Showing papers on "Liquation published in 2001"

Metallurgical factors contributing to HAZ cracking susceptibility in cast alloy 718 welds and its improvement by cerium addition

Abstract: A study was carried out to determine the effect of cerium addition on HAZ cracking susceptibility in cast alloy 718 welds. The cause of HAZ cracking was also investigated using commercial cast alloy 718 varying in grain size at three levels. The hot cracking test results for commercial cast alloy 718 indicated that the fine-grained alloy was less sensitive to HAZ cracking. Furthermore, cerium addition of up to 0.3 wt.% was found to have a beneficial effect in reducing HAZ cracking susceptibility. When cerium addition exceeds 0.3 wt%, HAZ cracking susceptibility increased further. The mechanism of HAZ cracking was found to be related to intergranular liquation caused by the eutectic melting of sulfur containing a laves cluster at the grain boundaries. The degree of grain boundary liquation in the fine-grained specimen was less than that of the coarse-grained specimen due to a decrease in the amount of laves cluster and sulfur segregation in it at the grain boundaries, which seems to be responsible for a reduction in hot cracking susceptibility in the fine-grained specimen. Microscopic observation suggested that the improved HAZ cracking susceptibility in cerium containing alloy could be attributed to a reduction in the amount of laves cluster and sulfur segregation in it at grain boundaries due to the grain size reduction and sulfur-scavenging effects of cerium.

Influence of electron beam welding parameters and metallurgical factors on intergranular liquation cracking susceptibility of cast alloy 718

Abstract: The factors affecting intergranular liquation cracking susceptibility in electron beam welds were investigated for cast alloy 718. The materials used were as-received plates and heat-treated plates with three different levels of grain size. Liquation cracking susceptibility in HAZ was evaluated by a bead-on-plate test and a restraint/relaxation U-type hot cracking test. The penetrated shapes in the welds were classified into wine cup-like Type W and nail head-like Type N. For a given beam current, Type w and Type N were observed at the lower and higher welding speeds, respectively. Welding defects, i.e., underfills and microcracks were seen in the electron beam welds. Compared with Type W, the liquation cracking was more sensitive for the Type N bead cross sectional shape. Furthermore, it easily occurred at grain boundaries in Region II, i.e., very near the nail head necked part. According to the restraint/relaxation U-type hot cracking test, the liquation cracking susceptibility decreased with decreasing grain size or with homogenization heat treatment. These results suggested that the liquation cracking susceptibility in cast alloy 718 electron beam welds could be improved by using the Type W bead cross sectional shape, a decreasing the grain size and using appropriate heat treatment before welding.

Hot ductility behavior and HAZ hot cracking susceptibility of 7475‐T7351 aluminum alloy

Abstract: The elevated temperature mechanical properties and hot crack susceptibility of the aluminum alloys, 7475-T7351 and 7005-T1 were evaluated using a Gleeble hot-ductility test. The 7475 alloy exhibited lower hot ductility and strength, significantly poorer ductility recovery on cooling from the nil-strength temperature, considerably wider brittle temperature range (BTR) and crack susceptible region (CSR), and thus greater susceptibility to heat-affected zone (HAZ) hot cracking, as compared to the 7005 alloy. Longitudinal samples of 7475 had a lower crack susceptibility than short-transverse 7475 samples. The results showed good agreement with those of the spot-Varestraint tests. Scanning electron microscopy (SEM) revealed that the ductility loss was accompanied by fracture transition from ductile transgranular mode to brittle intergranular mode. The HAZ hot cracking in alloy 7475 due to the loss of ductility was primarily caused by liquation of low-melting-point grain boundary segregates or eutectics that contained high amounts of Mg, Cu, and Zn.

Mathematical Simulation of Grain Boundary Liquation in HAZ during Welding. Study on Weldability of Cast Alloy 718. Report 5.

Abstract: In order to study the mechanism of liquation cracking in cast alloy 718, a mathematical model was developed to calculate grain boundary liquation in weld thermal cycle. According to microscopic observation of specimen heated at high tempera-tures, the laves cluster has started to melt at about 1383 K. Thus, it was considered on modeling that the grain boundaries were liquated by the melting of laves cluster which were located at the center of the grain boundaries. In this model, two factors were considered for solute diffusion in solid and liquid phase, namely, (1) high diffusivity at the grain boundary region, (2) the balance between the grain boundary energy and solid/liquid interfacial energy. Calculation has shown that the experimental values for the amount of grain boundary liquation fairly agree with the calculated results where the grain boundary diffusivity is to be the ten times value of the bulk solid diffusivity. The calculation has also indicated that the liquid strongly infiltrates along grain boundary with increasing the heating temperature, and with decreasing the heating rate. In addition, the amount of grain boundary liquation increases, as the grain size and laves cluster increases. On the basis of these calculations, it has been elucidated that the liquation cracking in cast alloy 718 strongly depended upon the factors, such as weld thermal cycle, grain size and the amount of laves cluster, etc..

Fracture toughness of various zones in weldments of 6082 aluminum alloy

Abstract: The effects of Mn addition on toughness of welded Al–Mg–Si alloys have been investigated. In-situ SEM observations of fracture toughness tests have also been used to characterize crack initiation and propagation behavior through the weldment. Recrystallization of HAZ is completely suppressed by adding Mn, while a sample without Mn exhibits recrystallized coarse grain structure. Recrystallization in HAZ can lead to drastic decrease of its fracture toughness comparing with the samples containing Mn. Many microcracks are distributed at grain boundaries within several mm ahead of a crack-tip where crack-tip stresses are relatively weak. Fracture-mechanical analysis reveals that the toughness degradation is attributed to such premature damage evolution before the onset of crack extension. However, the microcracking is not attributed to so-called liquation cracks, but degradation of the grain boundaries due to the formation of film-like Al–Mg intermetallic compounds. The microcracks ahead of the crack-tip aligned at an angle of approximately 60° from an initial notch direction. Tension obliquely applied to the HAZ next to welding pool appears to give rise to such inclination of the microcracks. It is suggested that the toughness degradation can be suppressed by utilizing such anisotropy even if the GB films are formed during welding.

Experimental and analytical investigations of fracture toughness in weldments of 6082 Al alloy

Abstract: 6082 Al alloys are commercial and medium strength alloys, widely used as materials for welded structures. The purpose of this study is to investigate the effects of Mn addition on toughness of welded Al-Mg-Si alloys. To evaluate microstructural effects quantitatively, in-situ SEM observations of crack initiation and propagation behaviors through weldment are carried out. For the consideration of in-situ observation of fracture toughness test, stress field at crack-tip is analyzed using elasto-plastic finite element method (Hereinafter, FEM.) assuming that a crack is near a boundary between a weld metal and heat affected zone (Hereinafter, HAZ.). When small amount of Mn is added, recrystallization is completely suppressed as compared to specimens to which no Mn is added, thereby fibrous grains are kept. On the other hand, recrystallization of HAZ causes drastic decrease in fracture toughness in the case of no Mn addition. With the extension of a main crack, many microcracks are formed at grain boundaries ahead of a crack-tip despite the fact that the stress is relatively low. Such microcracking is not attributed to so-called liquation cracks, but the degradation is caused by the formation of film like Al-Mg intermetallic compounds at grain boundaries. The microcracks are aligned ahead of the crack-tip at an angle of 60 degrees from an initial notch direction. This is attributable to the experimentally-observed direction of the intermetallic compound film, which is also confirmed by the numerical analysis.

Extraction of non-ferrous metal from metal scrap parts by liquation and tunnel furnace installation therefor

Abstract: of EP0869192Recovery of non-ferrous metals from objects (38) consisting of or containing both ferrous and non-ferrous materials takes place comprises heating such objects to a separation temperature which lies between the melting points of the ferrous and non-ferrous materials, and collecting the molten non-ferrous material. The objects (38) are transported continuously, without preliminary comminution, through a tunnel furnace (1), which is provided with an appropriate transport system (35) and are heated there to a separation temperature, with collection of the resulting molten non-ferrous material.