Direct chill rheocasting (DCRC) of AZ31 Mg alloy

Abstract

A new processing technology, direct chill rheocasting (DCRC), has recently been developed by BCAST at Brunel University, for production of high quality Mg alloy billets and slabs. In the present paper the authors present the DCRC process and experimental results on microstructure and mechanical properties of DCRC billets. Basically, the DCRC process consists of a high quality semisolid slurry supply system, continuously feeding a conventional direct chill caster, to produce billets or slabs. Experimental results show that the DCRC billets have a fine and uniform microstructure throughout the cross section, the average grain size being ～50 μm. Direct extrusion of the DCRC billets was conducted to assess the deformability and mechanical properties after extrusion. The average grain size is <3 μm in the as extruded state. The increased deformability of the DCRC billets has been attributed to the fine and uniform microstructure, while the improved mechanical properties of the extruded product is attributed to the fine grain size.     

Effect of process parameters on structure and macrosegregation upon direct chill casting of Mg–Nd–Zn–Zr alloy

Abstract

Direct chill (DC) semicontinuous casting process has been successfully used to produce sound Mg–3·0Nd–0·4Zn–0·4Zr (NZ30K) billets. The influence of process parameters such as casting speed, casting temperature on the microstructure and macrosegregation was studied. The results show that the casting speed affects the macrosegregation greatly while it has a slight influence on the grain size of the billet; the casting temperature has a slight influence on macrosegregation of the billet while the grain size of the billet increases as the casting temperature increases. The optimal process parameters have been experimentally determined as follows: casting temperature 700°C and casting speed 90 mm min^?1. The ultimate tensile strength, yield strength and elongation of billets cast at the optimal casting parameters are 196 MPa, 125 MPa and 16·5% respectively.     

Influence of liquid metal feeding on the flow and macrosegregation in DC casting

ABSTRACT

The influence of liquid metal feeding on the flow and macrosegregation during direct chill (DC) casting of Al-4.5wt-%Cu billets is investigated. An analytical study for constricted inlets predicts that the inflow only reaches the bottom of the sump for narrow inlets, a result confirmed by steady-state axisymmetric simulations of equiaxed solidification. For wide inlets, and inlets using a combo bag, the inflow is swiftly entrained towards the chill and the final macrosegregation predictions are all similar. For narrow inlets without a combo bag, penetration of the thermal jet to the bottom of the sump locally delays solidification producing an inclination in the packed solid interface and the possible growth of columnar dendrites in this region.     

Microstructural evaluation of melt conditioned twin roll cast Al–Mg alloy

Abstract

Aluminium alloy strip has been successfully cast using the melt conditioned twin roll casting (MC-TRC) process. The liquid metal to be strip cast was fed into a melt conditioner, where it was intensively sheared under conditions of high shear rate and high turbulence induced by corotating twin screws. Melt conditioning provides a melt which has a uniform temperature and chemical composition and well dispersed nuclei. The conditioned melt was then fed into a twin roll caster. The MC-TRC process produced high quality Al–Mg alloy strip with fine and uniformly distributed intermetallics throughout the cross-section with minimal centreline segregation compared to conventional TRC strip. The Erichsen height value of MC-TRC sheet was superior to that of TRC sheet. The lower formability of the TRC sheet is attributed to the severe centreline segregation.     

Experimental investigation of microsegregation in low frequency electromagnetic casting 7075 aluminum alloy

7075 aluminum alloy ingots were prepared by the process of low frequency electromagnetic casting (low frequency electromagnetic casting = LFEC) and conventional direct chill (direct chill = DC) casting, respectively. The effects of low frequency electromagnetic field on microsegregation were investigated from eutectic analysis and electro probe microanalysis (EPMA). It was found that the amount of the eutectic, which was composed of α‐Al (aluminum phase), T phase and MgZn₂ was decreased markedly. In contrast, the solute concentration profiles which depend on solid fraction were increased to a certain extent with the presence of low frequency electromagnetic field. The effective distribution coefficient k_e was calculated, and the values of k_e for solutes in the LFEC sample were bigger than those in the DC sample. The LFEC process alleviated the microsegregation in 7075 aluminum alloy.     

Comparison of texture evolution during cold rolling between direct chill and continuous cast aluminium alloy 5052

Abstract

Hot bands of direct chill cast (DC) and continuous cast (CC) aluminium alloy 5052 were cold rolled to different reductions after being annealed at 454 ° C for 4 h. The texture evolution during cold rolling was investigated for both DC and CC AA 5052 by determining the orientation distribution functions of the cold rolled specimens via X-ray diffraction. Texture evolution during cold rolling was predicted by empirical formulas of the variation of the texture components with true strain. The results show that the processing method (DC versus CC) has an effect on the texture of annealed hot bands and the texture evolution during subsequent cold rolling.     

Microstructure and texture of aluminium alloys 3105 and 3015 during cold rolling and annealing

Abstract

The microstructure and texture of industrially produced hot bands of direct chill (DC) cast AA 3105 and continuous cast (CC) AA 3015 during cold rolling and annealing have been studied. The textures were determined using an X-ray diffraction technique and then analysed using orientation distribution functions. The Copper, Brass, and S texture components were the major deformation texture components for both the DC and CC materials after the same cold rolling process. After an annealing process, the Cube component was found to be the major recrystallisation texture component for AA 3105 DC material. In contrast, it was difficult to obtain the Cube texture component in AA 3015 CC material after cold rolling followed by annealing. Instead, the P orientation {011} 〈566〉 with Euler angles of (?₁,Φ,? ₂) = (60°,45°,0°/90°) was found to be the major recrystallisation texture component of the AA 3015 CC material.     

Analysis of solidification in Bridgman furnace as simulation of DC casting of aluminium alloy slabs

Abstract

The vertical Bridgman directional solidification equipment has been used in several investigations to simulate direct chill casting of wrought aluminium alloys. As a basis for such investigations and alloy developments, it is important to have an understanding of the performance of the furnace used during simulation of the casting conditions. In this investigation the thermal conditions in the furnace have been analysed in detail, both by measurements and by mathematical modelling. The growth characteristics of the furnace, such as gradient, growth rates and cooling rates have been compared to conditions in large ingots. The direct chill casting conditions, which the simulations have been compared to, are casting of slabs of 330 and 600 mm thickness of an aluminium AA3003 type alloy. The results show that the experiments are able to simulate the cooling conditions in the ingots except from the surface zone. Comparisons of the microstructures have been made and a good agreement has been obtained for structure parameters such as grain size and DAS.     

Direct chill casting of magnesium alloy under pulsed magnetic field

To refine the grain size and improve the hot workability, the billets of AZ80 magnesium alloy were cast by a new process of the pulsed magnetic field-direct chill casting. The effect of pulsed magnetic field on the grain size, segregation and mechanical properties of cast billet were investigated by experiments. The results indicate that the grain size of cast billet with pulsed magnetic field was greatly refined, and the homogeneity of microstructure was improved significantly. Meanwhile, the macrosegregation and microsegregation of main alloying elements in the cast billet with pulsed magnetic field were suppressed. Compared with the conventional cast billet, the yield strength, ultimate tensile strength and elongation of the cast billet with pulsed magnetic field were increased obviously.     

Cold cracking in DC-cast high strength aluminum alloy ingots: An intrinsic problem intensified by casting process parameters

For almost half a century the catastrophic failure of direct chill (DC) cast high strength aluminum alloys has been challenging the production of sound ingots. To overcome this problem, a criterion is required that can assist the researchers in predicting the critical conditions which facilitate the catastrophic failure of the ingots. This could be achieved at first glance by application of computer simulations to assess the level and distribution of residual thermal stresses. However, the simulation results are only able to show the critical locations and conditions where and when high stresses may appear in the ingots. The prediction of critical void/crack size requires simultaneous application of fracture mechanics. In this paper, we present the thermo-mechanical simulation results that indicate the critical crack size distribution in several DC-cast billets cast at various casting conditions. The simulation results were validated upon experimental DC-casting trials and revealed that the existence of voids/cracks with a considerable size is required for cold cracking to occur.     

Finite element analysis for inductive reheating process of continuously cast steel billets

Abstract

Inductive heating has distinctive advantages for reheating of billets in continuous casting, but the success of this process requires good understanding of the operation principles and effective control of the process variables. In the present study modelling and finite element simulation are performed for the inductive reheating of continuously cast square billets. Important performance characteristics such as temperature profile, magnetic field distribution and the power absorption have been analysed. Representative temperatures obtained from the simulation are compared with experimentally measured temperatures. The results provide better understanding of the inductive reheating process and key information for process design.     

Role of grain refining in hot cracking and macrosegregation in direct chill cast AA 7075 billets

Abstract

Direct chill (DC) casting experimental results are presented for an Al–Zn–Mg–Cu alloy with and without grain refining by attempting the inoculant addition either in the furnace or in the launder at two different casting speeds. Despite considerable structural refinement macro segregation remains unchanged with grain refining at a lower casting speed. Hot cracking, on the other hand, is totally prevented at this casting speed. Depending on the grain refining practice, either hot tears do not appear at all or prior cracks existing in the non-grain refined billet are completely healed. At a higher casting speed, the severity of segregation increases. The hot cracking tendency, however, has shown a distinct difference with grain refining method, with furnace-refined alloy resulting in a crack-free billet. But the billet grain refined in the launder exhibited hot cracking. Overall, these experimental results confirm the beneficiary effect of grain refining on hot cracking.     

Mechanism of grain refinement of an Al–Zn–Mg–Cu alloy prepared by low-frequency electromagnetic casting

Low-frequency electromagnetic casting (LFEC) process had been developed and is being used for the past several years with the application of an induction coil placed outside the conventional direct chill (DC) casting mould. It has been demonstrated that the LFEC process has a significant grain refining effect on aluminium alloys. In the present study, temperature measurement and direct quenching from liquid and/or semi-solid were carried out to study the temperature field during casting process and to understand the mechanism of the grain-refining effect of the LFEC process. The experimental results showed that in contrast to the conventional DC casting process, the liquid melt from the launder, during the LFEC process, is cooled with very high cooling rate directly to 3–6 °C below the liquidus, and the temperature field of the entire melt in the mould, and the hot top is quite uniform, which results in the enhanced heterogeneous nucleation and improved survival rate of the nuclei. This is believed to be the main reason why the LFEC process can significantly refine the grain size of aluminium alloys.     

Effect of combined application of electromagnetic fields on horizontal direct chill casting of 7050 aluminium alloy

Abstract

Influence of combined electromagnetic field application on horizontal direct chill casting of 7050 aluminium alloy is investigated. Temperature measurement and structure observation are carried out to analyse the casting process under different electromagnetic fields. Combined electromagnetic field can reduce the harmful gravitational thermal effect in the horizontal direct chill casting process and improve the ingot properties. With application of combined electromagnetic field, temperature distribution in the melt is more uniform, sump of the ingot becomes flat and symmetric, surface quality of ingot improves markedly, grain morphology changes from feathery grains to equiaxed grains and grain size decreases. Distribution of copper and zinc in the transverse section of the ingot becomes more uniform.     

Influence of aluminium on solidification structure and initial deformability of continuously cast C–Mn–Si–N steel

Abstract

The solidification structure and the initial deformability of continuously cast steel were investigated by assessment of cracks on billets and on rolled product. Some billets were rolled directly off the casting machine and some cooled to ambient temperature, then reheated to rolling temperature. On direct rolled steels, the number of defects increases with increasing aluminium content, while virtually no defects are found on steel rolled after reheating. By increasing the aluminium content, the solidification structure of steel is highly modified and a columnar structure obtained over the entire section of the billet. It was shown by chemical analysis and fracture examination that the increased hot shortness is not related to the effect of AIN. It is concluded that the hot shortness is related to the effect of aluminium on the solidification structure.

MST/761     

Dispersion strengthening in vanadium microalloyed steels processed by simulated thin slab casting and direct charging Part 1 – Processing parameters,mechanical properties and microstructure

Abstract

A study simulating thin slab continuous casting followed by direct charging into an equalisation furnace has been undertaken based on six low carbon (0·06 wt-%) vanadium microalloyed steels. Mechanical and impact test data showed that properties were similar or better than those obtained from similar microalloyed conventional thick cast as rolled slabs. The dispersion plus dislocation strengthening was estimated to be in the range 80–250 MPa. A detailed TEM/EELS analysis of the dispersion sized sub 15 nm particles showed that in all the steels, they were essentially nitrides with little crystalline carbon detected. In the steels V–Nb, V–Ti and V–Nb–Ti, mixed transition metal nitrides were present. Modelling of equilibrium precipitates in these steels, based on a modified version of ChemSage, predicted that only vanadium rich nitrides would precipitate in austenite but that the C/N ratio would increase through the two phase field and in ferrite. The experimental analytical data clearly point to the thin slab direct charging process, which has substantially higher cooling rates than conventional casting, nucleating non-equilibrium particles in ferrite which are close to stoichiometric nitrides. These did not coarsen during the final stages of processing, but retained their highly stable average size of, ～7 nm resulting in substantial dispersion strengthening. The results are considered in conjunction with pertinent published literature.     

The Microstructure of Direct Squeeze Cast and Gravity Die Cast 7050 (Al–6.2Zn–2.3Cu–2.3Mg) Wrought Al Alloy

A 7050 (Al–6.2 wt% Zn–2.3 wt% Cu–2.3 wt% Mg) Al alloy, conventionally used for wrought products, has been successfully cast to near-net shape using direct squeeze casting. Squeeze casting with an applied pressure of 50 MPa removes the defects observed in gravity die cast billets, in particular, (1) shrinkage pipe, (2) poor die replication and waisting, and (3) microporosity. Squeeze casting results in considerable refinement of the microstructure due to an increase in cooling rate from 0.5°C s^–1 for gravity casting to 11°C s^–1 for squeeze casting in a tool steel die lined with porous insulation, and from 2.5 to 10°C s^–1, respectively, in an uninsulated die. A normal segregation pattern of increasing eutectic toward the center of the billet is found for squeeze casting, compared to an inverse segregation pattern of increasing eutectic toward the edge of the billet for gravity casting. This change in segregation pattern is due to a higher radial temperature gradient and reduced time in the semisolid state for squeeze casting.     

Microstructures and mechanical properties of milled and continuously cast lead sheet

Abstract

In Part 1 of this paper the microstructures generated in the Pb–Cu system at copper contents less than the eutectic content of 0·06 wt-% were studied for sheet produced both by the conventional method of rolling from heavy cast block and by a direct casting method without rolling. In Part 2 the mechanical properties associated with these microstructures are investigated and the mechanisms by which failure occurred are considered. Similar levels of tensile strength and thermal fatigue resistance are obtained but with lower tensile strain to fracture in the cast material. Both materials are considered satisfactory for use in flashing and roofing applications.

MST/1465     

Prevention of Scum Formation and Entrapment in High Carbon Steel Billets

Gradual build-up of scum over the liquid steel surface in the mould gives rise to entrapped exogenous inclusions and slag patch in the cast billets. The problem is more commonly observed during the open stream casting of continuous casting of high carbon (C > 0.6 mass%) steel billets. This problem has been quite commonly observed at Tata Steel during its billet casting. Present work aimed at eliminating the problem of scum formation during open stream billet casting of high carbon steel billets. This work involved experiments in the laboratory as well as in the plant. In addition, based on liquid steel composition thermodynamic calculations were carried out for predicting the possible oxide inclusions in Mn-Si deoxidised steels. Water model experiments were carried out in a full-scale billet caster physical model for investigating the influence of tundish outlet nozzle alignment on the entrapment of scum as slag patch over the billet surface. Based on these, cause of scum formation and its exact mechanism of entrapment in cast billets were established. Finally, countermeasures were recommended to the plant. Adequacy of the proposed countermeasures was established through a series of trials in the plant before implementation. Improvement obtained from the implementation over a period of time has been reported in the paper.

     

Microstructure and mechanical properties of hot extruded Mg–Al–Mn–Ca alloy produced by rapid solidification powder metallurgy

The microstructure and mechanical properties of hot extruded Mg–Al–Mn–Ca alloy was investigated. Both rapid solidified powders and cast billets were extruded at 573, 623 and 673 K to optimize the processing conditions for obtaining better mechanical response. Powder was consolidated to prepare the extrusion billets using both cold compaction and Spark Plasma Sintering at 473 K. The tensile properties of the extruded alloy were then evaluated and correlated to the observed microstructure. The results show that the use of rapid solidified powder could lead to effective grain refinement, which in turn resulted in the improved mechanical response, especially compared to the extruded conventional cast material.