The feasibility of using friction welding was used to join similar kinds of cast irons was studied. This study was carried out to examine influence of welding parameters on tensile properties of friction welded joints by similar materials of spheroidal graphite iron castings and gray iron castings. It was found that cast iron, which is difficult to join by fusion welding, can be joined by friction welding without resorting to special measures such as precheating or postheating. Under proper welding conditions, the friction welds are defect free. This paper describes process parameters during friction welding which governing mechanical properties. Materials were selected two types of cast irons, spheroidal graphite iron castings and gray iron castings with flaky graphites which are called ductile cast iron and gray cast iron hereafter in this paper. The friction welding conditions for similar kinds of cast irons in the same diameter can be defined either by the minimum heat input rate or by the friction upset speed. The tensile strength of the joint increases with decreasing heat input rate or upset speed and it is possible to obtain joint strengths equal to that of the base metals.
The varestraint cracking test was carried out to assess the hot cracking tendency of alloy 800 at different welding conditions and augmented strain levels. The test results revealed that the hot cracking susceptibility of alloy 800 increases with increasing the heat input and augmented strain levels. This emphasizes the role of constraint either internally induced or externally applied to the welded structure. A cracking threshold or the minimum augmented strain (∈min) below which no hot cracking takes place has been found to be 0.25 for alloy 800. Also, the Critical Strain Rate to Time (CSS) required to cause cracking was found to be 6.42×10-3 sec-1 at welding condition of 100 Amp, 13 Volt and 140 mm/min.
A study was carried out on solidification crack in Al-Mg-Si alloys type A6351 by means of Houldcroft test with GTAW process without filler metal and high pulsation frequency. Three test series were carried out taking into account the variation of heat input. The obtained crack lengths were measured and the effect of pulsation frequency, fraction of pulse time and welding speed were analyzed. The effect of grain structures on the susceptibility of solidification crack was evaluated as well as the welding pool geometry in which these structures developed. The "quasi-stationary" state was studied for the specimen geometry used and the process efficiencies were estimated using welding pool width method. An estimation of the necessary force to close the crack was made using ROGERSON's physical-mathematical model. The obtained results were compared with those presented in surveyed references.
Transformation hardening of cabon steel (S45C) by CO2 laser beam was analyzed by using a novel beam shaping optics, which provides linear-polarized beam spot with uniform intensity distribution and variable aspect ratios. At normal incidence, the absorptivity of CO2 laser beam was 20% in the bare specimen (in Ar), and 45% in graphite coated specimen (in air). The absorptivity in p-polarization, AP, increased significantly with increasing incident angle, θ, reaching 50% at θ=80 degrees with bare specimen in Ar, which exceeds the value of graphite coated specimen at θ=0. In the graphite coated specimen, AP also increased with increasing incident angle, reaching 80% at θ=80 degrees. In the bare metal, the case depth was approximately 0.4 mm at normal incidence, and increased with increasing incident angle, reaching 2.5 mm at θ=80 degrees. The depth of laser hardening was estimated by a simple approximation equation, showing satisfactory agreement with the experimental values. It was also shown that the hardened depth can be changed independently of the surface temperature by using the shaping optics, which can provide laser beam spot with variable aspect ratios.
Metal fiber reinforced metal matrix composites (FRM) which consist of the high strength ductile steel wire and aluminium or titanium foil were produced by a explosive bonding method. Strength property was evaluated by a tensile test. An optimum explosive loading (explosive mass/driver plate mass) increased with the volume fraction of fiber. For aluminium matrix composites, micrographs showed good bonding between the adjoining foils and non-bonding between the foil and the steel wire. The tensile strength of aluminium matrix composites were 80% of the value predicted by the law of mixture. For titanium matrix composites, micrographs showed both boned and unbonded area between the adjoining foils or between the foil and the steel wire. The tensile strength of titanium matrix composites were in agreement with the value predicted by the law of mixture.
Multilayered composites reinforced with steel sheets were tried to be fabricated using single-shot explosive welding technique. The experimental conditions for obtaining moderate bonding at every interface were investigated based on collision velocity (VP) and kinetic energy loss by collision (ΔKE) calculated using finite-difference analysis. In case of aluminum base composite reinforced with stainless steel sheets with the same thickness of multilayered plates, the amount of energy dissipated by collision (ΔKE) should be regulated at a certain range by changing stand-off distances between plates. In case of aluminum and titanium base composite reinforced with maraging steel sheets, moderated welding is unable to be achieved when using the multilayered plates with the same thickness due to a large difference of tensile strength between the components. The use of multilayered plates with the same mass per unit area by regulating the thickness of multilayered plates was useful for obtaining moderate bonding as a means to decrease the difference of ΔKE at each collision of the plates.
The accurate measurement of temperature is of great importance in the materials processing such as welding, thermal cutting and heat treatment of metals. The radiation thermometry has a distinct advantage over other temperature detecting method, where it requires no physical contact with the work. The reliability of the thermometry is, however, limited if the emissivity of the work to be measured is not accurately known. According to Planck's law of radiation, it is evident that the effect of temperature on spectral radiance is greater at the shorter wavelengths. In the present paper, the characteristics of UV (ultra-violet) thermometry has been discussed experimentally. The temperature measurement test by UV spot-sensor (spectral range : 0.25μm) shows that the melting point of Ni can be measured with an accuracy of ±1% for any change of the emissivity. The CCD camera with UV-filter, used in this work, made it possible to detect the temperature image on the plate in welding, nearly independent of the emissivity of object.
This paper deals with the problem concerning the sensing and the controlling of weld pool. In order to obtain the high quality of the welding result, it is important to control the weld pool depth in the robotic welding regardless of the external disturbance such as an irregular change of the groove gap. The method of controlling the weld pool depth without mathematical model is discussed. Since it is difficult to directly measure the depth, the depth is estimated from the surface shape of the weld pool, the groove gap, and the welding current. A neural network is used to estimate the depth without mathematical model. The weld pool depth is controlled from the output of the neural network by using the fuzzy controller. The validity of the neural network and of the fuzzy controller was verified by the welding experiments.
The effect of preheating on coating process was investigated in low pressure plasma spraying. In this study, microstructure, bonding strength and blast erosion property were examined for the coating which consists of Ni based superalloy as a substrate and CoNiCrAlY spraying material. The results are summarized as follows. (1) High bonding strength over 72 MPa of coating to the substrate by low pressure plasma spraying, is derived from the diffusion zone formed adjacent to the coating interface. (2) 1073 K preheating coupled with the plasma spraying causes short-time diffusion adjacent to the coating interface that is equivalent to the vacuum-furnace heating at approximately 1373K. (3) The blast erosion property of low pressure plasma sprayed CoNiCrAlY is close to the stainless steel rather than the atomospheric plasma sprayed coating. (4) Low pressure plasma sprayed coating which is preheated over 873 K, consists of the top layer with higher blast erosion rate and excellent layers with low blast erosion rate under it. It is considered that in-situ sintering phenomenon caused by sufficient preheating and the subsequent spraying would improve combining strength between sprayed particles. (5) Plasma flame treatment just after spraying improves the blast erosion property of the top layer in low pressure plasma coating.
The purpose of this study is to investigate the influence of extreamly low levels of oxygen and nitrogen partial pressure, PO2 and PN2 in pressurized TIG welding atmospheres on the welding phenomenon and properties of welds of zirconium alloy tubing. In TIG welding of Zircaloy-2 tubing in welding atmospheres with various PO2 and PN2 in total pressure (PT) of 0.32 MPa (optionally 0.55 MPa), the arc voltages were measured and the properties of welds (surface discoloration, oxygen and nitrogen contents) were examined. Although definite arc voltage change is not observable at welding in ≤ 12.9 Pa of PO2 and 15.6-67.2 Pa of PO2+N2 (PO2/PN2=1/4), a tendency bf arc voltage drop with increase of PN2 is observed at welding in 13.1-53.0 Pa of PN2 (PO2=0.3 Pa). The surface of weld metal and heat affected zone (HAZ) in the atmosphere of 0.3 Pa of PO2 and 1.3 Pa of PN2 remains bright. The surface discoloration is observable slightly on weld metal and HAZ in the atmosphere of 3.4 Pa of PO2, and with increase of PO2 the initial straw color becomes darker until it gets partially blue. No surface discoloration is observable on weld metals and HAZ in the atmospheres of PN2≤53.0 Pa with 0.3 Pa of PO2. The nitrogen content [N] in the weld metal increases linearly with increase of √PN2 and the increasing rate of [N] in inner part of weld metal is lower than that of [N] in outer part. The oxygen content [O] in outer part of weld metal increases linearly with increase of √PO2 and shows same relations as [N], although the values of [O] in the weld metals flucutuate more than [N]. The increasing rates of [N] and [O] in the weld metal under PT=0.32 MPa are lower than that of [N] and [O] in the weld metal under PT=0.10 MPa which is reported in Report 3.
To grasp the characteristics on TIG weld joints of Zircaloy-2 tubing, the properties of weld heat affected zone (HAZ) welded in welding atmospheres with extremely low level of oxygen and nitrogen partial pressures, PO2 and PN2 were examined as compared to weld metals. The original fine structures of Zircaloy-2 tubing are re-crystallized at heated zones to β phase and α+β phase range, and the grains grow gradually from end of HAZ to fusion boundary. The nitrogen content [N] in outer part of HAZ increases linearly with increase of √PN2 as same as weld metals and the increasing rate of [N] is higher at nearer portion (0-1 mm) to fusion boundary than that of next portion (1-2 mm). [N] in inner part of HAZ does not increase even in case of welding in atmospheres with higher PN2 (PN2=53.3 Pa) and also at the nearer portion (0-1 mm) to fusion boundary. Although the oxygen content [O] in HAZ increases in case of welding in atmosphere with higher PO2 (PO2=13.9 Pa), the values of [O] in HAZ fluctuate more than [N], as weld metals, and definite relations between PO2 and [O] in HAZ were not obtainable.
The decomposition of M-A constituent by PWHT (Post Weld Heat Treatment) and its effects on the toughness have been investigated for the weld CGHAZ (Coarse Grained Heat Affected Zone) of HSLA steels of 780-980 MPa classes. As the PWHT temperature and time were increased, the area of the M-A constituent was decreased for all the steels employed, but dependence of the toughness of the 980 MPa class steels on PWHT parameters was quite different from that of the 780 MPa class steels. For the weld CGHAZ of 780 MPa class steel, the toughness was increased, as the M-A constituent was decomposed into ferrite and cementite by the PWHT. For the weld CGHAZ of 980 MPa class steel, however, embrittlement occurred at PWHT temperatures above 823 K though the toughness was increased with PWHT time at temperature below 623 K. From these results, it can be concluded that the toughness of HSLA steels impaired by the high-heat-input welding can be improved significantly by PWHT at temperature about 623 K.
A type 329J1 duplex stainless steel was welded by gas tungsten arc welding process in argon-nitrogen gas mixture atmosphere. The microstructure, Vickers hardness and Charpy impact toughness of the weld metals were examined. The nitrogen content increased and the ferrite content decreased with an increase in the nitrogen partial pressure of atmosphere. The ferrite content decreased linearly with an increase in nitrogen content. The hardness in each of ferrite and austenite phase was nearly constant regardless of the nitrogen content of weld metal. The toughness increased with the nitrogen content at room temperature and more, while it was constant at low temperatures. The toughness of weld metal was affected by the presence of austenite and chromium nitride precipitates in ferrite. The nitrogen absorption of duplex stainless steel weld metal improved the impact toughness because of the increase in austenite content and the decrease in the amount of nitride.
Electron beam welding is featured in (1) capability of welding thick plates by a single pass, (2) less weld distortions, and it has been applied to welding of heavy section large structures. But, when a heavy section low alloy steel plate is welded, the weld metal toughness deteriorates. Therefore, it is very important to increase the weld metal toughness. Weld metal toughness is affected by weld metal structure and carbide precipitation. Then we have investigated the effects of the electron beam welding cooling rate and PWHT conditions on weld metal toughness and we have also studied the mechanism of toughness changes metallographically. Consequently, it has been clarified that in order to increase the toughness of electron beam weld metal for low alloy steel, the structure should be lower bainite by increasing welding cooling rate and a temper parameter of about 19 should be selected as the key for PWHT.
The influence of the low expansivity interlayer on the cracking susceptibility and properties of AIN -copper joints brazed with Ag-Cu-Ti filler metal was examined. The thermal stress and heat conduction numerical analyses suggest that not only crack-free joints but also the superior thermal conductive joints can be produced by employing tungsten or molybdenum interlayer for AIN to copper joining. The cracking of AIN-OFC joints can be prevented by increasing the tungsten and molybdenum interlayer thicknesses above 0.8 mm and 2.0 mm, respectively. The tensile strength of AIN-OFC joints brazed at 1078 K for 120 s with 1.0 mm thick tungsten interlayer is approx. 50 MPa. AIN-W-OFC joints using Ag-Cu-Ti filler metal indicate the superior reliability for thermal fatigue and heat conduction.
Microcracks developed in cleaning action region by GTA process have been investigated using an aluminum and aluminum alloys. Although no microcracks are observed in a pure aluminum and a commercially pure aluminum, microcracks are mainly developed in the region of Al-Mg alloys where the oxide film is broken due to the cleaning action. This tendency of cracks is quite the same as the weld solidification crack. The microcracks are easily developed with the increase in EN ratio of the welding current. Since dendrites are observed on the crack surface, the microcracks is intergranular due to the solidification crack. The cooling rate in this case was estimated to be about 104°C using the size of the dendrites. The microcracks are considered to be developed due to the phenomena that the region where the oxide film is broken is locally melted owing to the high current density and the cooling rate is very high. X-ray diffraction technique made clear that the continuous vague Debye-Scherrer rings are obtained from the region where the oxide film is broken even when using a single crystal. This shows that the single crystal changes to very small polycrystals in this region and the polycrystallized depth from the specimen surface was about 100μm. Fatigue strength at 1 × 107 cycles of specimens having the microcracks is lower than that of the base metal and decreases linearly with the increase in the crack depth.
A failure assessment diagram (FAD) approach is strongly expected to be useful for the simplified fracture assessment. The applicability of FAD approach to ductile fracture strength analysis was verified using a heavy section 9%Ni steel plate and its weld joints, in use for large scale LNG storage tank, in this paper. Various ductile fracture tests using compact and center cracked panel specimens were carried out in order to observe stable crack extension behavior and to estimate failure strength. The obtained results are summarized as follows : (1) When the assessment by means of Option 2 FADs is considered, an appropriate failure assessment curve (FAC) which takes account of actual fracture behavior should be selected. (2) The accuracy of assessment by FAD approach was strongly affected by the precision of estimation of limit load for Lr. (3) In case of 9%Ni steel, stable crack behavior agreed well with FAC regardless the specimen thickness. (4) Maximum load measured in experiments and its prediction by FAD were in good agreement regardless the specimen configuration. In case of 9%Ni weld joints, however, FAD approach showed a poor accuracy in the ductile tearing analysis and the maximum load prediction.
The basic fatigue strength with correspond the fatigue strength of welded structures containing high tensile residual stresses of yield strength magnitude was obtained by the σmax=σy test. This test method was confirmed to be useful even for specimens containing high tensile residual stresses of yield strength magnitude. The basic yield strength was decreased with the increase of the yield strength of material. Based on this result, following proposals are given in order to increase the fatigue strength of welded structures by reducing the tensile residual stresses. (1) Use of low yield ratio materials. (2) Use of low transformation temperature materials. (3) Use of low yield strength weld metals for the passes which form the part of fatigue crack initiation. (4) Use of welding methods which bring the early cooling of surface area of plate.
In this study, critical J-value was used as the parameter of bonding strength criterion of dissimilar material joints with an interface crack. J-value was calculated by finite element method. The effect of several factors (Young's Modulus ratio, Difference in coefficient of thermal expansion, Temperature variations, Crack length etc.) on J-value was discussed. Fracture tests were carried out using specimens that were bonded using an insert material. Critical J-value was analyzed from both breaking load of fracture test and thermal stress induced by temperature variations from bonding temperature to test temperature. This value was not affected by initial crack length and insert material thickness. Therefore, this value was reasonable as parameter of bonding strength criterion of joints. From the results, it was found that this method was valid as the evaluation method of bonding strength of dissimilar material joints.
Detection process on the degree of fillet formation at the micro soldered joints of QFP lead has been researched and developed. The shape of the joints is expressed by three parameters (fillet length, average thickness of solder, angle of flat part of lead). The fillet length can be detected by temperature of three part on the surface of lead (toe part, center part, bend part), furthermore both average thickness of solder, and angle of flat part of lead can be detected by them. This detection process is investigated by the computer simulation on thermal model of the joint of QFP between copper alloy lead with 150μm thickness and glass epoxy substrates (1.6 mm thickness).