The effects of welding wire composition on microstructure and mechanical properties of welded joint in Al-Mg-Si alloy were studied by electrochemical test, X-ray diffraction (XRD) analysis and metallographic analysis. The results show that the weld zone is composed of coarse columnar dendrites and fine equated grains. Recrystallized grains are observed in the fusion zone, and the microstructure in the heat affected zone is coarsened by welding heat. The hardness curve of welded joint is like W-shaped, the highest hardness point appears near the fusion zone, and the lowest hardness point is in the heat affected zone. The main second phases of welded joints are: matrix α-Al, Mg2Si, AlMnSi, elemental Si and SiO2. The addition of rare earth in welding wire can refine the grain in weld zone obviously, produce fine grain strengthening effect, and improve the electrochemical performance of weld.

Aiming at the high-degree difficult TIG welding problem of a titanium alloy curved surface weld,a series of contrast experiments under accurate intervals,different duty cycles and addition of superimposed pulse current were carried out.The performance of the curved surface welds was tested by using dimensional inspection,visual inspection and metallographic examination,the TIG welding technology was finally detemined,and the weld appearance and the length of weld fusion line meet the requirements.The result has good reference for welding technology research of curved surface weld with high technical requirement and other special welds.

The thickness of the intermetallic compound (IMC) layer that forms when aluminum is welded to steel is critical in determining the properties of the dissimilar joints. The IMC reaction layer typically consists of two phases (η and θ) and many attempts have been made to determine the apparent activation energy for its growth, an essential parameter in developing any predictive model for layer thickness. However, even with alloys of similar composition, there is no agreement of the correct value of this activation energy. In the present work, the IMC layer growth has been characterized in detail for AA6111 aluminum to DC04 steel couples under isothermal annealing conditions. The samples were initially lightly ultrasonically welded to produce a metallic bond, and the structure and thickness of the layer were then characterized in detail, including tracking the evolution of composition and grain size in the IMC phases. A model developed previously for Al-Mg dissimilar welds was adapted to predict the coupled growth of the two phases in the layer, whilst accounting explicitly for grain boundary and lattice diffusion, and considering the influence of grain growth. It has been shown that the intermetallic layer has a submicron grain size, and grain boundary diffusion as well as grain growth plays a critical role in determining the thickening rate for both phases. The model was used to demonstrate how this explains the wide scatter in the apparent activation energies previously reported. From this, process maps were developed that show the relative importance of each diffusion path to layer growth as a function of temperature and time.

Nb is a very important micro-alloying element in low-carbon steels, for grain size refinement and precipitation strengthening, and even a low content of Nb can result in a significant effect on phase transformation kinetics from austenite to ferrite. Solute Nb atoms and Nb precipitates may have different effects on transformation behaviors, and these effects have not yet been fully characterized. This paper examines in detail the effects of solute Nb atoms and Nb precipitates on isothermal transformation kinetics from austenite to ferrite. The mechanisms of the effects have been analyzed using various microscopy techniques. Many solute Nb atoms were found to be segregated at the austenite/ferrite interface and apply a solute drag effect. It has been found that solute Nb atoms have a retardation effect on ferrite nucleation rate and ferrite grain growth rate. The particle pinning effect caused by Nb precipitates is much weaker than the solute drag effect.

In this paper, a new model has been developed to predict the phase transformation behaviours from austenite to ferrite in Nb-containing low carbon steels. The new model is based on some previous work and incorporates the effects of Nb on phase transformation behaviours, in order to make it applicable for Nb-containing steels. Dissolved Nb atoms segregated at prior austenite grain boundaries increase the critical energy for ferrite nucleation, and thus the ferrite nucleation rate is decreased. Dissolved Nb atoms also apply a solute drag effect to the moving transformation interface, and the ferrite grain growth rate is also decreased. The overall transformation kinetics is then calculated according to the classic Johnson–Mehl–Avrami–Kolmogorov (JMAK) theory. The new model predictions are quite consistent with experimental results for various steels during isothermal transformations or continuous cooling.

A model has been developed to predict growth kinetics of the intermetallic phases (IMCs) formed in a reactive diffusion couple between two metals for the case where multiple IMC phases are observed. The model explicitly accounts for the effect of grain boundary diffusion through the IMC layer, and can thus be used to explore the effect of IMC grain size on the thickening of the reaction layer. The model has been applied to the industrially important case of aluminum to magnesium alloy diffusion couples in which several different IMC phases are possible. It is demonstrated that there is a transition from grain boundary-dominated diffusion to lattice-dominated diffusion at a critical grain size, which is different for each IMC phase. The varying contribution of grain boundary diffusion to the overall thickening kinetics with changing grain size helps explain the large scatter in thickening kinetics reported for diffusion couples produced under different conditions.

The early stages of formation of intermetallic compounds (IMC) have been investigated in dissimilar aluminum to steel welds, manufactured by high power (2.5 kW) ultrasonic spot welding (USW). To better understand the influence of alloy composition, welds were produced between a low-carbon steel (DC04) and two different aluminum alloys (6111 and 7055). The joint strengths were measured in lap shear tests and the formation and growth behavior of IMCs at the weld interface were characterized by electron microscopy, for welding times from 0.2 to 2.4 seconds. With the material combinations studied, the η (Fe 2 Al 5 ) intermetallic phase was found to form first, very rapidly in the initial stage of welding, with a discontinuous island morphology. Continuous layers of η and then θ (FeAl 3 ) phase were subsequently seen to develop on extending the welding time to greater than 0.7 second. The IMC layer formed in the DC04-AA7055 combination grew thicker than for the DC04-AA6111 welds, despite both weld sets having near identical thermal histories. Zinc was also found to be dissolved in the IMC phases when welding with the AA7055 alloy. After post-weld aging of the aluminum alloy, fracture in the lap shear tests always occurred along the joint interface; however, the DC04-AA6111 welds had higher fracture energy than the DC04-AA7055 combination.

This chapter contains sections titled: Introduction Experimental Results Discussion Conclusions Acknowledgement

A critical issue when joining aluminium alloys to dissimilar metals such as magnesium, titanium, and steel is to control the formation of brittle intermetallic compounds (IMCs) that occur due to the reaction at the joint interface. It is demonstrated that once the IMC exceeds a critical thickness, failure of welds always occurs in a brittle manner, with cracks propagating through the IMC. One approach to minimize IMC thickness is to use a solid state joining process, such as friction stir or ultrasonic welding. However, even using these processes, an IMC that exceeds the critical thickness can either form during welding or post-weld heat treatment. In this paper, a number of approaches are discussed to control IMC formation in welds between aluminium alloys and magnesium alloy. Modelling predictions indicate that interfacial reaction rates and grain size of the IMC phases are two critical factors to control layer growth. Experimental results demonstrate that the grain size of IMC layers changes as the layers grow. These modelling predictions and experimental results offer new design strategies to optimize dissimilar metal welding involving aluminium.

Niobium has an important effect on the transformation behaviour, grain size refinement and precipitation strengthening during hot rolling and subsequent cooling in low carbon steels, with even a low content of niobium having a strong effect on the transformation rate from austenite to ferrite. However, the effects of niobium on transformation behaviour have not been fully characterised and understood to date. This paper examines in detail austenite grain growth as a function of austenitisation time in high strength low alloy (HSLA) steels with three different niobium contents, together with the effect of niobium on the isothermal transformation kinetics from austenite to ferrite as a function of temperature. It is shown that austenite has the slowest grain growth rate in the steel with the highest niobium content. When austenite grain sizes are consistent, the steel with the highest niobium content was found to have the slowest transformation rate from austenite to ferrite.

In order to study the flow field characteristics of impingement-effusion cooling , a kind of impingement-effusion cooling structure was designed, the flow field characteristics of it was studied by PIV (particle image velocimetry). It was found that the flow characteristics between impingement plate and effusion plate is very complex, and the size of eddy between impingement plate and effusion plate is different with the change of blowing rati-o. On the side of effusion film, the flow from effusion hole will penetrate gradually main flow when blowing ratio is increased, that destroys the effusion film. There is a pair of "kidney eddies" on the vertical section of the outlet of effusion hole, and the "kidney eddies" will moves into the main flow when the blowing ratio is increased.

Surface coating modification was carried out by ultrasonic agitation in organic solution with surfactant,in order to improve the dispersion of nano-CeO2 particulates in eutectic-like Zn-Al alloy.The coating effects,including covering thickness,carbonization temperature of covering layer and aggregation status of CeO2 particulates,were characterized by means of AES,thermal-gravimetric analysis(TGA) and TEM,respectively.Then,the dispersion of CeO2 in nano-CeO2p/Zn-4.5%Al-RE composites was observed by FE-SEM.The results showed that the thickness of the covering surfactant layer,which improves particulates′ dispersion,was about 20 nm,and that a carbonization came into being on the nano-CeO2 surface at 495 ℃.Moreover,from classical thermodynamic point of view,reactions between the carbonization layer and zinc oxide,which can remove the block of oxygenation film coated on CeO2 and promote the wettability between CeO2 and the alloy,may take place on the CeO2 surface.As a result,nano-CeO2 particulates were dispersed homogeneously in the alloy.