Láser melting treatment of NiP surface alloys on mild steel : Influence of initial coating thickness and láser scanning rate ( # )

Different thickness Ni-P coatings deposited on mild steel are submitted to láser surface melting at different scanning rates. The microstructure of the alloys is characterized by optical and scanning electrón microscopy and microprobe analysis. It is shown that both the initial coating thickness and the láser scanning rate have an influence on the shape, extent and size of the different structures resulting from the solidification process. Thus, when the láser scanning rate increases a progressive refinement of the structure takes place that could even totally block the dendritic growth produced during solidification for a high initial coating thickness.

ductility and conductivity of metáis with the great resistance and chemical inertia of glasses which lack crystalline structure (1).
The solidification microstructure depends especially on the solidification rate and on the material composition (2).Thus the addition of a non metallic element, as phosphorous, in a proportion equal or higher than 20 at.wt.% leads to amorphous alloys (3 and 4).In the rapid solidification technics, the cooling rate is a function of both the sample thickness and the characteristics of the contact between the sample and the cooling substrate.In practice, the high cooling rates needed to get an amorphous structure (> 10 5 Ks 1 ) are only possible for low thickness alloys.Others technologies for manufacturing amorphous alloys are based on the predeposition of an amorphous coating on a metallic substrate with a low degree of dilution.Corrosión resistant surfaces with high mechanical performances are obtained by this procedure (5).During the processing of surface alloys, high levéis of residual stresses may be introduced in the coating that can lead to cracking of various forms; clearly for the satisfactory protection of the substrate, the cracking of the coating must be kept to a minimum.Láser surface melting offers an attractive means of achieving rapid solidification and the possibility of sealing cracks in predeposited amorphous alloys.
The objective is to obtain a surface homogeneous alloy with no cracks and a high degree of amorphicity.Different thickness electroless Ni-P layers deposited on mild steel samples are considered.These systems have been submitted to láser surface melting at different scanning rates and the resulting microstructures have been characterized.

Materials
Mild steel of the composition given in table I was used as substrate.Specimens (70 x 33 x 10 mm 3 ) were coated with an electroless Ni-P layer.The phosphorous contení was about 8 wt % and the nickel content about 92 %.Layers of various thicknesses were used: 45, 80, 120 and 180 jxm.

Láser processing
The melting treatments were carried out using a Spectra Physics 975 model.Coated specimens were irradiated with a continuous C0 2 láser having an elliptical spot of 4 x 6 mm, moving in the direction of the minor axis, and an output power of 3 kW, that corresponds to an energy density of 3,980 W/cm 2 .A black absorbing paint was used to increase the efficiency of the láser treatment.Thus, the energy absorbed by the samples was about 1,650 W/cm 2 (6).The scanning rates varied between 1,180 and 5,952 mm/min.

Techniques for the eharacterization of the microstructure
The resulting microstructures were investigated by several techniques: optical microscopy on cross sections, after etching with a 2 % nital solution, for mild steel substrate structure, and concentrated HN0 3 for the Ni-P alloy structure, and scanning electrón microscopy and microanalysis experiments through energy dispersión analysis of X-ray (EDX).

Characterization of Ni-P electroless samples
The carbón steel substrate microstructure is composed of ferrite grains and pearlite in bands (Fig. 1).
The electroless Ni-P deposit presents no microstructure and during the cross-section examination of the samples only a succession of layers could be observed (Fig. 2).The composition of the Ni-P alloy determined by EDX was 8 wt % P and 92 wt % Ni.The Ni-P coating shows longitudinal and transversal cracks due to internal stresses (Fig. 3).These stresses are generated in the absorption-desorption processes accompanying the electrodeposition reaction of nickel and in the codeposit of phosphorous that alters the crystallographic structure of nickel.

Láser treated samples
After optimization of the láser processing conditions, a cross-section observation of the sample reveáis the existence of different zones that have been characterizated.Figure 4 shows a schematic representation of these zones.

Steel substrate
The steel substrate structure is composed of martensite and ferrite grains in the zone 1 of figure 4 and of martensite in the zone 2, as can be seen in the cross section view presented in figure 5. Penetrations of melted metal appear in the steel substrate as can be seen at higher magnifications in figure 6.These penetrations are essentially composed by phosphorous as the EDX analysis revealed.

Ni-P láser treated alloy
A strip of material without a defined structure is observed in the interface (zone 3, Fig. 4) with the steel substrate.This strip corresponds to a plañe front growth as can be seen in figure 7. The analysis of the composition of this strip shows a high iron content, some nickel proportion and no presence of phosphorous.FIG. 3 The rest of the Ni-P alloy (zones 4 y 5, Fig. 4) presents a dendritic solidification structure as a consequence of the láser melting (Fig. 8).The dendritic size depends on the láser processing conditions and specially on the scanning rate: the higher that rate, the smaller the dendritic size.The composition of dendrites and interdendrites áreas is different.Thus, the dendrites are essentially composed by a Ni-Fe solid solution.Phosphorous does not take part in the dendritic growth being rejected to the interdendritic spaces as it could be deduced from the results of the EDX analysis presented in figure 9.
A careful SEM examination of the samples reveáis that the interdendritic spaces present a more complex structure (Fig. 10).Actually, the interdendritic áreas are an eutectic mixture of a Fe-Ni solid solution and a mixed phosphide (Fe-Ni^P, as could be inferred from the Fe-Ni, Ni-P and Fe-P phase diagrams (7).
The characteristics of this dendritic solidification process depend not only on the láser processing conditions but also vary with the initial scanning rate, the refinement of the structure is higher as a thicker coating.The structure shown in figure 11 d) is particularly interesting.For samples with the máximum Ni-P initial coating thickness -180 jxm-and láser treated at the máximum scanning rate -5,952 mm/min-, there is no iron at all in the dendritic composition and the eutectic structure does not appear in the interdendritic áreas.That is the only case in which after láser treatment, big áreas of a probably amorphous material appear (Fig. 12) with the same composition of the initial Ni-P alloy.The coating is free of defects and as there is no presence of penetration ñor dillution effect with substrate, the phosphorous content of the coating has not decreased.Finally, at the heat affected zone of the láser track (zone 6, Fig. 4), a globular precipitation takes place as it is possible to see in figures 13 and 14.The globular precipitates are composed essentially by nickel with some phosphorous.As the temperature reached at the borders of the láser track is not high enough to melt the material, this process must be a solid state transformation.Two samples of different thickness were heated up to a temperature a little below their melting point and left to cool to prove this hypothesis.The SEM examination of both samples reveáis a general globular precipitation similar to that shown in figure 14.
It should be mentioned that although the initial cracks of Ni-P amorphous alloy are sealed in the láser induced melting of the alloy, new cracks FIG. 12

DISCUSSION
Láser treatment has affected both the mild steel substrate structure and the electroless Ni-P coating.In the steel substrate, microstructure modifications have occurred during láser treatment by heating effect, through solid state phase transformations.Near the melt zone, the temperature reached during heating was high enough to promote the transformation a -> 7 with quenching to martensite during later cooling.In the vicinity of the unaffected substrate, the temperature reached is located in the a-y 2-phase field and because of the heating, the pearlite -only stable at low temperatures-is transformed to austenite that quenches to martensite during the cooling.However, the ferrite in bands with low carbón content only presents a modification in the grain size.
The solidification process starts with a plañe front composed by a solid solution of Fe-Ni in which phosphorous is rejected to the melt coating alloy and also penetrates the steel substrate producing a metal liquid attack.The reason for this phosphorous segregation is that the phosphorous is not soluble either nickel ñor in iron, but there is a full solubility between iron and nickel.When the solidification rate raises, the plañe front becomes unstable disappearing and dendritic growth of Fe-Ni solid solution appears.Again the phosphorous is segregated to the interdendritic áreas.When the solidification process finishes the interdendritic áreas have a higher phosphorous content and the composition of an eutectic of Fe-Ni solid solution and a mixed phosphide: (Fe, Ni)-(Fe, Ni) 3 P (8).
The different solid phases and the solidification mechanism proposed take place for all the samples láser treated independently of the conditions tested, except in the singular case of a coating thickness of 180 jxm and a scanning rate of 5,952 mm/min.In this case, there is almost no dilution with the steel substrate, the amorphous structure is kept and only a little degree of crystallization occurs.
The initial coating thickness and the láser scanning rate have an influence on the shape, extent and size of the different structures resulting from the solidification process.
In figure 16 the thickness of the melted coating is represented versus the láser scanning rate for the samples with the máximum and mínimum initial coating alloy thickness.It could be seen that for samples with 45 {xm thickness, the melted coating thickness is about five times the initial one for a low scanning rate.Here the interaction time has been so high that crystallization has been complete and a strong effect of dilution with the base metal has taken place.The higher the láser scanning rate, the lower the dilution with the steel and the degree of crystallization.
The low láser scanning rates have also a negative influence on the dilution effect with steel and the degree of crystallization for samples with the máximum coating alloy thickness (180 |xm).The optimum conditions tested are reached for samples with 180 juim láser treated at 5,952 mm/min.In this case the melted coating keeps its initial thickness and the degree of crystallization is mínimum, the adherence is good and the coating has no cracks.
In a further research it should be considered that to block the dendritic growth not only the láser scanning rate must be increased but also the láser incident beam power -a parameter not studied in the present paper-could be decreased in order to get enough heating effect to melt the coating but the minimum perturbation of the substrate material.

CONCLUSIONS
The beneficial effects of the láser melting treatment employed to improve the quality of Ni-P surface alloys on mild steel depend on the láser processing conditions -especially on the láser scanning rate-and the initial coating alloy thickness.
In the dendritic solidification, when the láser scanning rate increases a progressive reñnement of the structure takes place that could even block totally the dendritic growth.If high enough beam scanning rates are reached, the later cooling rate will be so high that it will not allow the diffusion of atoms to form the equilibrium phases and an amorphous structure could be obtained.