M.P. Sealy

Surface integrity and process mechanics of laser shock peening of novel biodegradable magnesium-calcium (Mg-Ca) alloy.

Current permanent metallic biomaterials of orthopedic implants, such as titanium, stainless steel, and cobalt-chromium alloys, have excellent corrosive properties and superior strengths. However, their strengths are often too high resulting in a stress shielding effect that is detrimental to the bone healing process. Without proper healing, costly and painful revision surgeries may be required. The close Youngs modulus between magnesium-based implants and cancellous bones has the potential to minimize stress shielding while providing both biocompatibility and adequate mechanical properties. The problem with Mg implants is how to control corrosion rates so that the degradation of Mg implants matches that of bone growth. Laser shock peening (LSP) is an innovative surface treatment method to impart compressive residual stress to a novel Mg-Ca implant. The high compressive residual stress has great potential to slow corrosion rates. Therefore, LSP was initiated in this study to investigate surface topography and integrity produced by sequential peening a Mg-Ca alloy. Also, a 3D semi-infinite simulation was developed to predict the topography and residual stress fields produced by sequential peening. The dynamic mechanical behavior of the biomaterial was modeled using a user material subroutine from the internal state variable plasticity model. The temporal and spatial peening pressure was modeled using a user load subroutine. The simulated dent agrees with the measured dent topography in terms of profile and depth. Sequential peening was found to increase the tensile pile-up region which is critical to orthopedic applications. The predicted residual stress profiles are also presented.

Fabrication and Characterization of Surface Texture for Bone Ingrowth by Sequential Laser Peening Biodegradable Orthopedic Magnesium-Calcium Implants

Biodegradable magnesium-calcium (Mg–Ca) implants have the ability to gradually dissolve and absorb into the human body after implantation. The similar mechanical properties to bone indicate that Mg–Ca is an ideal implant material to minimize the negative effects of stress shielding. Furthermore, using a biodegradable Mg–Ca implant prevents the need for a secondary removal surgery that commonly occurs with permanent metallic implants. The critical issue that hinders the application of Mg–Ca implants is the poor corrosion resistance to human body fluids. The corrosion process adversely affects bone ingrowth that is critical for recovery. Therefore, sequential laser shock peening (LSP) of a biodegradable Mg–Ca alloy was initiated to create a superior surface topography for improving implant performance. LSP is an innovative treatment to fabricate functional patterns on the surface of an implant. A patterned surface promotes bone ingrowth by providing a rough surface texture. Also, LSP imparts deep compressive residual stresses below the surface, which could potentially slow corrosion rates. Unique surface topographies were fabricated by changing the laser power and peening overlap ratio. The resultant effects on surface topography were investigated. Sequential peening at higher overlap ratios (75%) was found to reduce the tensile pileup region by over 40% as well as compress the overall surface by as much as 35 μm.

State-Of-Art, Challenges, and Outlook on Manufacturing of Cooling Holes for Turbine Blades

A cooling hole is important structure of turbine blades for high-performance aircraft engines. It is very challenging to manufacture cooling holes in superalloys including nickel-based and titanium alloys. This article aims to provide a critical assessment on the major types of machining processes for manufacturing cooling holes. The process mechanism, efficiency, form accuracy, and surface integrity of the state-of-art of four machining processes, i.e., mechanical drilling (MD), electrical discharge drilling (EDD), laser drilling (LD), and electrochemical drilling (ECD) have been thoroughly analyzed and compared in details. The future challenges and future potential research directions for the machining processes are also discussed.

Fabrication and Finite Element Simulation of Micro-Laser Shock Peening for Micro Dents

An innovative surface treatment process micro laser shock peening (μ -LSP) has been developed in this study to fabricate micro dents on Ti-6Al-4V surfaces for improving tribology performance. A 3D finite element simulation model was also developed to investigate transient laser/material interactions at nano timescale during peening. The dynamic mechanical behavior was modeled using a user material subroutine of the internal state variable plasticity model. The temporal and spatial peening pressure was modeled using a user load subroutine. The simulated dent geometry agrees with the measured ones in terms of profile and depth. The results suggested that there is an optimal peening time for the deepest dent. The maximum transient stress in peening direction occurred at a certain pulse time.

Fabrication and Tribological Functions of Microdent Arrays on Ti–6Al–4V Surface by Laser Shock Peening

Surface patterning has become a valuable technique for fabricating microdents, which may act as lubricant reservoirs to reduce friction and wear in sliding and rolling contact applications. In this paper, the use of laser shock peening (LSP) along with an automatic X–Y table proves to be an attractive and reliable method for producing microdent arrays with enhanced surface integrity. Surface topography and profiles of the fabricated microdent arrays on polished Ti–6Al–4V have been characterized. The effect of dent arrays with different density on friction reduction at low and high viscosity lubrication was investigated. An acoustic emission (AE) sensor was used to online monitor friction and wear processes. It was found that a surface with 10% dent density provides better effect in reducing coefficient of friction (CoF) than those of smooth surface and a surface with 20% dent density. It was shown that there is a strong correlation between AE energy signals and wear rate.

Fabrication and finite element simulation of sequential laser shock peening of biodegradable Mg-Ca implants

Current biocompatible metals such as steel and titanium alloys have excellent corrosive properties and superior strengths. However, their strengths are often too high and as a result have a negative effect on the body. Therefore, Magnesium (Mg) alloys with relatively low strengths are ideal biocompatible metallic materials. The problem with Mg implants is how to control corrosion rates so that the degradation of Mg implants may match with bone growth. The high compressive residual stress induced by laser shock peening (LSP) has a great potential to slow down the corrosion rate. LSP is a known surface treatment method to impart compressive residual stress in subsurface of a metal. Therefore, LSP was initiated in this study to investigate surface topography and integrity produced by peening a Mg alloy. A 3D semi-infinite simulation has also been developed to predict the topography and residual stress fields produced by sequential peening. The dynamic mechanical behavior was modeled using a user material subroutine of the internal state variable plasticity model. The temporal and spatial peening pressure was modeled using a user load subroutine. The simulated dent agrees with the measured dent topography in terms of profile and depth. Sequential peening was found to increase the tensile pile up region which is critical to tribological applications. The predicted residual stress profiles are also presented.© 2009 ASME

A review of additive manufacturing of magnesium alloys

Additive manufacturing (AM) distinguishes itself from the traditional manufacturing method in that the process is fast and there is no need for expensive tooling, mold, dies etc and the process is capable of fabricating complex, customized parts derived from digital data. Magnesium is one of the most highly important lightweight metals used in automotive application to reduce vehicle weight. Its applicability in implant sector for orthopedic application is also getting acceptance. Currently, polymers have occupied most of the implant applications but superior qualities like ductility, strength and biodegradability established Mg alloys as a better choice for orthopedic implants [1]. Biodegradable Magnesium induces very small amount of stress shielding and it eliminates the necessity of secondary surgery. Additive manufactured specimen, made of Magnesium alloy, is getting popularity in structural, aviation and automobile sectors and they can be a good replacement for expensive Aluminum, Cobalt and Titanium based alloys. The sector is promising but being new, very small amount of work has been done on evaluating mechanical properties. Mechanical behaviors like tensile strength, compressive strength and fatigue properties largely depend on cavities present in the specimen. In this paper, the author described selective laser melting (SLM), paste extruding deposition (PED), friction stir additive manufacturing (FSAM) and laser additive manufacturing (LAM) techniques to fabricate Magnesium alloy based 3D printed specimen and discussed their mechanical behaviors.

Prediction and analysis of residual stress and deflections of Almen strip by burnishing

Burnishing is a surface treatment process widely used to improve fatigue and corrosion resistance of metal components by introducing a compressive residual stress layer. However, the measurement of residual stress by X-ray diffraction is expensive, time consuming, and tedious. Hole drilling method is quick and simple, but it is destructive and provides limited resolution in depth direction. This work presents a quick method to determine the nature and magnitude of residual stress by using Almen strips. Inspired by the application of Almen strips in shot peening, the deflection of the burnished Almen strips under different burnishing conditions were measured. In addition, a finite element simulation model has been developed to predict residual stresses and deflection which were compared with the experimental results. Contact conditions with different combinations of sliding and rolling were investigated in the simulations to understand the contact nature in a burnishing process. It was found that the deflection of Almen strip reflects the magnitude and penetration depth into subsurface of the induced residual stress. The contact condition during burnishing is either pure sliding or a mixed mode of sliding and rolling, but not pure rolling.

A Strategy to Optimize Recovery in Orthopedic Sports Injuries

An important goal for treatment of sports injuries is to have as short a recovery time as possible. The critical problem with current orthopedic implants is that they are designed to be permanent and have a high complication rate (15%) that often requires removal and replacement with a second surgery; and subsequently a second rehabilitation cycle. This study was designed to test the feasibility of having a device that could provide the needed mechanical properties, while promoting healing, for a specified amount of time and then degrade away, to shorten the recovery time. The specific strategy was to create a surface layer on a degradable metal alloy with a controllable degradation rate. Previous studies have shown the feasibility of using surface treatments to alter the surface integrity (i.e., topography, microhardness, and residual stress) leading to increased fatigue strength and a decreased degradation rate. This study was an extension of these previous studies to look at the changes in surface integrity and mechanical properties initially as well as the degradation over time for two surface treatments (shot peening and burnishing). Although the treatments improved initial properties and the burnishing treatment slowed degradation rate, the faster degradation of the base material in vitro (compared to previous studies) probably reduced the overall impact. Therefore although the study helped support the feasibility of this approach by showing the ability of the surface treatment to modify surface integrity, initial mechanical properties, and degradation rate; the degradation rate of the base material used needs to be slower and/or the surface treatment needs to create a bigger change in the degradation rate. Further it ultimately needs to be shown that the surface treatment can produce a material that will allow orthopedic devices to meet the required clinical design constraints in vivo.

Real-time monitoring and prognosis of energy consumption in hard milling

Tool wear progression is inevitable in precision cutting. However, the effect of tool wear on energy consumption at machine, spindle, and process levels is yet to understand. In this study, specific energy in dry milling of hardened AISI H13 was studied at the machine, spindle, and process levels. The effect of process parameters and tool wear progression on energy consumption at each level was investigated. The results indicated that tool wear progression only has a predominant influence on energy consumption at the process level but not the machine and spindle levels. Energy consumption at machine level can be described with a traditional empirical model effectively. However, the traditional model is incapable of predicting energy consumption at the process level. The investigation in energy consumption at different levels can help improve energy efficiency. Since energy consumption at the process level is responsible for chip formation and surface generation, the study of energy consumption at this level is critical to understanding and optimizing of a machining process.