Grain refinement of stainless steel in ultrasound-assisted additive manufacturing

Metals and alloys fabricated by fusion-based additive manufacturing (AM), or 3D printing, undergo complex dynamics of melting and solidification, presenting challenges to the effective control of grain structure. Herein, we report on the use of high-intensity ultrasound that controls the process of solidification during AM of 316L stainless steel. We find that the use of ultrasound favours the columnar-to-equiaxed transition, promoting the formation of fine equiaxed grains with random crystallographic texture. Moreover, the use of ultrasound increases the number density of grains from 305 mm−2 to 2748 mm−2 despite an associated decrease in cooling rate and temperature gradient in the melt pool during AM. Our assessment of the relationship between grain size and cooling rate indicates that the formation of crystallites during AM is enhanced by ultrasound. Furthermore, the use of ultrasound increases the amount of constitutional supercooling during solidification by lowering the temperature gradient in the bulk of the melt pool, thus creating an environment that favours nucleation, growth, and survival of grains. This new understanding provides opportunities to better exploit ultrasound to control grain structure in AM-fabricated metal products

Grain refinement of an Al-2 wt%Cu Alloy by Al3Ti1B master alloy and ultrasonic treatment

Both inoculation by AlTiB master alloys and Ultrasonic Treatment (UT) are effective methods of refining the grain size of aluminium alloys. The present study investigates the influence of UT on the grain refinement of an Al-2 wt% Cu alloy with a range of Al3Ti1B master alloy additions. When the alloy contains the smallest amount of added master alloy, UT caused significant additional grain refinement compared with that provided by the master alloy only. However, the influence of UT on grain size reduces with increasing addition of the master alloy. Plotting the grain size data versus the inverse of the growth restriction factor (Q) reveals that the application of UT causes both an increase in the number of potentially active nuclei and a decrease in the size of the nucleation free zone due to a reduction in the temperature gradient throughout the melt. Both these factors promote the formation of a fine equiaxed grain structure

The role of ultrasonically induced acoustic streaming in developing fine equiaxed grains during the solidification of an Al-2% Cu alloy

Recent research and a simulation of heat transfer and solidification during acoustically generated convection showed that the location of the coolest liquid, and thus the place where the first grains are expected to form, is under the sonotrode. Further, the generated vigorous convection produces a very flat temperature gradient in the bulk of the melt facilitating the formation of a refined equiaxed structure throughout the casting. This study validates these findings through a series of experiments on an Al-2 wt pct Cu alloy, which evaluate grain formation under the sonotrode over time and relate this to the formation of the macrostructure of a cast ingot. Analysis of the results confirms the predictions of the simulation and shows that, for the conditions applied, most grains nucleated in the cavitation zone are swept into the melt by acoustically generated convection and, over a period of 70 seconds, the number of grains increase and they grow with spherical and globular morphology gradually filling the casting with refined equiaxed grains. It was found that the macrostructure of each casting is made up of three microstructural zones. A fine grained equiaxed zone forms from the bottom of the casting due to settling of grains during and after termination of ultrasonic treatment (UST), which increases in size with the increasing duration of UST. Above this zone, a coarse-grained structure is formed due to depletion of UST-generated grains on termination of UST. At the top of the casting, a zone of columnar grains growing from the top surface of the melt is formed. The latter two zones decrease in size with the increasing UST duration until 80 seconds, when the macrostructure consists entirely of the equiaxed zone. (C) The Minerals, Metals & Materials Society and ASM International 201

Grain refinement of Al-Si hypoeutectic alloys by Al3Ti1B master alloy and ultrasonic treatment

Al-Si alloys are widely used in automotive and aerospace industries due to their excellent castability, high strength to weight ratio and good corrosion resistance. However, Si poisoning severely limits the degree of grain refinement with the grain size becoming larger as the Si content increases. Generally the effect of Si poisoning is reduced by increasing the amount of master alloy added to the melt during casting. However, an alternative approach is physical grain refinement through the application of an external force (e.g. mechanical or electromagnetic stirring, intensive shearing and ultrasonic irradiation). This work compares the grain refining efficiency of three approaches to the grain refinement of a range of hypoeutectic Al-Si alloys by (i) the addition of Al3Ti1B master alloy, (ii) the application of Ultrasonic Treatment (UT) and (iii) the combined addition of A13Ti1B master alloy and the application of UT

The effect of alloy content on the grain refinement of aluminium alloys

A recent model that predicts the effect of solute content on grain size was shown to predict the effect of titanium additions on the grain size of pure aluminium and an AlSi7Mg0.3 alloy. The model assumes that nucleation on substrates is facilitated by constitutional undercooling in front of a growing grain. To determine how generally applicable the model is to a broad range of aluminium alloys, titanium additions were made to five wrought alloys containing the same level of TiB2. It was found that the grain size obtained is a function of the reciprocal of mc(o)(k-1), the growth restriction factor, and that there is a lower limit to the grain size that can be achieved for the casting conditions used in the experiments, It was also found that at least stoichiometric levels of titanium are required for TiB2 particles to be effective nucleants