Laser Induced Micro/Nano …
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which resulted in a 23.62% porosity, 8.68 μm surface roughness, 30 GPa modulus
of elasticity and 522 MPa UCS [23]. A cellular structure formed in the Ti64 alloy
during SLM had 24 and 32% higher kinetic friction coefficient and a large amount of
adhered bone compared to the dense and as-cast alloy thus showing an enhancement
in implant stability [24]. Besides, the surface roughness and pore size formed in the
SLM component significantly affected its property, and the effective bone ingrowth
and vascularization were reported for a pore size in the range of 200-300 μm [25, 26].
In spite of its enhanced mechanical performance, the internal defects due to the
SLM process suggests developing more strategies to achieve better stents. The use
of SLM for biodegradable implant materials is growing steadily in recent days.
Biodegradable metallic materials are advantageous owing to their superior mechanical properties when compared to the polymeric implant materials. There are three
majors, but not limited biodegradable implant materials that are highly focused such
as Mg, Zn, and Fe based alloys. Among them, Mg has a very high corrosion rate
in physiological solution, and Fe has a slower corrosion rate, while Zn has the
nominal corrosion rate to satisfy the implant requirement particularly for cardiovascular stents [27–29]. SLM technique has been used to manufacture the Mg-based
degradable implant materials [30], and only a few reports on the Fe based alloys [31]
are available, however, Zn based alloys are not widely reported [27, 32].
2.3 Laser Surface Melting (LSM) of Implant Materials
LSM is widely used to enhance the surface-related properties such as hardness,
wear resistance and corrosion resistance of various alloys [33–35]. In the laser
melting process, the metal surface is irradiated with a high energy laser beam which
results in the homogenization of microstructure, grain refinement, and dispersion of
second phases, solubility, and phase transformation (Fig. 2b). The refined microstructure alters the mechanical and corrosion properties in physiological solution, thus
enhances the biocompatibility. Usage of laser source and processing parameters vary
depending upon the materials used due to their reactivity difference with different
laser sources, thus control the properties of the materials.
The LSM alters the surface microstructure and chemical composition which
greatly affect the biological response of the metallic implant materials. Laser
remelting of Mg 65 Zn 30 Ca 5 alloy was performed by Zhang et al. using a low power
fiber laser at scan speeds from 500 to 4000 mm/s. The depth and width of the melted
layer were reduced as the scanning speed was increased and the fraction of the crystalline phase was also decreased [36]. As the scan speed was increased the direct
contact between the laser beam and substrate was reduced, therefore the heat transfer
to the surface was also reduced. A positive shift in the corrosion potential (E corr ) and
a decrease in corrosion current density (i corr ) were observed as the laser melting
scan speed was increased. The formation of the amorphous phase with chemical
homogeneity and absence of microstructural defects was mainly attributed to the
better corrosion resistance of the remelted layer. Recently, we have reported that
445
which resulted in a 23.62% porosity, 8.68 μm surface roughness, 30 GPa modulus
of elasticity and 522 MPa UCS [23]. A cellular structure formed in the Ti64 alloy
during SLM had 24 and 32% higher kinetic friction coefficient and a large amount of
adhered bone compared to the dense and as-cast alloy thus showing an enhancement
in implant stability [24]. Besides, the surface roughness and pore size formed in the
SLM component significantly affected its property, and the effective bone ingrowth
and vascularization were reported for a pore size in the range of 200-300 μm [25, 26].
In spite of its enhanced mechanical performance, the internal defects due to the
SLM process suggests developing more strategies to achieve better stents. The use
of SLM for biodegradable implant materials is growing steadily in recent days.
Biodegradable metallic materials are advantageous owing to their superior mechanical properties when compared to the polymeric implant materials. There are three
majors, but not limited biodegradable implant materials that are highly focused such
as Mg, Zn, and Fe based alloys. Among them, Mg has a very high corrosion rate
in physiological solution, and Fe has a slower corrosion rate, while Zn has the
nominal corrosion rate to satisfy the implant requirement particularly for cardiovascular stents [27–29]. SLM technique has been used to manufacture the Mg-based
degradable implant materials [30], and only a few reports on the Fe based alloys [31]
are available, however, Zn based alloys are not widely reported [27, 32].
2.3 Laser Surface Melting (LSM) of Implant Materials
LSM is widely used to enhance the surface-related properties such as hardness,
wear resistance and corrosion resistance of various alloys [33–35]. In the laser
melting process, the metal surface is irradiated with a high energy laser beam which
results in the homogenization of microstructure, grain refinement, and dispersion of
second phases, solubility, and phase transformation (Fig. 2b). The refined microstructure alters the mechanical and corrosion properties in physiological solution, thus
enhances the biocompatibility. Usage of laser source and processing parameters vary
depending upon the materials used due to their reactivity difference with different
laser sources, thus control the properties of the materials.
The LSM alters the surface microstructure and chemical composition which
greatly affect the biological response of the metallic implant materials. Laser
remelting of Mg 65 Zn 30 Ca 5 alloy was performed by Zhang et al. using a low power
fiber laser at scan speeds from 500 to 4000 mm/s. The depth and width of the melted
layer were reduced as the scanning speed was increased and the fraction of the crystalline phase was also decreased [36]. As the scan speed was increased the direct
contact between the laser beam and substrate was reduced, therefore the heat transfer
to the surface was also reduced. A positive shift in the corrosion potential (E corr ) and
a decrease in corrosion current density (i corr ) were observed as the laser melting
scan speed was increased. The formation of the amorphous phase with chemical
homogeneity and absence of microstructural defects was mainly attributed to the
better corrosion resistance of the remelted layer. Recently, we have reported that