Showing papers on "Liquation published in 2011"

Formability and failure mechanisms of AA2024 under hot forming conditions

Abstract: Aluminium alloy 2024 (AA2024) is extensively used as a structural material in the aircraft industry because of its good combination of strength and fatigue resistance. However, complex shaped components, particularly those made from sheet, are extremely difficult to form by traditional cold forming due to its low ductility at room temperature. A possible solution of this problem is to form sheet workpieces at elevated temperature. The aim of the work described in this paper is to determine the relationship between formability and temperature for AA2024 by conducting a series of tensile tests at elevated temperatures ranging from 350 to 493 °C. Ductility of AA2024 was found to increase gradually with increasing temperature up to 450 °C, followed by a sharp decrease with further increase in temperature. So-called cup tests confirmed that the formability of AA2024 is very high at a temperature of about 450 °C. Fracture surfaces and longitudinal sections of formed samples were examined by scanning electron microscope. It was found that fracture occurred in three different modes depending upon the temperature, and the sharp decrease in ductility when the temperature exceeds 450 °C was caused by softening of grain boundaries by solute enrichment (at higher heating rates liquation may be involved) and softening of the matrix around inclusion particles.

Analysis of Microstructural Changes Induced by Linear Friction Welding in a Nickel-Base Superalloy

Abstract: A detailed microstructural analysis was performed on a difficult-to-weld nickel-base superalloy, IN 738, subjected to linear friction welding and Gleeble thermomechanical simulation, to understand the microstructural changes induced in the material. Correlations between the microstructures of the welded and simulated materials revealed that, in contrast to a general assumption of linear friction welding being an exclusively solid-state joining process, intergranular liquation, caused by nonequilibrium phase reaction(s), occurred during joining. However, despite a significant occurrence of liquation in the alloy, no heat-affected zone (HAZ) cracking was observed. The study showed that the manufacturing of crack-free welds by linear friction welding is not due to preclusion of grain boundary liquation, as has been commonly assumed and reported. Instead, resistance to cracking can be related to the counter-crack-formation effect of the imposed compressive stress during linear friction welding and strain-induced rapid solidification. Moreover, adequate understanding of the microstructure of the joint requires proper consideration of the concepts of nonequilibrium liquation reaction and strain-induced rapid solidification, which are carefully elucidated in this work.

On preventing HAZ cracking in laser welded DS Rene 80 superalloy

Abstract: The heat affected zone (HAZ) cracking behaviour in a laser beam directionally solidified (DS) Rene 80 nickel based superalloy subjected to preweld heat treatments was studied. The HAZ cracks in the alloy are grain boundary liquation cracks caused by liquation reaction of both non-equilibrium secondary solidification product, MC carbides and equilibrium solid state reaction product, γ′ precipitates. In contrast to theoretical prediction based a preweld heat treatment that reduced grain boundary liquid film thickness did not result in a lower HAZ cracking, which can be related to concomitant reduction in the ability of the base alloy to relax welding stress. In addition, formation of intergranular M5B3 boride particles in preweld alloy appeared to have aided cracking susceptibility by lowering grain boundary liquation temperature and widening the brittle temperature range in the HAZ during cooling. Based on the analysis of the results, application of a new preweld heat treatment that prevents the fo...

Crack-Free Welding of IN 738 by Linear Friction Welding

Abstract: Inconel 738 (IN 738), like other precipitation-hardened nickel-base superalloys that contain a substantial amount of Al and Ti, is very difficult to weld due to its high susceptibility to heat-affected zone (HAZ) cracking during conventional fusion welding processes. The cause of this cracking, which is usually intergranular in nature, has been attributed to the liquation of various phases in the alloy, subsequent wetting of the grain boundaries by the liquid and decohesion across one of the solid-liquid interfaces due to on-cooling tensile stresses. In the present work, crack-free welding of the alloy was obtained by linear friction welding (LFW), notwithstanding the high susceptibility of the material to HAZ cracking. Gleeble thermomechanical simulation of the LFW process was carefully performed to study the microstructural response of IN 738 to the welding thermal cycle. Correlation between the simulated microstructure and that of the weldments was obtained, in that, a significant grain boundary liquation was observed in both the simulated specimens and actual weldments due to non-equilibrium reaction of second phase particles, including the strengthening gamma prime phase. These results show that in contrast to the general assumption of LFW being an exclusively solid-state joining process, intergranular liquation is possible during LFW. However, despite a significant occurrence of liquation in the alloy, no HAZ cracking was observed, which can be partly related to the nature of the imposed stress during LFW

Hot Cracking in Welds of Aluminum and Magnesium Alloys

Abstract: Hot cracking in fusion and solid-state welding of aluminum alloys, magnesium alloys, and aluminum alloys to magnesium alloys is discussed. This includes: (1) liquation cracking in the partially melted zones of arc and resistance spot welds, (2) liquation cracking in the stir zones in friction stir welds, and (3) solidification cracking in the fusion zones of arc and resistance spot welds. Recent data of hot tearing (that is, solidification cracking during casting) in magnesium castings, which can be used to study solidification cracking in magnesium welds, are presented. These include the hot tearing susceptibility of new creep-resistant magnesium alloys and the test method used for evaluating the susceptibility.

Metallurgical Response of Electron Beam Welded Allvac® 718Plus™

Abstract: Electron beam welding of forged Allvac 718Plus superalloy has been carried out without any visible cracks in weld cross-sections. Healed cracks in the heat affected zone were, however, seen in most cross-sections with the healing as well as the cracking believed to be due to the constitutional liquation of the δ-phase. The δ-phase undergoes constitutional liquation in the Heat Affected Zone (HAZ) and consequently decreases the ductility of the material and renders cracks in the HAZ but due to the large amount of eutectic liquid produced at the same time the healing of the opened cracks takes place.

Tensile creep testing of an Al–Cu alloy above solidus with a dynamic mechanical analyser

Abstract: A dynamic mechanical analyser (DMA) was used to perform tensile creep tests at constant load and temperature to improve the accuracy in the measurement of mechanical properties above the solidus for an aluminium alloy. The binary Al–5.78 wt%Cu alloy was chosen for the investigation and specimens having columnar dendritic or equiaxed globular microstructures were tested to assess the validity of the technique and to see how the distribution of the liquid phase can affect the mechanical properties above solidus. It was found that the equiaxed globular microstructure was more rigid than the columnar dendritic microstructure with 8–9 vol% liquid phase. This was explained by the small thickness of the dendrite arms, which make the grains behave like sponges. Failure of the specimens occurred when the strain rates exceeded a critical level. Microstructure examinations performed before and after the creep tests gave indications that liquation occurred even at coalesced grain boundaries. Grain boundary sliding was promoted by liquid films at the contact points and generated cavities which accelerated the failure. The DMA was proven to be a very sensitive and reliable tool, able to heat specimens with good temperature uniformity while providing accurate measurements of the displacement and excellent control of the applied load.

Numerical simulation of solidification and liquation behavior during welding of low-expansion superalloys

Abstract: Low-expansion superalloys are susceptible to weld solidification cracks and heat-affected zone (HAZ) microfissures. To predict solidification cracking, QBasic procedures were developed and solidification reaction sequence, type, and amount of eutectic product were calculated. As manifested, primary solidification is followed by L → (γ + NbC) and L → (γ + Laves) eutectic reaction sequentially for GH903 and GH907; hence, the terminal eutectic constituents are made up of γ/NbC and γ/Laves. While for GH909, only reaction L → (γ + Laves) occurs and more γ/Laves eutectic forms. Therefore, GH909 is more sensitive to solidification cracking. To predict HAZ liquation, cracking Visual FORTRAN procedures were developed, and constitutional liquation of NbC was simulated. As shown, solid dissolution of NbC prior to liquation decreases, and initial liquid film increases with the rate of thermal cycle. Higher rate of thermal cycle promotes the melting of the matrix adjacent to the liquid film and postpones the solidification of the liquid by the liquid-to-γ mode. Thus, more residual liquid film remains at the eutectic point, which will promote HAZ microfissuring. The increase in original grain size and peak temperature also promotes liquation. Finally, these conclusions were verified indirectly by hot ductility tests.

Liquation Cracking of Dissimilar Aluminum Alloys during Friction Stir Welding

Abstract: A liquation cracking mechanism of dissimilar Al alloys during the friction stir welding (FSW) is suggested in this study. To identify the mechanism, the precipitates were analyzed and Al-Mg-Cu phase diagrams were calculated. Electron backscattering diffraction (EBSD) analysis and electron probe microanalysis (EPMA) were also conducted. In the same manner as constitutional liquation, at high heating rate, the main liquation-inducing precipitates were not dissolved in the matrix and reacted with Al to form the partially melted zone (PMZ), after which liquation cracking occurred where strain was applied to the PMZ. [doi:10.2320/matertrans.M2010343]

Prediction of Liquation Crack Initiation in Al-Mg-Si Alloy Welded Joints

Abstract: Heat-treatable 6005-T6 alloy welded joints were studied experimentally and theoretically. The joints were gas tungsten arc and gas metal arc welded, bead-on-plate with 4043 and 5356 alloy filler metals. The temperature dependences of mechanical properties (yield stress, elastic modulus, thermal strain and characteristic ductility curve) under welding conditions were obtained using the 3800 Gleeble System. The unknown volume heat source parameters were found by application of inverse modeling. The three-dimensional heat conduction and thermomechanical problems were solved numerically. Liquation cracking criterion in terms of accumulated plastic strains was proposed. Strain vs temperature curves for the partially melted zone were calculated and compared with the ductility curve. The proposed technique allows the prediction of liquation cracking in arc welding of the 6005-T6 alloy.

Weldability Evaluation of a Cu-Bearing High-Strength Blast-Resistant Steel

Abstract: BlastAlloy160 (BA-160) steel, with a nominal composition of Fe-0.05C-3.65Cu-6.5Ni-1.84Cr-0.6Mo-0.1V (wt pct), is strengthened by Cu-rich precipitates and M2C carbides. This alloy was subjected to several weldability tests to assess its susceptibility to certain weld cracking mechanisms. Hot ductility testing revealed a liquation cracking temperature range (LCTR) of 148 K (–125 °C), which suggested moderate susceptibility to heat-affected zone (HAZ) liquation cracking. The enrichment of Ni and Cu was measured along the prior austenite grain boundaries in the simulated partially melted zone (PMZ) and was consistent with similar enrichment at interdendritic boundaries of the simulated fusion zone (FZ). Good wetting and penetration of liquid films along the austenite grain boundaries of the PMZ was also observed. Associated with that finding were thermodynamic calculations indicating a completely austenitic (face-centered cubic) microstructure at elevated temperatures. In testing to determine reheat cracking susceptibility, ductility values of 41 to 78 pct RA were established for the 723 K to 973 K (450 °C to 700 °C) temperature range. The good ductility values precluded susceptibility to reheat cracking according to the test criterion. Dilatometric measurements and thermodynamic calculations revealed the formation of austenite in the reheat cracking temperature range, which was attributed to the high Ni content of the BA-160 alloy.

Features of structural changes through the cross section of sheet rolled from high strength skelp steels

Abstract: The effect of liquation through the cross section of rolled sheet and the properties of ferrite-bainite high strength pipe steels of categories Kh70 and Kh80 are studied. Auger electron spectroscopy reveals features of carbon and manganese distribution through a sheet cross section. x-Ray photoelectron spectroscopy is used to study the change in manganese distribution between component phases through a sheet cross section of steels Kh70 and Kh80. The effect of liquation on the microstructure of these steels and the change in position of critical points on TTT and CCT curves is determined. A connection is determined between the dispersion of cementite and bainite colonies and steel hardness. It is shown that the strength of steel Kh70 is determined by dispersion hardening of bainite, and in steel Kh80 strength is determined to a considerable degree by strain hardening.

Additional of REM effect for prevention of micro cracking in dissimilar multipass welds of Alloy 690 to type 316L stainless steel

Abstract: The microcracking susceptibility in dissimilar multipass welds of alloy 690 to type 316L stainless steel was investigated by the Varestraint test and the multipass welding test using the four different stainless steels varying the amounts of impurity elements such as P and S, and using the nine different filler metals of alloy 690 varying the REM(La) content. In order to simulate the dissimilar weld metals, stainless steels were gas tungsten arc (GTA) welded using alloy 690 filler metals with varying the dilution ratio. Microcracks occurring in the reheated weld metals were classified into ductility dip, liquation and solidification cracks. The ductility dip cracking susceptibility decreased with increasing the La content in the weld metal, while the liquation and solidification cracking susceptibilities increased contrarily when the La content in the weld metal exceeded ∼0.02 mass%. The relation among the microcracking susceptibility, the (P+S) and La contents in every weld pass of the multipass welding was investigated. Ductility dip cracks occurred in the compositional range (atomic ratio) of La/(P+S) 0.55, while any cracks did not occur at La/(P+S) being between 0.13-0.55. The ductility dip cracking susceptibility could be improved by adding La due to the scavenging of the impurity elements. The excessive La addition to the weld metal resulted in the solidification/liquation cracking alternatively attributed to the formation of Ni-La intermetallic compounds with low eutectic point. The optimal filler metal for the dissimilar multipass welding of alloy 690 to stainless steel was selected as La/(P+S) would be 0.13-0.55 for every weld pass.

Weldability of Low-Carbon ASTM A356 CA6NM Martensitic Stainless Steel Casting for Power Plants

Abstract: Weldability, especially HAZ cold cracking, weld metal solidification cracking, and HAZ liquation cracking susceptibilities, of ASTM A356 CA6NM martensitic stainless steel casting was investigated and compared with that of 9-12% Cr ferritic steel castings. Irrespective of the Cr and Ni content in the castings, the HAZ maximum hardness increased with an increase of carbon content. CA6NM steel, which has the lowest carbon content, had the lowest HAZ hardness and showed no cold cracking in y-slit cracking tests. CA6NM steel, meanwhile, showed the largest weld metal solidification cracking susceptibility in varestraint tests because of its higher amount of impurity elements, phosphorus, and sulfur. All castings investigated had good high temperature ductility in hot ductility tests and showed little difference in liquation cracking susceptibility. (Received June 30, 2010)

Liquation Cracks in Hot Upset Low Alloy Steel Screw Heads

Abstract: A thermally upset screw head had numerous cracks at the intersection screw head / screw shaft. The screw consisted of steel that usually does not tend to hot crack. The screw failed when tightening it by applying a torque lower than the nominal torque. The screw head was simply sheared off. A hot crack has clearly been identified as damage-causing mechanism of the material, or, more precisely, liquation cracks. Other damage hypotheses such as liquid metal induced stress corrosion cracking have reliably been excluded in the course of examinations. The material used was 1.7711, 40CrMoV4–6, a material especially used for creep resistant fasteners such as screws and nuts for operating temperatures up to 540°C. This material belongs to the group of low alloy chromium molybdenum vanadium steels. It is typically used in the tempered condition. In Europe, it corresponds to standard EN 10269. In the USA, this steel is known as ASTM A193 GR B16.

Weldability of a New Ni-Cu Welding Consumable for Joining Austenitic Stainless Steels

Abstract: A new welding consumable, based on the Ni-Cu metallurgical system, was developed to reduce hexavalent Cr (Cr6+) emissions during welding of austenitic stainless steels. However, dissimilar metal welds between Ni-Cu alloys and stainless steels have not been thoroughly evaluated for susceptibility to elevated temperature cracking during welding. This study was performed to determine susceptibility of Ni-Cu/304L dissimilar weld deposits to solidification, liquation, and ductility-dip cracking using the Transvarestraint and cast pin tear tests, the hot ductility test, and the strain-to-fracture test, respectively. Solidification experiments were performed using a button melting apparatus in conjunction with the single sensor differential thermal analysis (SS DTA™) technique to determine phase transformations during solidification. Transvarestraint testing revealed the Ni-Cu weld deposits had a higher solidification cracking susceptibility than conventional stainless steel weld deposits (such as E308L). This was attributed to fully austenitic solidification and stabilization of the TiC eutectic reaction at the end of solidification. Button melts and thermodynamic-based Scheil predictions showed a widening solidification range as weld metal dilution by 304L increased. Solidification cracking susceptibility increased at higher dilutions of 304L due to the increase in solidification temperature range. Hot ductility testing of multipass weld deposits revealed a narrow liquation cracking susceptible region (33–54°C) indicating good resistance to weld metal liquation cracking, although a ductility dip was observed in the temperature range of 800–1,100°C. This ductility trough is likely related to the solid-state weld cracking phenomenon known as ductility-dip cracking. Strain-to-fracture testing revealed that the ductility-dip cracking threshold strain was approximately 2–3% in weld deposits, indicating a moderate to high susceptibility. Weld deposits were considered to have similar weldability to other Ni-base and fully austenitic stainless steel alloys based on the current evaluation.

Prevention of microcracking by REM addition to alloy 690 filler metal in laser clad welds.

Abstract: Effect of REM addition to alloy 690 filler metal on microcracking prevention was verified in laser clad welding. Laser clad welding on alloy 132 weld metal or type 316L stainless steel was conducted using the five different filler metals of alloy 690 varying the La content. Ductility-dip crack occurred in laser clad welding when La-free alloy 690 filler metal was applied. Solidification and liquation cracks occurred contrarily in the laser cladding weld metal when the 0.07mass%La containing filler metal was applied. In case of laser clad welding on alloy 132 weld metal and type 316L stainless steel, the ductility-dip cracking susceptibility decreased, and solidification/liquation cracking susceptibilities increased with increasing the La content in the weld metal. The relation among the microcracking susceptibility, the (P+S) and La contents in every weld pass of the laser clad welding was investigated. Ductility-dip cracks occurred in the compositional range (atomic ratio) of La/(P+S) 0.99(on alloy 132 weld metal), >0.90 (on type 316L stainless steel), while any cracks did not occur at La/(P+S) being between 0.21-0.99 (on alloy 132 weld metal) 0.10-0.90 (on type 316L stainless steel). Laser clad welding test on type 316L stainless steel using alloy 690 filler metal containing the optimum La content verified that any microcracks did not occurred in the laser clad welding metal.

Physical and chemical factors controlling sulfide-silicate liquation in unhydrous mafic systems (from result of computational modeling)

Abstract: Reaction 1 suggests the relationship: lnXs = -(A + βP)/T – B – DlgfO2 ∑EiXi (2) where the XS is the sulfur concentration at sulfide saturation (SCSS) in silicate melt. This reaction and a corresponding equation of the SCSS well reproduce the experimental data. The coefficients in the equation (2) were found by multiple linear regression of experimental data using the option \"solver\" in MS Excel. The coefficients of the equation are given in Table 1.