The tested series are indicated in Table 1。 

Table 1。 

Tested series 

Series 

pH 

Temperature (°C) 

Current density (A/dm2) 

1 4。2 ± 0。2 55 2。22

2 3。9 ± 0。2 45 5。56

3 4。0 ± 0。2 45 10。00

4 4。0 ± 0。2 45 22。22

Fig。 3 illustrates the surface of a sample of the series after the first etch。 It shows the roads originated by the FDM machine, that is to say that there is a good reproducibility。 It cannot be still noticed the rounded grain structure。 In Fig。 4, series 2, after a second etch, it can be observed a line of the road in a way less clear than in the previous case。 In Fig。 5, series 3 and 2° etch it begins to appear the rounded grain structure although it is very difficult to check the roads at this time。 Besides, the most darkened areas indicate the presence of pitting by inadequate conditions of process and bath composition。 

Fig。 3。 Series 1 (×150), etch 1。

Fig。 4。 Series 2 (×300), etch 2。

Fig。 5。 Series 3 (×300), etch 2。

   This behavior indicates that, working at a low current density and a high temperature, shells with a good reproducibility of the model and with a small grain size are obtained, that is, adequate for the required application。 

   If the analysis is carried out in a plane transversal to the deposition, it can be tested in all the samples and for all the conditions that the growth structure of the deposit is laminar (Fig。 6), what is very satisfactory to obtain a high mechanical resistance although at the expense of a low ductibility。 This quality is due, above all, to the presence of the additives used because a nickel sulfamate bath without additives normally creates a fibrous and non-laminar structure [9]。 The modification until a nearly null value of the wetting agent gave as a result that the laminar structure was maintained in any case, that matter demonstrated that the determinant for such structure was the stress reducer (Allbrite SLA)。 On the other hand, it was also tested that the laminar structure varies according to the thickness of the layer in terms of the current density。 

Fig。 6。 Plane transversal of series 2 (×600), etch 2。

6。 Internal stresses

   One of the main characteristic that a shell should have for its application like an insert is to have a low level of internal stresses。 Different tests at different bath temperatures and current densities were done and a measure system rested on cathode flexural tensiometer method was used。 A steel testing control was used with a side fixed and the other free (160 mm length, 12。7 mm width and thickness 0。3 mm)。 Because the metallic deposition is only in one side the testing control has a mechanical strain (tensile or compressive stress) that allows to calculate the internal stresses。 Stoney model [10] was applied and was supposed that nickel substratum thickness is enough small (3 μm) to influence, in an elastic point of view, to the strained steel part。 In all the tested cases the most value of internal stress was under 50 MPa for extreme conditions and 2 MPa for optimal conditions, an acceptable value for the required application。 The conclusion is that the electrolitic bath allows to work at different conditions and parameters without a significant variation of internal stresses。