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Solidification of GTA Aluminum Weld Metal: Part I - Grain Morphology Dependent upon Alloy Composition and Grain Refiner Content
Abstract
The solidification conditions during welding strongly influence the weld metal microstructure and mechanical properties of a weld. In the first part of this study, the grain morphology of gas tungsten arc (GTA) bead-on-plate welds was investigated for the aluminum Alloys 1050A (Al 99.5), 6082 (Al SilMgMn), and 5083 (Al Mg4.5Mn0.7). The experiments revealed that increasing welding speed and alloy content allow the growth of small, equiaxed grains, particularly in the weld center. Furthermore, increasing grain refiner additions led to a strong reduction of the weld metal mean grain size and hence facilitated the columnar to equiaxed transition (CET). In addition, wavelength dispersive X-ray spectroscopy (WDS) and transmission electron microscopy (TEM) analysis revealed in the weld metal TilB2 particles that were surrounded by Al 3Ti. This suggests the duplex nucleation theory for nucleation of aluminum grains in GTA weld metal.
... These calculations agreed with experimental data Int J Adv Manuf Technol stray grain formation at the weld centerline increases but is simultaneously in competition with the refinement tendency of the microstructure due to very high undercooling. The solidification condition in the weld is controlled in part by the thermal gradient (G) and solidification growth rate (R) [4,16]. Thermal gradients (G) are minimum at the weld centerline and maximum on the weld side because of the large heat extraction along the colder base metal, thus inducing the elongation of the weld pool from circular to teardrop shape at fast welding speeds. ...
... Small G R ratios are associated with high constitutional undercooling ahead of the solid-liquid interface and equiaxed grain structures [17]. Hence, equiaxed solidification tends to occur unaided in the centerline region of the weld pool, where solidification rates (R) are the highest and the thermal gradients (G) are the flattest due to the distance of the arc (Fig. 5), and is generally associated to fast welding speeds and important undercooling [9,16,18]. Columnar grain structure was always found predominantly at the weld interface where high G R ratios exist. The variation in both local solidification rate and thermal gradient in a single weld on moving around the fusion boundary from the side to the weld center causes a progressive change in solidification substructure across an individual weld bead [9]. ...
... Faster welding velocities were investigated for autogenous GTA full-penetration welds on 3-mm-thick sheets of 1050, 6082 and 5083 aluminum alloys [16]. The welding speed was varied from 2 to 11.5 mm s −1 , and the current was adjusted between 170 and 200 A to maintain a constant full-penetrated weld bead size. ...
Solidification cracking is a weld defect common to certain susceptible alloys rendering many of them unweldable. It forms and grows continuously behind a moving weld pool within the two-phase mushy zone and involves a complex interaction between thermal, metallurgical, and mechanical factors. Research has demonstrated the ability to minimize solidification cracking occurrence by using appropriate welding parameters. Despite decade’s long efforts to investigate weld solidification cracking, there remains a lack of understanding regarding the particular effect of travel speed. While the use of the fastest welding speed is usually recommended, this rule has not always been confirmed on site. Varying welding speed has many consequences both on stress cells surrounding the weld pool, grain structure, and mushy zone extent. Experimental data and models are compiled to highlight the importance of welding speed on solidification cracking. This review is partitioned into three parts: part I focuses on the effects of welding speed on weld metal characteristics, part II reviews the data of the literature to discuss the importance of selecting properly the metrics, and part III details the different methods to model the effect of welding speed on solidification cracking occurrence.
... TiB 2 has been used as grain refiner for aluminum ingot casting for decades. The transmission of refining welds with TiB 2 did not find industrial application, although it was demonstrated to be efficient at restricting solidification cracking [114]. The solution is directly transferable to WAAM [179,180]. ...
... An increased v induces equiaxed grain growth and suppresses cracking. Reproduced with permission from Schempp et al., Welding Journal; published by the American Welding Society, 2014[114]. ...
Processing of aluminum alloys by wire arc additive manufacturing (WAAM) gained significant attention from industry and academia in the last decade. With the possibility to create large and relatively complex parts at low investment and operational expenses, WAAM is well-suited for implementation in a range of industries. The process nature involves fusion melting of a feedstock wire by an electric arc where metal droplets are strategically deposited in a layer-by-layer fashion to create the final shape. The inherent fusion and solidification characteristics in WAAM are governing several aspects of the final material, herein process-related defects such as porosity and cracking, microstructure, properties, and performance. Coupled to all mentioned aspects is the alloy composition, which at present is highly restricted for WAAM of aluminum but received considerable attention in later years. This review article describes common quality issues related to WAAM of aluminum, i.e., porosity, residual stresses, and cracking. Measures to combat these challenges are further outlined, with special attention to the alloy composition. The state-of-the-art of aluminum alloy selection and measures to further enhance the performance of aluminum WAAM materials are presented. Strategies for further development of new alloys are discussed, with attention on the importance of reducing crack susceptibility and grain refinement.
... Martin et al. (2017) have found that improving the heterogeneous nucleation by adding ZrH 2 nanoparticles as nucleant promotes equiaxed grain structures and high-strength aluminium alloys during AM. Schempp et al. (2014) have reported that a constitutionally supercooled zone was formed in the solute-enriched liquid ahead of the solid/liquid (S/L) interface. ...
... The achieved 3D fine equiaxed β-grain structure is shown in Fig. 5b. In the first several layers, the ratio between the thermal gradient G and the solidification rate R, i.e., an important indicator of the solidification patterns (Liu et al., 2018;Schempp et al., 2014), is large due to cold substrate, thus producing columnar grains. On the contrary, the ratio G/R becomes much smaller away from the substrate, indicating high nucleation probability. ...
A computational framework is developed to investigate the process-structure-property relationship for additive manufacturing (AM) of Ti–6Al–4V alloy. The proposed model incorporates experimentally informed two-phase α+β morphologies within prior β-grains, which are widely observed in the as-built AM components. Specifically, the temperature-dependent phase-field model (PFM) is used to simulate the evolution of various grain morphologies, e.g., columnar and equiaxed grain structures. The proposed PFM taking into account both of the epitaxial grain growth and the constitutional cooling-driven heterogeneous nucleation enables us to capture the columnar to equiaxed transition (CET) of grain structures. The thermal fields concerned with the scanning strategies and manufacturing parameters are simulated using a finite-element model (FEM). The Burgers orientation relation (BOR) is further utilized to generate two-phase α+β morphologies within prior β-grains, accompanied by the transformation of crystal orientations, i.e., (0001)α//{101}β and <112‾0>α//<111>β. Finally, a fast Fourier transform-based elasto-viscoplastic (EVP-FFT) model is employed to predict the micromechanical behaviors and properties for the two-phase α+β microstructures. The presented PFM-based formulation is generally applicable to predict the process-structure-property relationship for additive manufacturing of a variety of alloy systems, e.g., titanium alloys, aluminum alloys and nickel-based superalloys.
... Another important phenomenon is the grain nucleation ahead of the solidification front, which is associated with local thermal conditions (i.e., thermal gradient G and solidification rate R). In general, low G/R values at the pool top will result in large constitutional undercoolings that encourage grain nucleation [50,51]. The introduction of new grains would directly interrupt the original epitaxial grain growth and even give rise to equiaxed structures in extreme cases. ...
... The columnar/equiaxed delineation in Fig. 6 suggests a small-equiaxed region in the space of low power and high velocity. This is consistent with the previous experimental observation of finer grains with increasing beam speed [76], as well as the indication of the classical nucleation criteria [50]. That is, in this P-v space, correspondingly low-thermal gradient and high solidification rate tend to be developed, thus facilitating grain nucleation and equiaxed grain structure formation as indicated by the classical nucleation criteria. ...
The presence of various uncertainty sources in metal-based additive manufacturing (AM) process prevents producing AM products with consistently high quality. Using electron beam melting (EBM) of Ti-6Al-4V as an example, this paper presents a data-driven framework for process parameters optimization using physics-informed computer simulation models. The goal is to identify a robust manufacturing condition that allows us to constantly obtain equiaxed materials microstructures under uncertainty. To overcome the computational challenge in the robust design optimization under uncertainty, a two-level data-driven surrogate model is constructed based on the simulation data of a validated high-fidelity multi-physics AM simulation model. The robust design result, indicating a combination of low preheating temperature, low beam power and intermediate scanning speed, was acquired enabling the repetitive production of equiaxed-structure products as demonstrated by physics-based simulations. Global sensitivity analysis at the optimal design point indicates that among the studied six noise factors, specific heat capacity and grain growth activation energy have largest impact on the microstructure variation. Through this exemplar process optimization, the current study also demonstrates the promising potential of the presented approach in facilitating other complicate AM process optimizations, such as robust designs in terms of porosity control or direct mechanical property control.
... In addition, columnar grain morphology having epitaxial growth tendency occurring in the weld bead also causes inhomogeneous mechanical properties in the structure. Thus, this region is a significant point to be emphasized that also determines the performance and service life of the workpiece [71,72]. Therefore, grain refining, which enhances mechanical properties, is an important issue for arc-welded joints [70]. ...
Joining aluminum alloys with arc welding methods is frequently subject to literature and industrial applications. Although aluminum alloys have different difficulties in the arc welded process, the formation and elimination of solidification cracking defects is a more complex phenomenon. Since avoidance of this defect requires specific approaches and methods, special attempts and improvements have been studied frequently on this subject in recent years.
Studies in the literature have clearly shown that this defect, which is often encountered in aluminum alloys, takes place along the grain boundaries. Therefore, the major approach to eliminate this defect is activating nucleation and decreasing the grain size. In this context, modification approaches in the literature, which are frequently used for arc welding of aluminum alloys, have been developed to use three different mechanisms including heterogeneous nucleation, dendrite fragmentation, and grain detachment. While it is aimed to increase heterogeneous nucleation by reinforcing filler metals with compounds in the inoculation approach; dendrite fragmentation and grain detachment are also aimed in the approaches where external effects and forces are used. Within the frame of references, it is also possible to review the external factors aiming to improve weld pool convection and thermal conditions under two headings, which are weld pool stirring and pulsed arc current approaches. The weld pool stirring approach also includes ultrasonic treatment and magnetic arc oscillation methods.
In this study, solidification cracking defect that frequently occurs in the arc welding of aluminum alloys is explained fundamentally and the attempts to eliminate this defect are presented as a review paper in a comprehensive manner.
... Thus, it cools slowly, resulting in low G and high R. With low G/R, a columnar dendrite growth with distinctively developed secondary arms is formed in the center part of the weld bead [28]. This microstructural transition was observed during the melt pool solidification process of several different materials. ...
The microstructural and mechanical evaluation of 9% Ni steel with flux-cored arc welding (FCAW) was performed with two different Ni-based weld metals: Inconel 625 and Hastelloy 609. The weld metals showed microstructural changes depending on the temperature gradient and crystal growth rate for each region during the cooling after welding. A cellular/planar growth was exhibited at the bottom of the weld metal, which was rapidly cooled in contact with the cold base metal. Columnar dendrites were exhibited in the central region that cooled relatively slowly, and precipitates were observed in the interdendritic region. The weld joints between the base metal and weld metal have a compositional transition region due to dilution. The transition region comprised a martensite layer and a γ-phase cellular/planar layer, according to the compositional distribution. In the low-temperature toughness test, the absorbed impact energies were 89 and 55 J for Inconel 625 and Hastelloy 609, respectively. When Inconel 625 is used as the weld metal compared to Hastelloy 609, the high content of the γ-stabilizer and martensite start temperature decreasing elements leads to the formation of a thicker γ-phase layer and thinner martensite layer in the transition region. In addition, the high content of these elements suppresses the martensite transformation and maintains the stability of the weld joint interface even at low temperatures, resulting in the higher absorbed impact energy. The microstructure of weld joints and its influences on mechanical properties help to improve the practical application of 9% Ni steel FCAW.
... intermediate alloy [13], which has a better refinement effect on silicon containing hypoeutectic aluminum alloy. Several experiments have found duplex nucleation [14][15][16][17][18][19][20][21], especially at the core of primary silicon. L.N. Yu et al. [18,19] found that TiB 2 or Al 4 C 3 particles can absorb AlP particles and have a peritectic-like coupling with them, forming a large number of coupling compounds that serve as heterogeneous nucleating substrates for primary silicon grains. ...
Duplex nucleation and its effect on the grain size and properties of near eutectic Al-Si alloys were investigated in this work. It is found that the grain size of Al-13Si-1Cu alloy can be greatly refined by addition of Al-2Ti-0.5B-0.5C and Al-P master alloys, and TiB2/AlP duplex nucleation was observed at the nuclei of primary silicon in the process of studying the refinement mechanism. TiB2 phase coated by Al co-existed with AlP in the nuclei of primary Si. The existence of Al phase should not only guarantee the promotion of TiB2 particles on the nucleation of AlP particles, but also made TiB2 particle stable in the core of primary silicon through the interaction with the Si phase. Duplex nucleation can not only affect the grain size of Al-Si alloy, but also effect of the distribution of strengthening phases in Al-Si alloy and improve the properties. Compared with single Al-P master alloy treatment, the tensile strength of Al-12Si-4Cu-2Ni-1Mg alloy after duplex nucleation treatment are improved obviously. The fatigue performance of Al-12Si-4Cu-2Ni-1Mg alloy is also improved significantly by the duplex nucleation.
... Especially, they proposed an overlap welding procedure for identifying the nucleation mechanism in welds. Applying heterogeneous nucleation mechanism, Schempp et al. [7][8][9] realized remarkable grain refining and performance improvement through the addition of grain refiner in GTA welds. For magnetically stirred GTA welding, Pearce and Kerr [10] suggested grain detachment as the nucleation mechanism of equiaxed grains. ...
The nucleation mechanism in laser welds of aluminum alloys is still unclear. Here, we identified it for the first time by adopting an overlap welding procedure. The commonly used 5083 aluminum alloy was taken as an example. The variation of grain structure and grain size in different overlap welding cases was characterized by electron-backscattered diffraction. Results revealed that the nucleation mechanism was heterogeneous nucleation, rather than grain detachment and dendrite fragmentation. Furthermore, the chemical composition and phase structure of the heterogeneous nuclei were tested by energy-dispersive spectrometer and transmission electron microscopy, which indicated that they were D022 Al3Ti phase. The present study is significant in that it can provide a theoretical basis for grain refining in laser welds of aluminum alloys.
Over the last decade, developments in metal additive manufacturing (AM) have opened up new possibilities in various industries. Current metal AM technologies are now capable of processing a larger selection of metals, including steel mold materials such as H13 and P20. In the injection molding industry, mold makers have implemented metal AM technologies to 3D print steel molds. The main challenge currently faced by mold makers is to 3D print steel molds with mechanical properties that are comparable with conventionally made ones. Research on the microstructure evolution in 3D printed steel molds provide the necessary information for tailoring the mold’s microstructure and improving its mechanical properties. This review presents a unique perspective on the microstructure evolution in 3D printed steel molds. The microstructure evolution is discussed according to two major processing stages. Stage 1 describes the formation of the mold’s microstructure in as-built condition after it has solidified from its molten state. Subsequently, Stage 2 describes the changes in the mold’s microstructure post-heat treatment. This review also summarizes the various experimental techniques and numerical models used to study the microstructure evolution in 3D printed components. Advances in experimental microstructure characterization techniques enable researchers to investigate microstructure evolution in situ during metal AM processes. Coupled thermal-microstructure numerical models serve as an alternative approach for predicting grain growth in 3D printed components. The review concludes by summarizing future prospects in mold making and metal AM research in general.
In order to simulate the columnar-to-equiaxed transition (CET) in the entire molten pool of Al–Cu alloy laser welding, a phase-field model was established by considering crystalline orientations and heterogeneous nucleation. Simulation results revealed that crystals initialized as planar ones from the molten pool edge and were then transformed into columnar dendrites during the solidification process. It was found that dendrites grew toward the center of the fusion zone irrespective of their crystalline orientations. Equiaxed grains grew ahead of columnar dendrites. They gradually formed a belt ahead of columnar dendrites and stopped columnar dendrites from growing. The highest number of equiaxed grains was found at the top edge of the cross-section of the molten pool due to the fastest pulling velocity. The variation of undercooling in the molten pool was analyzed to investigate CET during the solidification process. Heterogeneous nuclei were formed throughout the solidification process. They melted during the early stage of the solidification process and started to grow when the undercooling ahead of columnar dendrites was large enough. With the prolonged solidification, the number of heterogeneous nuclei gradually increased and the size of equiaxed grains decreased. Experimental results were consistent with simulations.
The mechanism of grain refinement by the addition of small amounts of titanium, molybdenum, zirconium, tungsten, and chromium to aluminum was investigated. The results indicate that the grain refinement is caused by the peritectic reaction which, by transforming the intermetallic compound into crystals of aluminum solid solution, seeds the melt with nuclei above the freezing point of aluminum.
The basic weldability of the alloy Al2.2Li-2.7 Cu was investigated and compared with two common aluminum alloys: 2024 and 5083. The basic weldability was characterized by the susceptibility of the alloy to hot tearing as determined by the trans-Varestraint test. The hot tearing susceptibility was characterized through plots of total crack length and maximum crack length versus augmented strain. These data were correlated with thermal profiles taken of the weld to provide additional characterization through the use of the brittle temperature range and critical strain rate for temperature drop parameters. The second part of the investigation explored the effect of grain refiner additions upon enhancing the weldability of the alloy. The results of a study showed the Al-2.2Li-2.7Cu alloy to have a high susceptibility to hot tearing. Additions of titanium and zirconium were found to enhance weldability through refinement of the grain structure and alteration of the shape and distribution of the eutectic phase in the weld.
The effect of scandium and titanium–boron (Tibor) additions on the solidification behaviour of castings and welds of aluminium alloy 7108 has been investigated. A circular patch test was adopted to evaluate the effects of these elements on the hot cracking suscepti bility of welds made on cast coupons treated with different grain refiner additions. It was observed that grain size, as well as cracking susceptibility, decreased with increasing amounts of scandium and that hot cracking was completely eliminated at scandium additions above 0·25 wt-%. A more pronounced grain refining effect in welds was observed with Tibor and, in addition, no hot cracking was observed with Tibor additions as low as 0·02 wt-%Ti (0·004 wt-%B). Castings, however, were more effectively grain refined with scandium, achieving a finer grain size than with Tibor
This article conveys the general strategy in controlling the grain size of various commercial aluminum alloys. Methods for refining common alloys are discussed, as well as the manner in which these apply to the increasing use of high-silicon and Zrcontaining alloys. A comprehensive grain refinement strategy involves adding inoculant particles that provide seed crystals for primary aluminum, as well as additions of other elements that control the growth rate of the aluminum grains. The present work explains the importance of these different goals and gives general advice on how to approach them in an industrial environment. Difficulties encountered with the various inoculant systems are also discussed from a casthouse standpoint.
Control of grain size involves control of nucleation as well as growth. A critical review involving criteria for heterogeneous nucleation like particle size, abundance, stability, epitaxy and so forth will be given. Some potential candidates for heterogeneous nucleants are discussed in detail. Being able to supply a sufficient number of potent nuclei constitutes the basis for grain refinement. But the final grain size will be determined by the subsequent growth rate of α-Al crystals. This has been found to be controlled by a growth restriction factor [(k i-1)m iC o] which, at least for low alloying levels, seems to be additive in nature. Experimental evidence shows a range of composition where optimum grain size can be achieved. This can be related to a change over from cellular to dendritic growth in the equiaxed zone.
Solidification, in the sense used in this context, is the process by which a liquid is transformed into a crystalline solid. In crystal growth the solid that forms first is solvent rich as distinct from crystallisation, in which the crystals that are formed are solute rich. It is not always possible to make a clear distinction. Solidification is important as the process employed in the widely used process of casting, in all its forms from large ingots of steel to small crystals of silicon. While in principle it would seem simple to convert a homogeneous liquid into an equally homogeneous perfect crystal, this is extremely difficult, if not impossible to achieve in practice. Thorough understanding requires that the process be studied at various levels, which can be conveniently described as the angstrom level, the micron level and the centimetre level.
The thermal expansion coefficients for Al and TiB2 have been determined by refining the unit cells measured by means of high-temperature powder X-ray diffraction. The lattice expansion coefficients were found to be linear within the measured temperature range 300 to 900 K: 2.79 · 10-5 K-1 for Al; 3.56 · 10-6 K-1 for TiB2 in the a-axis direction and 6.13 · 10-6 K-1 for TiB2 in the c-axis direction. These data have been used to calculate the lattice mismatch between the two phases at the melting point of aluminium. This is of interest since TiB2 particles often are added to Al melts as a nucleating agent in order to induce grain refinement. It was found that the fit is better at the melting temperature of aluminium than at room temperature.
A microstructural study was carried out to better understand one of the most important aspects of weld metal solidification, i. e. , nucleation and grain refining. The microstructure in the welding zone was preserved by quenching during welding and was examined afterward. Heterogeneous nuclei in the weld pool, as well as partially melting grains along the leading portion of the weld pool boundary and growing dendrites along the trailing portion of the weld pool boundary, were clearly revealed. Based on the microstructure observed, three different mechanisms of weld metal nucleation and grain refining were discussed, i. e. , dendrite fragmentation, grain detachment and heterogeneous nucleation. A method was presented for identifying the nucleation and grain refining mechanism of the weld metal, by changing the microstructure around the weld pool boundary and dissolving heterogeneous nuclei through overlap welding, and by examining the grains of the weld metal with electron microscopy. Welds of commercial 6061 aluminum alloys were used as an example for demonstrating this mechanism identification method.
An overview is given of grain refinement in aluminum casting alloys. The mechanisms involved and the benefits of refinement are described. The review shows that current practices were developed long before modern Al-Ti-B refiners became available, and are employed now largely for historical reasons.
The results of tests in Al-Si, Al-Si-Cu, Al-Cu, Al-Mg and Al-Zn-Mg alloys are presented. The grain refining response is different for each alloy system. It is important to understand that titanium can be present in two forms. One dissolves in aluminum; the other is nearly insoluble. Each must be controlled separately.
With today’s powerful Al-Ti-B refiners, there is no reason for large additions of soluble titanium in most alloys. In fact, it is better to say we grain refine with boron, not titanium. The recommended addition is 10–20 ppm of boron, preferably in the form of Al-5Ti-1B or Al-3Ti-1B rod. Lower dissolved titanium levels provide better grain refinement and an improved resistance to hot cracking in some alloys. Al-Si casting alloys which contain copper are an exception. In alloys such as 319 or 355, it is best to have a minimum of about 0.1% Ti.
A study is made of the various metallurgical phenomena (competitive grain growth, constitutional supercooling, etc. ) which occur during the solidification of a weld pool in TIG welding of various types of steels and alloys. The resulting solidification sub structures are examined. On the basis of the different phases of solidification, an analysis is made of the origin of the defects which occur in the weld, such as microsegregation, hot cracking, and hydrogen cracking. The role of delta ferrite in austenitic stainless steels is also studied and the different zones of the welded joint are redefined.