Jyoti Prakash et al Int J Engg Techsci Vol 1(1) 2010,1-17 ... · Jyoti prakash , S.P. Tewari, Bipin...

Nucleation ,Graingrowth ,Solidification and Residual Stress Relaxation Under Stationary and Vibratory Welding Condition

-A Review

Jyoti prakash , S.P. Tewari, Bipin kumar Srivastava

Institute of Technology, Banaras Hindu University, Varanasi

Abstract Understanding of grain nucleation and grain growth becomes necessary that are influenced under welding conditions. After completion of nucleation, the solidification process will continue with nucleus growth. Increasing the growth rate will reduce the grain size of metal. In vibratory welding, work piece vibrates in the whole welding process and it mainly effects the welding solidification which improves the quality. Vibration facilitates the release of dissolved gases and the resulting weld beads greatly exhibit reduced porosity. Mechanical properties of the welds prepared under vibratory conditions are dependent on the structural changes of the welds. This paper presents the nucleation, grain growth, solidification behaviour and residual stress relaxation under stationary and vibratory welding condition.

Key Words: Nucleation, Heat affected zone, Solidification, Vibratory welding condition 1. INTRODUCTION In welding, as the heat source interacts with the material, the severity of thermal excursions experienced by the material varies from region to region, resulting in three distinct regions in the weldment (Figure 1). These are the fusion zone (FZ), also known as the weld metal, the heat-affected zone (HAZ), and the unaffected base metal (BM). The FZ experiences melting and solidification, and its microstructural characteristics are the focus of this article.

Fig.1 A schematic diagram showing the interaction between the heat source and the base metal. 2. NUCLEATION In certain welds, where filler metals are used, inoculants and other grain-refining techniques are used in much the same way as they are in casting practices. In addition, dynamic methods for promoting nucleation such as weld-pool stirring and arc oscillation have been used to refine the weld metal solidification structure.2 Although the mechanisms of nucleation in weld metal are reasonably well understood, not much attention is given to modeling this phenomenon. Often, weld solidification models assume epitaxial growth and for most of the cases the assumption

Jyoti Prakash et al Int J Engg Techsci Vol 1(1) 2010,1-17

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seems to be appropriate. However, to describe the effects of inoculants, arc oscillations, and weld pool stirring, heat and mass transfer models[4,5,6] 2.1. NUCLEATION THEORY

Fig.2 Schematic representation of a heterogeneous nucleation of a spherical cap on a solid substrate. A crystal can nucleate from a liquid on a flat substrate if the energy barrier ΔG is over come, according to Turnbull’s equation

. where γLC is the surface energy of the liquid-crystal interface Tm is the equilibrium melting temperature ΔHm is the latent heat of melting. ΔT is the undercooling temperature below Tm

θ is the contact angle If the liquid wets the substrate completely, θ= 0 ΔG=0

2.2. NUCLEATION MECHANISM 2.2.1. Dendrite Fragmentation Weld pool convection causes fragmentation of dendrite tips in the mushy zone and then carried into the bulk weld pool, acting as nuclei for new grains. 2.2.2. Grain Detachment Weld pool convection also causes partially melted grains to detach themselves from the solid-liquid mixture surrounding the weld pool giving nuclei for new grains. 2.2.3. Heterogeneous Nucleation Foreign particles present in the weld pool can act as heterogeneous nuclei. 2.2.4. Surface Nucleation Surface nucleation is induced by applying cooling gas or by instantaneous reduction or removal of heat input at the weld pool surface.



Fig. 3 Nucleation mechanisms during welding (a) top view, (b) side view. 3. GRAIN GROWTH During growth of the solid in the weld pool, the shape of the solid-liquid interface controls the development of micro structural features. The nature and the stability of the solid-liquid interface is mostly determined by the thermal and constitutional conditions (constitutional super cooling) that exist in the immediate vicinity of the interface.[7,8] Depending on these conditions, the interface growth may occur by planar, cellular, or dendritic growth. The simulation of grain growth during solidification has progressed since many stochastic approaches such as Monte Carlo（MC）and Cellular Automaton (CA) techniques were introduced into this field [12-17] .Similar to a casting process, the microstructure in the weld zone is expected to significantly change due to remelting and solidification of metal at the temperature beyond the effective liquidus temperature. However fusion welding is much more complex due to physical interactions between the heat source and the base metal. Nucleation and growth of the new grains occur at the surface of the base metal in welding rather than at the casting mould wall.

Fig. 4(a) cast structure Fig. 4(b) welding structure



Refer to reference [61] Fig.5 Epitaxial and columnar growth near the fusion line in an iridium alloy electron-beam weld.The figure also shows the grain-growth selection process of the grains from the fusion line. 3.1 Effect Of Welding Speed On Weld Structure

Fig.6 GTAW of 99.96% aluminium (a) 1000 mm/min and (b) 250 mm/min welding speeds. Axial grains of GTAW (c) 1100 aluminium at 12.7 mm/s welding speed, (d) 2014 aluminium at 3.6/s welding speed. Refer to reference [61]

3.2 Effect Of Heat Input On Weld Structure A slight tendency for the elements C, Mn, Si to decrease (in the composition of the weld) when the heat input increases. when heat I/P increases then weld bead size and HAZ size also increases. During welding process the cooling speed is rapid so that the grain growth velocity is correspondingly high.

Refer to reference[1]



Fig. 7 Typical macrosegregation of multipass welds deposited with different heat inputs Refer to reference [61]

There are many factors which influence the dendrite spacing during solidification process. But in weld solidification process, the main factors which determine the dendrite spacing include competitive growth, undercooling conditions, etc. [10] With grain growing the solutes accumulate in the front of the S/L interface and accordingly the constitutional undercooling increase. Therefore, the grain growth velocity tends to increases. It is obvious that the growth velocity of the primary dendrite trunk increases along with the solidification process. Stochastic modelling of columnar dendritic grain growth in weld pool of Al-Cu alloy

Fig.8 Simulated columnar dendritic grain morphologies at different time: (a) 70000CAs, (b) 120000CAs, (c) 180000CAs, (d)225000CAs. (Online color at ww.crt-journal.org) Refer to reference [10]

4. SOLIDIFICATION Since solidification of the weld metal proceeds spontaneously by epitaxial growth of the partially melted grains in the base metal, the FZ grain structure is mainly determined by the base metal grain structure and the welding conditions[9] 4.1.Solidification Mode Different soloidification modes are shown in fig.9 and fig. 10.From fig. 9 it can be seen that grain formation takes place from planer to equiaxed dendritic. Fig. 10 shows planar to cellular and cellular to dendritic transitions in 1100 Al welded with 4047 filler.



Fig. 9 Steps involved in equiaxed grain formation Refer to reference[61]

Fig.10 Planar to cellular and cellular to dendritic transitions in 1100 Al welded with 4047 filler. Refer to reference[61]

The fusion zone microstructure depends on the solidification behaviour of the weld pool, which controls the size and shape of the grains, segregation, and the distribution of inclusions and porosity. As constitutional supercooling increases, the solidification mode changes from planar to cellular and then dendritic. From fig.11, We observed that when supercooling increased then heterogeneous nucleation takes place which promotes equiaxed grain formation. When supercooling time increases then size of dendrite also increases. When heat input and welding speed increases then amount of equiaxed grain also increases.



Fig.11 Effect of welding speed and heat on heterogeneous nucleation

Fig.12 Variation in solidification mode across the fusion zone.

Grains grow in the planar mode along the easy growth direction <100> of the base meatl grains.

Fig.13 (a) 70Ax11V heat input and 5.1 mm/s welding speed, (b) 120Ax11V heat input and 12.7 mm/s welding speed. Orientation of columnar grains can be manipulated through low frequency arc oscillation (~ 1 Hz)



Fig.14(a) Transverse arc oscillation Fig. 14(b) Circular arc oscillation 4.2 EFFECT OF ELECTRODE DIAMETER ON WELD STRUCTURE:

Fig 15. Refer to reference[61] for Fig. No. 11,12,13,14 & 15

Table 1



Increase the electrode diameter will increase the heat input and this also increase the cooling time. We get coarse microstructure. When electrode diameter increased then weld bead size and HAZ size increases but amount of weld bead decreased. 5. STRESS RELAXATION Significant reductions of welding residual stress have been achieved in vibratory SAW, the axial residual stress in vibratory SAW is found to be slightly lower than that in normal SAW. It can be explained that the residual stress in axial direction with no constrains can be released more easily than that in the radial direction with constrains. It may be felt that the axial residual stress is fully relieved. So, the vibration applied during welding has less influence on the axial residual stress than the radial one[49]. Result of toughness measurement is given in Table 2 and also shown in fig. 16(a) and 16(b). During annealing process these toughness measurements compared for specimens under stationary and vibratory condition.

fig. 16 (a) stationary condition fig. 16 (b) vibratory condition

6. REVIEW ON NUCLEATION, GRAIN GROWTH , SOLIDIFICATION AND STRESS RELAXATION 6.1 STATIONARY CONDITION S.A. David et al.[1] concentrated on parameters that control the solidification of castings also control the solidification and microstructure of welds. However, various physical processes that occur due to the interaction of the heat source with the metal during welding add a new dimension to the understanding of the weld pool solidification.

Table 1

Refer to reference[24]



S. Kou [2] introduced important aspect of weld solidification is the dynamics of weld pool development and its steady-state geometry. Weld pool shape is important in the development of grain structure and dendrite growth selection process. T. DebRoy et al. [3]explained the recent theoretical developments include the formulation of a free-surface computational model to investigate coupled conduction and convection heat-transfer models to predict not only weld pool geometry but also thermal profiles to estimate thermal gradients and cooling rates critical to determining solidification structure. X. H. Zhan et al.[10] simulated the results and indicate that the average primary dendrite spacing changes during the solidification process in the weld pool because of the complicated thermal field, solute diffusion field and competitive growth. T. Koseki et al.[11] states that in recent years, with the advancements of computer technology, welding metallurgy and material science, it is feasible to simulate the grain morphology evolution during weld solidification process. X. H. Zhan et al.[18] have experimentally found that CA model is coupled with finite difference technique to constitute the multi-scale model which is used to simulate the solidification process especially the columnar dendritic grain growth in the weld pool. The finite difference technique is used to solve the thermal field of welding process and the microscopic model to calculate the dendrite growth in the weld pool. The advantage of the CA-FD model is that the macroscopic temperature field analysis and the microstructure simulation can be performed simultaneously. W. Kurz et al.[19]defined that the growth velocity at dendrite tip is important to simulate grain growth of metal solidification. In the simulation of grain growth with CA method, Kurz–Giovanola–Trevedi (KGT) model has been widely used to determine the S/L interface velocity. However, the KGT model is based on the assumption that the dendrite tip has an ideal parabolic shape and advances with a steady state velocity. E. Folkhard [20,21] states that Solidification cracking, also known as hot cracking, consists of fractures at the interdentritic and/or intergranular weld metal boundaries in the solidification process, during which the liquid phase of the mushy melt becomes rich in impurities, mainly sulphur (S) and phosphorus (P). Brooks, et al. [22] also states that stainless steels with a value of Creq/Nieq = 1.5 are susceptible to solidification cracking, while stainless steels with values of Creq/Nieq > 1.5 are immune to solidification cracking, or nearly so. The assessment of the propensity for cracking susceptibility of austenitic stainless steels is based on the concentration of P + S and on the values of the Creq/Nieq ratio, because the Creq quantifies the influence of the ferritizing elements while the Nieq quantifies the austenizing compounds. J. R. Davis[23] introduced that High-energy beam processes reduce the overall heat input. The high thermal gradient from the weld into the base metal creates limited metallurgical modifications and is least likely to cause intergranular cracking in butt joints when no filler metal is added Y. M. Zhang and S. B. Zhang [24]studied that the conventional gas tungsten arc welding process is modified by disconnecting the workpiece from the power supply and placing a second torch on the opposite side of the workpiece. Such a modification changes the direction of the current flow, improves the weld penetration and reduces the heat input. Using this modified process, 6061-T651 alloy was welded without filler metals. Analysis suggested the reduced heat input, the changed direction of the current flow and the symmetric heating were responsible for the observed reduction of the cracking sensitivity. Rappaz, M. et al. [25]states about new models arising from a better understanding of nonequilibrium solidification have suggested that strain rate may play a more direct role in the actual liquid fracture mechanism, e.g., controlling the pressure drop in the interdendritic liquid to initiate cracks . Braccini, M. et al. [26] defined that crack growth mechanisms have been less studied, but it appears that strain rate may likewise play a direct role, influencing the balance between transverse displacement, liquid feeding, and crack advancement .



A. Kostrivas, J.C. Lippold [27] derived following conclusion from their experimentation and found that Aluminium alloys exhibit a variety of microstructures within the fusion zone adjacent to the fusion boundary. Under conventional weld solidification conditions, epitaxial nucleation occurs off grains in the heat-affected zone (HAZ) and solidification proceeds along preferred growth directions. In some aluminium alloys, such as those containing Li and Zr, a non-dendritic equiaxed grain zone (EQZ) has been observed along the fusion boundary that does not appear to nucleate epitaxially from the HAZ substrate. C.E. Cross, T. Boellinghaus[28] explained that the construction of a special welding fixture has allowed the welding of an aluminium alloy to be performed under variable levels of restraint. It has normally been assumed that conditions of high restraint are most conducive to solidification cracking. This reasoning has no doubt evolved, in part, from the well defined behaviour of cold cracking in steels, assumed to apply equally to hot cracking. The difference is that cold cracking is caused by residual stresses after welding, whereas hot cracking is caused by local strains during welding. Experimental evidence in this study has demonstrated that conditions of low restraint are most likely to result in solidification crack initiation and growth. H. Yunjia et al.[29] investigated the effect of epitaxial growth which requires a minimal degree of undercooling prevail. In contrast the nucleation of new grains both at and near the fusion boundary necessitates a free energy barrier to be overcome. Consequently, no undercooling is necessary for nucleation. To initiate nucleation in the weld deposit and concurrently promote epitaxial grain refinement, it is essential to either increase the driving force, i.e., degree of undercooling or reduce the free energy barrier by introducing trace amounts of zirconium or titanium to the aluminium weld pool . Reddy et al. [30] observed the formation and presence of equiaxed grains in aluminum alloy 8090, in the solution heat-treated and aged condition, under gas tungsten arc (GTA) welding process. However, no such zone was evident in the as-cast condition of the alloy. No convincing hypothesis was put forth for the nucleation and growth of equiaxed grains. 6.2 VIBRATORY CONDITION S. Kou and Y. Le [31]proposed a new mechanism for reducing weld solidification cracking , based on the concept of the crack path and resistance to crack propagation, and its effectiveness was verified in magnetically oscillated GTA welds of a rather crack susceptible material 2014 aluminium alloy. This mechanism i.e., alternating grain orientation, was most pronounced in welds made with transverse arc oscillation of low frequency and high amplitude, and solidification cracking was dramatically reduced in these welds. S. Kou and Y. Le [32]investigated the effect of arc oscillation on grain structure and solidification cracking in GTA welds of 5052 aluminium alloy using a four-pole magnetic arc oscillator and a modified fish-bone crack test. Two different mechanisms of crack reduction were identified: one in the low frequency range of arc oscillation and the other in the high frequency range. The former was the alteration of the orientation of columnar grains, while the latter was grain refining. Neither mechanism was operative in the intermediate frequency range and solidification cracking was severe, especially when the amplitude of arc oscillation was small. Alteration of grain orientation was obtained in welds made with transverse and circular arc oscillations, but not longitudinal arc oscillation. Grain refining, on the other hand, was achieved in welds made with all three types of arc oscillation patterns. A S M Y Munsi et al. [33]investigated the effect of torsional vibration on residual stresses. Three types of shaft specimen were processed, namely (a) a homogeneous shaft, (b) a shaft welded on a circumferential line and (c) a spot-welded shaft. The first two types of shaft showed some redistribution in the residual stresses under applied torsional loads. On the spot-welded



shafts the residual stresses were found to decrease significantly at a very low level of vibration induced stress. Ch. Vives [34] studied the influence of electromagnetic vibrations (50 Hz) imposed during solidification on grain refinement in the 1085 and 2214 Al alloys. He observed extensive grain refinement in both the alloys due to imposed vibration. Claxton, R.A. et al. [35, 41 ] analyzed the effect of vibration on residual stresses of welded joints and concluded that vibration reduced residual stresses in welds. Few of the researchers used vibration processing before and after welding, the latter at low frequency (80 Hz) and measured residual stresses at several points in the middle section of the weld face. Their finding was that reduction in residual stresses in welds depended not on processing but on conditions of ensuing operations in use. They recommended that heat treatment is not always necessary. Dvornak et al. [36] while studying the solidification under vibratory conditions concluded that the grain refinement so observed was due to the lower energy required for the nucleation of the solid phase. However, the rapid removal of latent heat of solidification from the solid-liquid interface played a minor part in the grain refinement under vibration. Galyash et al.[37] applied low frequency vibration treatment for stabilizing welded and cast products. They developed and automated measuring system for vibrotreatment efficiency control. Control method was based on variation of amplitude-frequency characteristics of the structure(displacement of resonance peaks, changes of their width and loss coefficients) prior to and after vibrotreatment. They concluded that vibrotreatment does not reduce cyclic durability of welded joints and resulted in a considerable reduction of deformations. The complex (technique) developed is recommended for size stabilization of structures. Izdinska [38,39] studied the effect of ultrasonic treatment upon fatigue properties of welded joints. His finding was that due to ultrasonic reduction macroscopic residual stresses had a favourable effect on the fatigue properties. This finding was rather favourable when compared with annealing. Miclosi et al. [40] found good effect of electromagnetic oscillation upon the characteristics of weld. They found that the present of the electromagnetic axial pulsation of the electric arc led to smaller penetration and enlarged the width of the weld. This is favourable effect from the point of view of heat cracking because deep and narrow weld run a higher risk of heat cracking. Sobolev [42] studied the effect of liquid metal solidification during welding process in the ultrasonic field and analyzed that critical undercooling provoked volumetric nucleation and critical pressure which was near the cavitation bubbles caused breakage of the formed crystals. Watanabe and Nakamura [44] investigated the effect of electromagnetic stirring on microstructure of SVS 310S. They examined the parameters to achieve grain refinement like magnetic field intensity, the frequency of alternating stirring and the relative distance from electrode to magnetic field centre. Bead on plate TIG weld were made under the condition that welding current was 60A and travel speed was 3 cm/minute. A significant decrease in the grain size of weld metal could be achieved when the electrode was located 1-2 cm apart from the magnetic field centre in the welding direction and the stirring frequency was 0.5-1 Hz. This might be due to fragmentation and the increase in constitutional supercooling ahead of solidification interface due to the molten metal stirring assisted by the weld metal in grain refining. Wei [45] introduced longitudinal steady state sinusoidal vibrations into the unidirectional dendrite solidification process of Al-3 Mg alloy in first (470 Hz), second (1050 Hz) and third (1736 Hz) order resonant frequencies of solidification system to produce strong vibrational



response out of less exciting energy. He observed that alloy mechanical properties are appreciably improved if the second and third order resonant frequencies are applied. Wuich [46] while discussing the vibration method (VSR) explained that welding, forging and other processes induced internal stresses in Fe based structural parts which led to deformations and dimensional tolerance reductions. The vibration method is an alternative to heat treatment and offers a number of technical and economical advantages. The iron and steel parts weighing from 50 Kg upto 200 tons were vibrated-100 Hz for 30 minutes to relieve the internal stresses for the material structure. Yamamoto et al. [47] vibrated molten puddle with low frequency pulsed MIG welding process. The degree of this vibration depended on switching frequency of unit pulsed conditions and there was an optimum frequency range of 10-30 Hz. Large difference between two pulsed current values promoted the molten puddle vibration. This gave rise to molten puddle stirring which resulted in remarkable grain refinement of weld structure of commercially available Al-Mg alloy base metal (A 5052) and wire within the optimum frequency range, especially 30 Hz. Yoneda et al. [48] investigated the primary crystal morphology and the mechanical properties in hypoeutectic Al-Cu alloys (e.g. Al-6Cu, Al-11Cu, Al-15Cu) vibrated mechanically during primary solidification. The relationships between primary crystal morphology and the strength were also considered. Due to vibration primary Al morphology is refined and distributed uniformly. They also observed that with increasing frequency the tensile strength of specimen increases. However, it was more frequent that the fracture propagated through the primary crystal and the eutectic, consequently the tensile strength of vibrated specimen increased. Lu Qinghua et al. [49]observed the finer subgrain structure and higher dislocation density in V-SAW specimen. Because of the existence of fine subgrain, there are more subgrain boundaries where the dislocations are hindered and piled up. These sub-boundaries act as barriers for the crack to propagate. Therefore, vibration induced in welding favours this dynamic recrystallization and improves weld properties. The vibratory welding method has produced some of the other useful changes effects besides grain refining. Lu Qinghua et al.[52] found that the microscopic structure has dramatically changed after V-SAW. Vibratory energy breaks up the growing dendritic grains in the weld and the HAZ. A significantly higher weld pool velocity which leads to a faster the heat removal during solidification is produced in VWC. Thus, the higher the cooling rate, the more the nuclei coming into play, and the smaller the grain size. Lu Qinghua et al.[49]states that Vibration facilitates the release of dissolved gases and the resulting weld beads greatly exhibit reduced porosity. Another beneficial effect is to facilitate drifting inclusions to the slag with the imposed vibration from the molten welding pool. Moreover, the mechanical energies provided by external vibration increase the boundary plastic deformation and dislocation density. Then the dislocations with high density will tangle and pile up to combine to make small-angle sub-boundaries .The welding maximum and minimum residual stresses have been achieved reduced through the application of vibratory welding and the vibration applied during welding has less influence on the axial residual stress than the radial one due to no constraint in axial direction. Jijin Xu et al. [51] found VWC can reduce the residual hoop stresses at the outer surface and the maximum residual stresses; but VWC has only a slight effect on the residual axial stresses at the outer surface. The residual stresses are lower than the yield strength when using VWC, which improves the safety of welded structures VWC makes the residual stresses decrease and their peak values are lower than the yield strength. Therefore, VWC can decrease the susceptibility of a weld to fatigue damage, stress corrosion cracking and fracture, and improve the safety of welded structures.



S. Spooner et al[53] analyzed 300-type stainless steel plates and found that the residual stresses within the HAZ and base metal in the conventionally welded plate and in the vibratory-treated plate exhibit small differences which are comparable to the estimate of experimental error. This comparison is limited to results in the HAZ and base metal. On the basis of these residual stress measurements no degradation of mechanical properties from vibratory treatment during welding is expected. J.S Hornsey[54] found that The Vibratory stress relieving can be employed for stabilisation of the size of suitable weldments prior to their machining and servicing as a replacement of stress relief annealing. The VSR process is used for lowering of residual stresses and stabilisation of the size of different weldments such as frames of forming machines, machine frames, grey cast iron castings, etc. which were up to now subjected to stress relief annealing. VSR does not negatively affect the static dynamic strength of welded joints and weldments, fracture and notch toughness and homogeneity of welded joints. Based on the attained data the implementation of VSR procedures as a replacement of stress relief annealing for the stabilisation of weldments, castings and forging leads to high savings of production costs to our national economy. I. A. Shulyak et al.[55] Compared with the existing SM-402 vibratory sieves, the SNV sieving machine is characterized by a larger specific throughput, a noise and external vibration level reduced by between one-half and two-thirds, and effective self-cleaning of the screening surface. The anticipated saving resulting from the adoption of this machine is 14,800 rubles per annum. William F. Hahn [56]studied that both resonant and sub-resonant vibrations can relieve residual stresses in parts. Resonant VSR produces the greatest residual stress relief. Subresonant VSR can get stress relief, whose effect depends on the driven frequency employed. The larger tip deflection the driven frequency produces, the greater reduction of residual stresses. Larger excitation amplitude produces greater residual stress relief. Stress reduction is greater for parts with lower level of initial residual stresses. B.Pucko,V.Gliha[57] introduced that there is a positive effect of vibration during welding on impact toughness. Vibration stabilizes microstructures to become more resistant to heat affects that could minimize impact toughness. Type of fracture turns more ductile with vibrating during welding. Tseng, C.F et al. Shows [64]studied the influence of vibration after welding which intensifies effects of heat affect like microstructural changes in some temperature ranges. Vibration during welding holds back such changes. the fracture toughness is proportional to the size of subgrains. Kou, S., Le, Y., [65]explained the subgrains formation by VWC in the presence of forced vibration in the melt. The vibration of liquid metal can contribute to increasing the rate of heat transfer and the removal of liquid superheat, which decreases the likelihood of remelting of initial solid grains. The temperature gradient from the centre to the edge of the pool is decreased and the undercooled zone is dispersed in the entire bulk liquid. Fragmentation, nucleation and growth can happen within the entire melt, which give rise to the refinement of grains. In addition, the velocity in VSAW with the help of vibration is greater than that of non-vibratory weld pool . Flemings, M.C. [66]found that significant higher weld pool velocity apparently produces a higher cooling rate during solidification. According to the principles of solidification. 7. DISCUSSIONS AND CONCLUDING REMARKS The current study indicated that the primary dendrite spacings have influence on the growth of the secondary dendrite arms. In the zones of dense nucleation, the secondary and tertiary dendrite arms do not generate substantially. Owing to the nucleation is dense, the primary dendrite spacing is relatively small and it should be pointed out that this condition is common in the edge of the weld pool. The lack of space between dendrites induces solute accumulation and remarkably reduces the solute concentration gradient. It can be found that the grain morphologies and their evolution process in the weld pool are influenced not only by the cooling



conditions but also by the nucleation conditions, competitive growth and the existing microstructure, etc. The study of the previous work reviews states that a vibration reduces residual stresses in welds .Some researchers described that removal of latent heat of solidification from the solid –liquid interface played a minor part in the grain refinement under vibration and also recommended that heat treatment is not always necessary, whereas some researchers applied low frequency vibration treatment for stabilizing welded and cast product and also found that vibration facilitates the release of dissolved gases and the resulting weld beads greatly exhibit reduced porosity. Some researchers derived different theory to initiate nucleation in the weld deposit and concurrently promote epitaxial grain refinement, it is essential to either increase the driving force, or reduce the free energy barrier. Few of the researchers observed that alloy mechanical properties are appreciably improved if the second and third order resonant frequencies are applied. Thus the higher the cooling rate, the more the nuclei coming in to play and the smaller the grain size. Finer grain size benefits the mechanical properties . Vibration stabilizes microstructures to become more resistant to heat affects that could minimize impact toughness and fracture turns more ductile with vibration during welding. Some researchers realizes that vibratory weld condition can reduce the residual hoop stresses at the outer surface and the maximum residual stresses; but VWC has only a slight effect on the residual axial stresses at the outer surface. The vibration method is an alternative to heat treatment and offers a number of technical and economical advantages. 8. REFERENCES [1] S.A. David, S.S. Babu,and J.M. Vitek ,Welding: Solidification and Microstructure , The Minerals, Metals &

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