1. Introduction
Medical implants and prostheses that replace different parts of the human body are made of biocompatible materials, which have mechanical and physical-chemical properties specific to these applications [
1,
2,
3,
4]. The materials used in medical implants manufacturing are, at best, inert to the bone tissue having the possibility to become bioactive by controlling the morphology and chemical composition at surface. The elements that must be considered when choosing the material used for medical implants such as compatibility, the material nature and degree of alloying, process and manufacturing method, mechanical properties, processing conditions significantly influence the interaction between the material and bone tissue. The long-term stability of the implant depends largely on all of the aspects presented above but also on its ability to integrate into the adjacent bone tissue [
5].
Considering the possible problems caused by the incompatibility between the materials used to make the prosthesis on the bone tissue, scientists collaborate to create new biocompatible materials with a much higher acceptance rate [
6,
7].
Currently, a wide variety of biocompatible materials are used to make medical implants, such as metals and alloys (stainless steel, Ti-Al-V, Co-Cr-Mo) ceramics and glasses (alumina, zirconia), polymers (polyethylene, polyamides), composites (PMMA-glass fillers) [
8,
9].
In present, most medical prosthesis are made of titanium alloys, alloys that offer high mechanical properties (tensile strength, yield strength), low Young’s module, low specific weight (half the specific weight of stainless steel). In addition, such materials are not toxic (in comparison, stainless steel can cause several allergies), they are not ferromagnetic (the possibility of performing magnetic resonance imaging investigations) and have a good corrosion resistance in specific working environments [
10,
11].
The Ti6Al4V titanium alloy, is the most common titanium alloy used for making medical implants, being an α + β type of alloy initially used in the aerospace industry [
12,
13,
14,
15]. As in the case of stainless steels, the corrosion resistance of titanium alloys is given by the formation of an oxide layer on the surface of the material, in this case being the TiO
2 layer [
16]. The metallic materials used in the manufacture of medical implants are inert to human bone tissue [
17,
18]. To obtain customized implants having shapes and sizes specific to their use in case of surgery in areas with continuous growing tumors has led to the need of using new manufacturing methods. The classical methods of obtaining implants and medical instruments (forging, stamping) do not offer a high dimensional accuracy of the product [
19]. To meet the dimensional and quality requirements for customized implants, the new trend is the use of additive manufacturing processes (3D printing), using the information obtained from the imaging examinations to which the patient was subjected [
20,
21]. Popov et al. [
22] present the possibility of healthcare digitalization in the Industry 4.0 revolution using 3D printing which allows on-site printing of freeform shapes, which are potentially useful to develop custom-sized implants or prostheses.
Direct metal laser sintering (DMLS) is an additive manufacturing process used for printing metal products, which is based on using the energy of a laser beam to melt layers of metal powder (sintering) which, following solidification, finally forms the 3D model of the desired product. The size of the metal powder particles is between 20 and 40 μm. The size of the particles, the surfaces of the products, as well as their geometric configuration influence the final printing resolution [
23,
24,
25,
26,
27].
In certain situations, when implants are made by classical technological processes, there are disturbing factors that appear and influence the result of the process. In such cases, the resulting product has a series of dimensional or shape (geometric) imperfections that may lead to the decision to classify the resulting product as scrap. In these situations, the prosthesis material must be remelted and subjected to a new processing process, which leads to increased costs related to obtaining implants in the final form [
28,
29]. In the case of additive manufacturing processes, the reuse of scrap is no longer possible, so the financial, time or resources loss is significant.
In the case of recoverable scrap, there is the possibility of using welding reconditioning to restore the geometric configuration and the functional properties of the implants.
Through the advantages offered by the welding processes, it is possible to reduce the costs related to the production of implants, but also those related to the long-term use of surgical instruments or cosmetic implants. The costs related to welding reconditioning of instruments and medical implants, depending on their complexity and the type of defect, are between 15 and 50% of the costs of new products.
In the literature, there is a limited number of studies that refer to welding reconditioning possibilities of different types of products, made of steel, cast iron [
30,
31,
32,
33,
34,
35,
36,
37,
38] or non-ferrous materials [
39]. In terms of welding reconditioning products made of titanium alloys, Petrik I.A. [
40] presents the possibility of reconditioning by welding the components of gas turbine rotors and Paton B.E. [
41] uses various welding or reconditioning technologies for welding components made of titanium alloys. Graf B. and Liu Q. show [
42,
43] the possibility of welding repair with the help of laser metal deposition technology. Yu J.H. [
44], Onuike B. [
45] and Rahito [
46] use the notion of additive manufacturing for laser metal-layer deposit reconditioning.
Following the desktop research, it was found that there is no research conducted on the possibility of modifying the characteristics of the products obtained with the help of AM processes. In some cases, and especially in the case of medical implants, the rapid evolution of malignant bone tumors can lead to changes in the bone area that must be ex-removed and replaced. In such cases, considering the rapid evolution of the tumor, the time from the analysis of the imaging results to the design and realization of the implant with the help of an AM procedure, makes it necessary to modify the constructive form of the implant, by adding additional elements to compensate the excised bone area or of some additional elements for the attachment of the implant on the healthy bone structure.
The purpose of this paper is to analyze the possibilities of restoring the geometric characteristics by welding of implants made by additive manufacturing processes that would otherwise be unusable and unrecoverable. In this case, the use of the DMLS additive manufacturing process of a custom hip implant, resulted in a geometric configuration that did not meet the requirements of the fabrication specifications. To restore the geometric characteristics of the implant, the tungsten inert gas (TIG) welding deposition process was used.
The novelty of the proposed research consists in the restoration and/or modification of the geometric and functional characteristics of an AM made implant with manufacturing errors with the help of welding reconditioning processes by. Welding reconditioning is presented, in some cases, as a fast and inexpensive alternative for remanufacturing the implant according to the new functional requirements.