Spray forming of high density sheets

Abstract

Porosity is one of the most important quality criteria of spray‐formed materials in the as‐sprayed condition. Typically, spray‐formed sheets have a porous rim close to the substrate and depending on the spray conditions cold or hot porosity may also be present in the core of the deposit. This porosity has to be removed or minimized to make further processing steps such as rolling, forging or extrusion possible. In this paper, the influence of both substrate temperature and deposit surface temperature on porosity in spray‐formed sheets is studied. For this purpose spray forming experiments (sheet size 1000 mm × 250 mm) were carried out using three different materials: aluminium‐bronze, tin‐bronze and a nitriding steel. For the copper‐base alloys preheated steel‐substrates with different temperatures were moved through a scanning spray cone. In the case of steel a ceramic substrate at room temperature was used. In addition to the variation of the substrate temperature, the gas to metal mass flow ratio (GMR) was varied to achieve different deposit surface temperatures. During the run the surface temperature in the deposition zone was measured using a scanning, multi‐wavelength pyrometer. Samples of the deposits were polished and rasterized by light microscopy. The local porosity was characterized by digital image analysis. The influence of the substrate temperature and the GMR on the porosity in the vicinity of the substrate is evaluated and discussed in detail. The impact of the deposit surface temperature on the porosity was analyzed and is discussed as well. It was found that the deposit surface temperature has a strong impact on porosity for spray‐formed sheets. Finally, experimental results were used to develop a new approach to predict the porosity in spray‐formed sheets. The results clearly show the dependence on material properties. This approach can be used to identify process parameters to generate high density sheets in the future.

Solidification Sequence of Spray-Formed Steels

“…It has been reported in the literature that sprayformed products with a low porosity level and high yield are achieved when the deposit surface temperature at the deposition zone is kept above the solidus temperature of the spray-formed alloy [7,22,23] This paper proposes a solidification model for spray-forming in which the equilibrium solidification path takes place in the deposition zone. Two cases of spray-formed products with low porosity and high homogeneity, processed in laboratory and on industrial-scale spray-forming equipments, have been used to support the proposed model.…”

“…It was extensively reported that the non-stationary deposition conditions that occur at the initial stages of spray-forming lead to a microstructure with very high cold porosity (porosity caused if the fraction of liquid is too low and insufficient to fill the interstices between the solid particles in the deposition zone) in the vicinity of the substrate. [7,22] When the process reaches its steady-state condition, the deposit surface temperature remains constant and should be above solidus temperature to achieve a dense material. This model does not consider the transient deposition period and the microstructure evolution will be described from the point where the constant surface temperature is attained.…”

“…[2] Several types of alloys such as steels, cast irons, Ni-alloys, Al-alloys, and Cu-alloys, as well as different geometries (billets, tubes, and rings) have been successfully produced by spray-forming. [3][4][5][6][7][8][9][10][11][12][13][14] In all cases, the spray-formed microstructures present noticeable features: (1) equiaxed grains with diameters from 10 to 50 lm; (2) a high level of microstructural homogeneity and macrosegregation free; (3) uniform and homogeneous distribution of eutectic and second phases. The advantageous microstructure produced by the spray-forming process is well-known from research; however, solidification at the deposition zone and the creation of such a characteristic microstructure are still an open discussion.…”

Microstructure and Properties of 7075Al Alloy Fabricated by Directly Combined of Spray Forming and Continuous Extrusion Forming under Different Atomization Gas Pressures

“…On the one hand, more heat was removed from the spray droplets through higher gas pressure. On the other hand, the droplets, which are atomized under higher gas pressure, own bigger impulse when impinging on the substrate [31]. A faster cooling rate could be obtained with more heat taken away from the spray, which would contribute to the increase of the driving force for the nucleation in each single droplet.…”

Development of micro rotary swaging tools of graded tool steel via co-spray forming

“…The irregular shape of the pores in the gradient zone indicates relatively cold spray condition. During the spray forming of multilayer materials, a porous zone is frequently formed at the interface between the layers of the deposits [9,10]. When the first layer is spray-formed, the upper surface of the layer often shows high amount of porosity since the surface temperature is relatively low and the solidification shrinkage as well as the interstices between the droplet fragments and solid particles in the surface area cannot be refilled by sufficient liquid phase.…”