Study on the factors in creating the IMC-free region: dissimilar metal joining of aluminium alloys to steel by a MIG-braze welding by using the advanced hot-dip aluminized steel sheet

Dissimilar metal joining of aluminium alloys to steel is generally difficult to be in practical use because of a formation of brittle intermetallic Fe–Al compound (IMC) at the interface of the joint. The authors have been researching in order to minimize the thickness of this brittle IMC in order to get excellent joint strength and have found that the formation of this brittle IMC is regionally prevented by using the advanced hot-dip aluminized steel sheet and by adopting suitable joining conditions. In particular, this paper focuses on the mechanism of creating this IMC-free region in the case of MIG-braze welding and the results obtained are as follows. (1) The creation of IMC-free region is initiated as the first process by the dissolution of the τ₅ phase (Fe–Al–Si) in the aluminized layer into the weld metal, and temperatures of more than 886 K for dissolution during MIG-braze welding and the use of filler metal for dilution of Fe and Si in τ₅ phase have significant effects. (2) In the second process, the diffusion between aluminium-alloy weld metal and base steel is restricted by AlN on the surface thin layer of the base steel which existed under 908 K temperature conditions during MIG-braze welding.     

Friction stir welding between extrusions and laminates

Current market demands drive companies to innovate their production techniques to improve products and simultaneously hold down costs. In the search for solutions aimed at an optimization of production processes, semi-finished products in aluminium alloys very definitely play a significant role. Moreover, the assembly of extruded parts with laminates may result in considerable difficulties, especially in relation to distortions or generated residual stress and process productivity. A solution to these issues may be found in the application of friction stir welding (FSW), characterized by low thermal input and high productivity. The asymmetry of the FSW process determines a different thermal input to the edges of the pieces to be joined. This aspect can be exploited when it is necessary to join pieces characterized by a very different thermal lag, as occurs in the case of joining extruded products and sheeting. This study aims to identify optimal FW parameters, placing particular attention on heat transferred to materials so as to provide an adequate thermal input that will allow for compensation of the different thermal capacity of the pieces in question.     

Effectiveness of cold metal transfer process for welding 7075 aluminium alloys

Abstract

Cold metal transfer (CMT) welding is a promising process to improve the mechanical characteristics of the hard to weld 7075-T6 aluminium alloy owing to its unique advantages in contrast to conventional metal inert gas welding process. The welded joints, using ER5356, were identified and characterised by means of optical microscopy, scanning electron microscopy and energy dispersive spectroscopy. Mechanical properties were measured by tensile and hardness tests. Results indicate that CMT provides better joint tensile strength and ductility compared to metal inert gas welding. The softness in the heat affected zone was very low, but a big hardness gap was recognised in the welded metal compared to base metal. The joint had mechanical property coefficients of 77%, 60% and 69% for yield strength, ultimate tensile strength and elongation respectively.     

An analysis of shrinkage defects in aluminium and aluminium-copper castings

The influence of casting parameters and alloy composition on the formation of shrinkage defects has been studied using a T-piece test casting. Macro shrinkage defects were found in pure aluminium and aluminium 0.4wt% copper castings, but not in aluminium 1, 2, and 4.5 wt.% copper alloys. The thermal data which were in general, reproducible to +/-5% showed that a negative temperature gradient prevailed in the pure and aluminium 0.4wt% aluminium alloy right up to the end of freezing. In the aluminium 1, 2 and 4.5 wt.% copper alloys the initial negative gradient changes to become positive after approximately one half of the total solidification time has elapsed. The gradient was only of the order of 2°C/mm. Variations in pouring temperature of about 50°C and filling times of a factor of three changed the solidification times by 10% only. Such variations did not change the relative freezing times at different locations in the uniform section castings. Solidification times for the 15 mm horizontal section of the castings (a/b = 0.6) were similar to those of the 25 mm sections (a/b = 1) showing that even in this simple case the modulus concept is invalid. The form of the cavities in the feeder heads is a function of the metal cast, the gas content and the grain structure. The results formed the basis of validation tests for a computer shrinkage model.**     

Quench modelling of Al-7Si-Mg casting alloys

Quench factor analysis was applied to A356 and A357 aluminium casting alloys in order to model time-temperature-yield strength curves, to predict T6 yield strengths as a function of continuous cooling quench rate and to examine quench sensitivity. The predicted T6 yield strengths for given quench curves were typically within ± 10 MPa of experimentally measured yield strengths. It was found that A356 is more quench sensitive than A357 at high quench rates, whereas A357 is more sensitive at low quench rates. This is explained in terms of silicon clustering, which alters the matrix Mg/Si ratio and subsequent precipitation events. These observations were confirmed by differential scanning calorimetry.     

The enhancing effect of surface coatings on cast-in bonding of steel and titanium inserts in aluminium castings

Insertion bonding of mild steel and titanium pipes in aluminium castings was performed in two ways; either by holding paired pipe and aluminium cylinder specimens at constant high temperatures, or by pouring molten aluminium into sand moulds containing inserts. The surfaces of the inserts were electroplated using Ag or Au, or spray coated with Al-Si alloy. The bonding was evaluated by shear strength measurements and microstructure observations. To verify the effect of surface coatings, the contact between the molten aluminium and the specimens was observed. At constant high temperatures, coated steels showed good metallurgical bonding when they were maintained for about 200 s in contact with molten aluminium. The time necessary for bonding was shorter with titanium inserts. In bonded specimens, alloyed microstructures were observed at the interfaces. There was no bonding with uncoated specimens. All coatings used were effective when poured, but preheating of the moulds was necessary to allow sufficient solidification time for the melt/insert volume ratios which were 32 and 36. Process analysis by the finite difference method showed that bonding required a period of liquid phase contact of the melt with the inserts. Observation of aluminium droplet contact with the inserts showed that uncoated surfaces of steel and titanium are not wetted by the melt due to stable oxide skins. With Ag electroplating, aluminium droplets can make wet contact with the specimens. The contact angle decreased from about 140° in uncoated specimens to 15°-45° with Ag plating. The plating prevents surface oxidation of the inserts and promotes wet contact when the active aluminium liquid surface is exposed. The Al-Si alloy spray coating has some effect in promoting wettability, but it is inferior to Ag plating.     

Galvanic corrosion of aluminium alloy members of bridge guiderails under severe atmospheric exposure conditions

ABSTRACT

Aluminium alloys are nowadays preferred as materials for bridge guiderails especially for bridges connecting oceanic islands or spanning inlets due to their inherent corrosion resistance. But because of the limited mechanical strength of aluminium alloy, fasteners of guiderail members are made from steel materials. It has been found that contact between bare steel fasteners and aluminium alloy members can cause galvanic corrosion in the aluminium alloy. Research was carried out to investigate the capability of different surface treatments on fasteners and aluminium alloy members to inhibit galvanic corrosion under atmospheric exposure for periods of one and three years. It was found, among other results, that stainless steel fasteners treated with zinc flake coating were the most effective inhibitors of galvanic corrosion on aluminium alloy members.     

Effects of low frequency electromagnetic field on efficiency of refiner and hydrogen concentration in pure aluminium

Abstract

For aluminium alloys, the sizes of the grains can be important in determining the performance in structural applications. Al–Ti–B master alloy is a good grain refiner for aluminium alloys, but the fraction of particles which nucleate grains is small. The possibility of increasing the efficiency of refiner is studied in this research. The experimental results show that the efficiency of refiner is improved by applying low frequency electromagnetic field. In addition, hydrogen concentration in the melt is reduced from 0.19 mL/100 g to 0.16 mL/100 g when the current intensity is 100 A.     

Diffusion in FSW Joints by Inserting the Metallic Foils

The aims of this investigation were to study diffusion phenomenon in friction stir welding （FSW） joints and its influence on the tensile strength of joints. To study diffusion in stir zone, various metallic foils were inserted between two pure aluminium plates. The thin foils of pure copper, pure zinc, brass and Cu-Zn-Ni alloy with the thickness of 250 μm were used as metallic foils. The transversal cross sections of welds were observed by optical microscopy and scanning electron microscopy （SEM） equipped with an energy dispersive X-ray spectroscopy （EDS） system. The spot analyses near the metallic foils indicated that diffusion occurs from foils into the aluminium plates during welding; consequently, the strength of stir zone increases even by 50%. Besides, the metallic foils could reveal the flow of metal after FSW process.     

屋面防护落石冲击缓冲材料评述

屋面的防护缓冲材料对落石冲击下建筑物的安全有着至关重要的影响。对不同的缓冲材料从两个方面进行了评述,一是提高混凝土屋面板本身的抗冲击性能,主要介绍了橡胶混凝土、橡胶改性沥青混凝土、纤维混凝土、层布式混杂纤维混凝土和蜂窝状钢管约束混凝土5种具有较好抗冲击性能的改良混凝土。二是在混凝土屋面板上加缓冲垫层,就垫层碎石土体的选择、热压聚苯乙烯泡沫塑料垫层和废旧轮胎垫层进行了评述。有助于了解屋面板抗冲击缓冲材料的发展趋势,为相关的研究和工程应用提供参考。