Effect of reactive elements on porosity in spray-formed copper-alloy Billets

Abstract

In spray-formed preforms, porosity is unavoidable. This is because the atomising gas becomes entrapped during the process. Normally, nitrogen is used for atomisation and therefore the voids are filled with this gas, which is not soluble in copper. To obtain a dense material after further forming processes such as extrusion, forging or drawing, porosity has to be limited to a specified level.

There are means to influence porosity inside the spray-formed billet. To a certain extent, porosity can be influenced by process parameters. Reactive elements inside the alloy have an additional effect, that is reaction of elements such as Zr, Ti, Al with the entrapped nitrogen. Until now, a detailed explanation has not been given, however, a theory of hot porosity formation is presented in this paper. The calculated content of nitrogen and titanium-nitride is verified using analytical methods. The addition of titanium into the melt can reduce porosity by a factor of three.

Introduction

Quality issues require limitation of as-sprayed porosity to a certain level for subsequent processing of spray-formed copper-alloy billets. This porosity can only be minimised when the parameters influencing porosity are first identified. These have been described previously by various authors. To a certain extent, porosity can be influenced by process parameters such as melt temperature, metal mass flow and gas to metal flow rate ratio G/M. Other parameters include travelling distance of the droplets, gas and droplet velocity, spray cone angle and scanning angle. The most important parameter is the gas to metal flow rate ratio as reported elsewhere [1]. It is not only important for porosity but for segregation too. In production plants the minimum G/M ratio is limited by increasing porosity, which is caused by the lower viscosity of the mushy layer, resulting in increased gas entrainment [2]. The limiting factor is damage to the billet by centrifugal forces, whereby large pieces of partly solidified material are expelled.

Pores can be formed inside the droplets by gas entrapment during droplet formation, dissolution of gas from the molten metal, solidification shrinkage or collision between larger liquid and smaller solidified droplets [3]. During deposition, the fraction of solid f_s is most important. At a high fraction of solid the pores arise from poor spreading of droplets and insufficient liquid feeding [3]. Interstices between adjacent droplets arriving at the surface leave irregular cavities [4]. This effect can be intensified by a high heat extracting substrate, which makes the layers near the substrate more porous than the bulk material [4], [5], [6].

At a low fraction of solid the interstices between solid particles can be fed by liquid metal and porosity decreases. However there are some contrary results. Akhlaghi et al. [3], Doherty et al. [7] and Warner et al. [5] describe that a low liquid fraction f_l results in high percentages of pre-solidified droplets and high porosity, yet, Chu [2] reports that bulk porosity increases with increasing fraction of liquid in the spray cone when impacting on mushy surface. This result is explained by the state of the mushy layer on the top of the deposit. At a high fraction of liquid the mushy layer is thick and hot and therefore the viscosity is low. The mushy layer is continuously disturbed by the high velocity gas jet and atomised droplets, thus when droplets hit the mushy surface, they entrain the surrounding gas [8]. It is therefore expected that f_l has an upper and a lower limit for preventing either gas porosity or cold porosity.

Another type of porosity is caused by the so-called cauliflower-effect. This sort of porosity occurs during spray deposition in the centre of the billet when many solidified droplets are collected [9].

Reactive elements inside the alloy have an additional effect, as reported previously by Watson [4] and Watson et al. [10], and Cookey and Wood [11]. It is explained by reaction of reactive elements such as Zr, Ti, Al with the entrapped nitrogen. Cookey and Wood [11] recommend strong nitride formers such as silicon and chromium, while Watson named as preferred elements aluminium, silicon, titanium, chromium and zirconium for reducing porosity. The formation of nitrides changes the surface tension of the droplets. This influences the behaviour of the droplets during the impact of the mushy layer and therefore the entrainment of gas [8].

All the reported types of porosity can be classified into two classes: cold and hot porosity (Fig. 1).