Grant Brandal

Laser Induced Porosity and Crystallinity Modification of a Bioactive Glass Coating on Titanium Substrates

Functionally graded bioactive glass coatings on bioinert metallic substrates were produced by using continuous-wave (CW) laser irradiation. The aim is to achieve strong adhesion on the substrates and high bioactivity on the top surface of a coating material for load-bearing implants in biomedical applications. The morphology and microstructure of the bioactive glass from the laser coating process were investigated as functions of processing parameters. Laser sintering mechanisms were discussed with respect to the resulting morphology and microstructure. It has been shown that double layer laser coating results in a dense bond coat layer and a porous top coat layer with lower degree of crystallinity than an enameling coating sample. The dense bond coat strongly attached to the titanium substrate with a 10 lm wide mixed interfacial layer. A highly bioactive porous structure of the top coat layer is beneficial for early formation of a bone-bonding hydroxycarbonate apatite (HCA) layer. The numerical model developed in this work also allows for prediction of porosity and crystallinity in top coat layers of bioactive glass developed through laser induced sintering and crystallization. [DOI: 10.1115/1.4029566]

Experimental and Numerical Investigation of Laser Forming of Closed-Cell Aluminum Foam

Aluminum foams are generally very attractive because of their ability of combining different properties such as strength, light weight, thermal, and acoustic insulation. These materials, however, are typically brittle under mechanical forming, and this severely limits their use. Recent studies have shown that laser forming is an effective way for foam panel forming. In this paper, the laser formability of Al–Si closed-cell foam through experiments and numerical simulations was investigated. The bending angle as a function of the number of passes at different laser power and scan velocity values was investigated for large- and small-pore foams. In the finite element analysis, both effective-property and cellular models were considered for the closed-cell foam. Multiscan laser forming was also carried out and simulated to study the accumulative effect on the final bending angle and stress states. The maximum von Mises stress in the scanning section was on the order of 0.8 MPa, which was lower than the yield strength of the closed-cell foam material. This paper further discussed the reasonableness and applicability of the two models.

Beneficial Interface Geometry for Laser Joining of NiTi to Stainless Steel Wires

Joining the dissimilar metal pair of NiTi to stainless steel is of great interest for implantable medical applications. Formation of brittle intermetallic phases requires that the joining processes used for this dissimilar pair limits the amount of over-melting and mixing along the interface. Thus, because of its ability to precisely control heat input, laser joining is a preferred method. This study explores a method of using a cup and cone interfacial geometry, with no filler material, to increase the tensile strength of the joint. Not only does the cup and cone geometry increase the surface area of the interface, but it also introduces a shear stress component, which is shown to be beneficial to tensile strength of the wire as well. The fracture strength for various cone apex angles and laser powers is determined. Compositional profiles of the interfaces are analyzed. A numerical model is used for explanation of the processing parameters.

Effects of Laser Radiation on the Wetting and Diffusion Characteristics of Kovar Alloy on Borosilicate Glass

The purpose of this study was to investigate the advantages of laser surface melting for improving wetting over the traditional approach. For comparison, kovar alloy was preoxidized in atmosphere at 700 C for 10 min, and then wetted with borosilicate glass powder at 1100 C with different holding time in atmosphere. The proposed approach used a Nd:YAG laser to melt the surface of the kovar alloy sample in atmosphere, then wetted with borosilicate glass powder at 1100 C with the same holding time. The laser melted surface shows a decrease in contact angle (CA) from 47.5 deg to 38 deg after 100 min. Xray photoelectron spectroscopy (XPS) analysis shows that the surface and adjacent depth have higher concentration of FeO for laser treated kovar (Kovar(L)) than that on traditional thermal treated kovar (kovar(P)). This is attributed to the following improved wetting and diffusion process. The adhesive oxide layer formed on kovar (L) may enhance the oxygen diffusion into the substrate and iron diffusion outward to form an outside layer. This is an another way to enhance the wetting and diffusion process when compared to the delaminated oxide scales formed on kovar (P) surface. The diffusion mechanisms were discussed for both approaches. Scanning electron microscope (SEM) revealed that an iron oxide interlayer in the joint existed under both conditions. Fayalite nucleated on the iron oxide layer alloy and grew into the glass. In both cases, neither Co nor Ni were involved in the chemical bonding during wetting process. The work has shown that laser surface melting can be used to alter the wetting and diffusion characteristics of kovar alloy onto borosilicate glass. [DOI: 10.1115/1.4037426]

Laser Autogenous Brazing of Biocompatible, Dissimilar Metals in Tubular Geometries

The successful joining of dissimilar metal tubes would enable the selective use of the unique properties exhibited by biocompatible materials such as stainless steel and shape memory materials such as NiTi, to locally tailor the properties of implantable medical devices. The lack of robust joining processes for the dissimilar metal pairs found within these devices, however, is an obstacle to their development and manufacture. Traditional joining methods suffer from weak joints due to the formation of brittle intermetallics or use filler materials that are unsuitable for use within the human body. This study investigates a new process, Laser Autogenous Brazing, that utilizes a thermal accumulation mechanism to form joints between dissimilar metals without filler materials. This process has been shown to produce robust joints between wire specimens but requires additional considerations when applied to tubular parts. The strength, composition, and microstructure of the resultant joints between NiTi and Stainless Steel are investigated and the effects of laser parameters on the thermal profile and joining mechanism are studied through experiments and numerical simulations.

Biocompatibility and Corrosion Response of Laser Joined NiTi to Stainless Steel Wires

The biocompatibility of nickel titanium (NiTi) wires joined to stainless steel (SS) wires via laser autogenous brazing has been evaluated. The laser joining process is designed to limit the amount of mixing of the materials, thus preventing the formation of brittle intermetallic phases. This process has the potential for manufacturing implantable medical devices; therefore, the biocompatibility must be determined. Laser joined samples underwent nickel release rate, polarization, hemolysis, and cytotoxicity testing. Competing effects regarding grain refinement and galvanic effects were found to influence the corrosion response. After 15 days of exposure to a simulated body fluid, the total nickel released is less than 2 ug/cm. Numerical modeling of the corrosion currents along the wires, by making use of polarization data, helped to explain these results. Microbiological testing found a maximum hemolytic index of 1.8, while cytotoxicity tests found a zero toxicity grade. All of these results indicate that the autogenous laser brazing process results in joints with good biocompatibility. [DOI: 10.1115/1.4029766]

Laser Shock Peening for Suppression of Hydrogen-Induced Martensitic Transformation in Stress Corrosion Cracking

The combination of a susceptible material, tensile stress, and corrosive environment results in stress corrosion cracking (SCC). Laser shock peening (LSP) has previously been shown to prevent the occurrence of SCC on stainless steel. Compressive residual stresses from LSP are often attributed to the improvement, but this simple explanation does not explain the electrochemical nature of SCC by capturing the effects of microstructural changes from LSP processing and its interaction with the hydrogen atoms on the microscale. As the hydrogen concentration of the material increases, a phase transformation from austenite to martensite occurs. This transformation is a precursor to SCC failure, and its prevention would thus help explain the mitigation capabilities of LSP. In this paper, the role of LSP-induced dislocations counteracting the driving force of the martensitic transformation is explored. Stainless steel samples are LSP processed with a range of incident laser intensities and overlapping. Cathodic charging is then applied to accelerate the rate of hydrogen absorption. Using XRD, martensitic peaks are found after 24 h in samples that have not been LSP treated. But martensite formation does not occur after 24 h in LSP-treated samples. Transmission electron microscopy (TEM) analysis is also used for providing a description of how LSP provides mitigation against hydrogen enhanced localized plasticity (HELP), by causing tangling and prevention of dislocation movement. The formation of dislocation cells is attributed with further mitigation benefits. A finite element model predicting the dislocation density and cell formation is also developed to aid in the description. [DOI: 10.1115/1.4036530]

Comparative study of laser scribing of Sno2:F thin films using Gaussian and top-hat beams

Laser scribing is a common method used for manufacturing large-scale solar cells to increase cell efficiency by subdividing large cells into small mini-modules connected in series, decreasing the current produced and therefore reducing the ohmic losses. Introducing large temperature gradients causes thermal expansion and subsequently thermally induced stresses, and these stresses can be used to cause mechanical fracture and material removal. Gaussian beams, which are commonly used in existing scribing practice, have high energy intensity in the center of the beam resulting in unwanted substrate damage as well as excess energy toward the edge of the beam spot. This contributes to the formation of a heat affected zone and partial melting also resulting in large sidewall taper and residual material. The top-hat distribution, having a much more rapid decrease in intensity at the spot edges and more uniform intensity throughout, greatly reduces the likelihood of melting or partial removal on the spot edges, as well as the risk of damage to the glass substrate. However, little work has been done to quantify the differences in the resulting scribe quality of these laser beam intensity distributions. In this study, experiments were carried out on 400 nm thick SnO2:F TCO layer irradiated from the glass side using a 1064 nm Nd:YAG laser with both Gaussian and top-hat intensity distributions. Samples were processed using pulse energies ranging from 5µJ to 30µJ. Pulse repetition rates of 10 kHz were used. Scribe geometry was observed using AFM scans and SEM images. Possible negative effects such as delamination and crack formation resulting from abrupt intensity changes are investigated. A coupled thermo-mechanical finite element model is used to analyze the spatial temperature and stress distributions within the film during the scribing process. Our results find that the top-hat beam profile improves the uniformity and depths of the scribes, but increased thermal effects along the walls are experienced.Laser scribing is a common method used for manufacturing large-scale solar cells to increase cell efficiency by subdividing large cells into small mini-modules connected in series, decreasing the current produced and therefore reducing the ohmic losses. Introducing large temperature gradients causes thermal expansion and subsequently thermally induced stresses, and these stresses can be used to cause mechanical fracture and material removal. Gaussian beams, which are commonly used in existing scribing practice, have high energy intensity in the center of the beam resulting in unwanted substrate damage as well as excess energy toward the edge of the beam spot. This contributes to the formation of a heat affected zone and partial melting also resulting in large sidewall taper and residual material. The top-hat distribution, having a much more rapid decrease in intensity at the spot edges and more uniform intensity throughout, greatly reduces the likelihood of melting or partial removal on the spot edges, as...

Effects of Interfacial Geometry on Laser Joining of Dissimilar NiTi to Stainless Steel Wires

Joining of the dissimilar metal pair NiTi to stainless steel is of great interest for implantable biomedical applications. Formation of brittle intermetallic phases requires that the joining processes limit the amount of over-melting and mixing along the interface. Thus, laser joining is a preferred method due to its ability to precisely control heat input. This study explores a method of using a cup and cone interfacial geometry, with no filler material, to increase the tensile strength of the joint. Not only does the cup and cone geometry increase the surface area of the interface, but it also introduces a shear component, which is shown to be beneficial to tensile strength of the wire as well. The fracture strength for various cone apex angles and laser powers is determined. Compositional profiles of the interfaces are analyzed. A numerical model is used for explanation of the processing.

Material Influence on Mitigation of Stress Corrosion Cracking Via Laser Shock Peening

Stress corrosion cracking is a phenomenon that can lead to sudden failure of metallic components. Here, we use laser shock peening (LSP) as a surface treatment for mitigation of stress corrosion cracking (SCC), and explore how the material differences of 304 stainless steel, 4140 high strength steel, and 260 brass affect their mitigation. Cathodic charging of the samples in 1 M sulfuric acid was performed to accelerate hydrogen uptake. Nontreated stainless steel samples underwent hardness increases of 28%, but LSP treated samples only increased in the range of 0–8%, indicative that LSP keeps hydrogen from permeating into the metal. Similarly for the high strength steel, LSP treating limited the hardness changes from hydrogen to less than 5%. Mechanical U-bends subjected to Mattsson’s solution, NaCl, and MgCl2 environments are analyzed, to determine changes in fracture morphology. LSP treating increased the time to failure by 65% for the stainless steel, and by 40% for the high strength steel. LSP treating of the brass showed no improvement in U-bend tests. Surface chemical effects are addressed via Kelvin Probe Force Microscopy, and a finite element model comparing induced stresses is developed. Detection of any deformation induced martensite phases, which may be detrimental, is performed using X-ray diffraction. We find LSP to be beneficial for stainless and high strength steels but does not improve brass’s SCC resistance. With our analysis methods, we provide a description accounting for differences between the materials, and subsequently highlight important processing considerations for implementation of the process. [DOI: 10.1115/1.4034283]