid rice, cotton and other advanced in the world. GM technology Applied on cotton, rice and other crops on behalf of China's development status of GM crops. As a cotton producing countries, the industrialization of Chinese insect-resistant cotton has brought huge economic and ecological benefits to Chinese

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