2. Literature review

Firstly, we will start from the materials and their resistance using AM. This is the first topic investigated in the international literature since its investigation is the first point to demonstrate the feasibility of this kind of technology in the industrial world.

The keywords investigated were several and reported below: (i) AM materials, (ii) AM tolerances in production, (iii) AM metallic materials, and (iv) AM polymeric materials and similar or combinations of the previous words.

The materials more studied using AM are the titanium alloys, even if also other materials are studied such as the Cr-Co alloys, stainless steel, and other metallic materials.

The first investigation about the materials is about the tolerances in production. This is a very felt issue in the production environment since it demonstrates the feasibility of the process, from a first point of view, and the need to integrate the technology with other old technologies to realize a finished part.

The part orientation was demonstrated as influencing variable in the tolerances realization using AM since 2011 as it was reported in a paper in that year [4]. The paper focused the attention on the geometric tolerances related to the orientation of the part and, in fact, it was the first to do so. Before this study other study appeared, for example, the one by (i) Arni and Gupta in 1999 in which the planarity tolerance using an AM technology [5] was investigated and by Hanumaiah and Ravi for other linear geometric tolerances [6]. The tolerances were investigated also in relation with the production parameter by another study [7] that investigated the circularity in the AM in relation with the cutting angle of the starting point (i.e., the error is minimized if the starting cut angle is equal to 0); going in deeper analysis on this relation, it is also possible to find another paper about the relation between geometric tolerances and production parameters by Lynn-Charney and Rosen [8] who studied and defined a new decision support system (DSS) capable to minimize the errors for positioning of the part in the production chamber and for some geometric tolerances.

After this first contribution and many years, the 3D printing, or as it is named in a more detailed way the AM, arrived to the practitioners' world and many of the theoretic hypothesis and problems became real. The application of this technology in real cases or in the technological laboratories let to burst not only a lot of opportunities but also a lot of criticalities in its

In last years, the AM is employed in some first pilot production systems for the aerospace and aeronautic sector with application laboratories in collaboration with the MIT [2, 3]. Obviously, their productions are focused on small volume products since it is just a first attempt of experiments in this new field. From these first applications the industries, together with primary universities, are trying to understand the applicability of this new technology to substitute or at least to integrate their traditional production systems with these new ones, recognizing in the AM an opportunity to optimize their processes with particular attention to

The main problem for the practitioners and researchers, nowadays, is how to integrate this technology with the old, and which are the new paradigm to optimize the production using it. The aim of this chapter is to present firstly a literature review that is able to cover three relevant sectors of the industrial systems management problem, two of these are related to the possibility to apply this technology to the industrial world (analyzing the mechanical characteristics of the materials worked and the tolerance of working achievable through AM) and a third, that is more referred to the management methods for the AM and in particular the measurement of costs for the processes of AM production methods and the scheduling models if present. After this literature review, a possible model to measure the costs using the AM will be presented and after a mathematical model to schedule the AM activities will be presented, giving the possibility to the reader to understand the decision problem and so to apply the resolution method preferred by him.

Firstly, we will start from the materials and their resistance using AM. This is the first topic investigated in the international literature since its investigation is the first point to demon-

The keywords investigated were several and reported below: (i) AM materials, (ii) AM tolerances in production, (iii) AM metallic materials, and (iv) AM polymeric materials and similar

The materials more studied using AM are the titanium alloys, even if also other materials are

The first investigation about the materials is about the tolerances in production. This is a very felt issue in the production environment since it demonstrates the feasibility of the process, from a first point of view, and the need to integrate the technology with other old technologies

strate the feasibility of this kind of technology in the industrial world.

studied such as the Cr-Co alloys, stainless steel, and other metallic materials.

application.

72 3D Printing

2. Literature review

to realize a finished part.

or combinations of the previous words.

the design, engineering, production, and logistic.

Another problem in the AM production is the surface definition in the software file of the part, a paper that investigates this issue appeared in 2016 [9], in this chapter, it is defined a mathematical model to minimize the dimensional errors. Using this model, in an experimental campaign, the reduction of the 70% of the surface defects was eliminated.

Other authors investigated the effects on geometrical tolerances due to the thermal deformation of the melting process for the materials in the production chamber [10]. Other researchers [11] investigated the geometric errors of cylindrical shape dependent by the passage from the CAD model to the triangular shape for stereolithography and they built a new procedure to minimize it.

The geometrical and dimensional tolerances are a topic very felt also in other sectors such as the medical products. In this sector, it was found a contribution of 2015; in this chapter, it performed a sensitivity analysis on the several possible causes of the dimensional and geometrical errors that are possible in the production of this particular kind of objects [12]. The main elements of problem are (i) the quality of the image acquisition and printing, (ii) triangulation density, and (iii) segmentation threshold.

From the papers presented before, it is quite easy to understand that the AM technology is capable and maturely produces parts with good quality, so it is applicable to the industrial sector.

Once that the chapters on the tolerances in production were analyzed, they will be analyzed chapters referred to the mechanical properties of the materials produced with the AM technology.

The Ti-6Al-4 V material is used in many aerospace and mechanical sectors. In fact, as it is reported in a publication of 2015, the alloy Ti-6Al-4 V AM production is good to improve the buytofly index from a typical value of 15 to 1 for the aerospace industry [13]. This ratio is obtained using the raw materials compared with the weight of the components at the end of their production. For the Ti-6Al-4 V, using an electron beam melting (EBM) method is possible to have a deposition rate of 500 mm/s, with a moderate operational cost. In the paper by Szost, they reported the following defects for the EBM:


The porosity of the materials produced with AM is a very important issue and this is evident since 2011 when a paper by Baufeld and others appeared [14]. In this chapter, it is fixed a limit value for the porosity of the materials to avoid the gas capture phenomenon and this limit is 6%. Always on 2015, another paper appeared demonstrated the goodness of the parts realized in

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75

Another material used with the AM technology is the AlSi10Mg, that is, a no-metallic material. This material, when produced using the AM, has very similar mechanical characteristics when compared to the traditional production methods, as it was demonstrated in a paper where this

From the literature analysis about the mechanical characteristics of the materials worked with AM technology is quite evident that the materials worked with this kind of technology are comparable at all with the traditional ones, except for some defects about the porosity of the materials, that can be faced with several adjustments and practices in the cycle or using a post-

Therefore, in conclusion, speaking in terms of quality of the final product, the AM processes are able to produce good parts, and so, it is possible to consider it in a production environment.

After the presentation of the process capability to realize good parts, let us continue with the

Actually, different attention has to be paid to the operations management themes, since it is a research field for the AM not so much developed as the one about the materials and tolerances. The operations management are the themes related to the: (i) production organization, (ii) production balancing, (iii) production quality, (iv) life cycle management, and (v) production

The keywords investigated were strictly connected to the themes above mentioned with the

For the purpose of this chapter, they have analyzed 16 papers from several journals of the main

A first contribution analyzed is from an Indian study about the costing in the operations for the prototypes production with metallic materials. In the paper [19], it is evidenced that the rapid prototyping (RP) finds great advantages from the AM application since it allows reducing

Another contribution also in 2010 appeared in USA, the paper investigated the sustainability impact of AM production system [20]. In the paper, in particular, it is presented an assessment framework for the sustainability assessment for an AM production system, considering the following elements: (i) energy consumption, (ii) waste production, (iii) water usage, and (iv)

After these two first papers, for some years no particular research paper was published, maybe because the theme was too unripe. The theme was looked with interest once again in 2013, that is, after few years with the rising of the first possibility to bring the AM in the industrial

dramatically the contribution of the production costs for a single piece.

titanium alloys using the AM [17].

fact was demonstrated using a DMLS technology [18].

processing phase with traditional technologies.

operations management literature review.

scientific data base available nowadays.

sustainability.

adding of the AM words.

environmental impact.

context.

Another paper reports the results of several mechanical tests of the alloy Ti-6Al-4 V when produced with AM (laser-beam deposition (LBD) and the shaped metal deposition (SMD)). In particular, it is interesting that for both the products it is not present any particular remark about the comparison with the traditional ones; they have a critical break tension of 900–1000 MPa and a starting tension for the plastic behavior to 770 MPa.

The production methods such as the EBM, the LBD, and the SMD characteristics are summarized in Table 1.

To remark what before stated it is possible also to refer to a paper appeared on 2015 [15] on the mechanical properties of the alloy Ni-Ti produced using the technology of shaped memory alloy (SMA). In this chapter, the controls performed are several reported below:


In all the tests, the products produced with the Ni-Ti alloy with SMA presented a very close behavior to the ones produced with traditional methods, confirming the goodness of the AM methods.

In 2015, a chapter focused its attention on the laser powder deposition (LPD) method that is very used in the spare parts prototyping; the paper mainly investigated a method to recognize defects during the production run that is ongoing [16].


Table 1. Production parameters used for EBM, LBD and SMD in Baufeld et al.

Always on 2015, another paper appeared demonstrated the goodness of the parts realized in titanium alloys using the AM [17].

i. Vacancies of melting

74 3D Printing

rized in Table 1.

ii. High-porosity of the materials.

i. Scanning electron microscopy (SEM)

iii. X-ray diffraction (XRD)

iv. Micro-hardness test.

methods.

ii. Differential scanning colorimeter (DSC)

defects during the production run that is ongoing [16].

Table 1. Production parameters used for EBM, LBD and SMD in Baufeld et al.

The porosity of the materials produced with AM is a very important issue and this is evident since 2011 when a paper by Baufeld and others appeared [14]. In this chapter, it is fixed a limit value for the porosity of the materials to avoid the gas capture phenomenon and this limit is 6%. Another paper reports the results of several mechanical tests of the alloy Ti-6Al-4 V when produced with AM (laser-beam deposition (LBD) and the shaped metal deposition (SMD)). In particular, it is interesting that for both the products it is not present any particular remark about the comparison with the traditional ones; they have a critical break tension of

The production methods such as the EBM, the LBD, and the SMD characteristics are summa-

To remark what before stated it is possible also to refer to a paper appeared on 2015 [15] on the mechanical properties of the alloy Ni-Ti produced using the technology of shaped memory

In all the tests, the products produced with the Ni-Ti alloy with SMA presented a very close behavior to the ones produced with traditional methods, confirming the goodness of the AM

In 2015, a chapter focused its attention on the laser powder deposition (LPD) method that is very used in the spare parts prototyping; the paper mainly investigated a method to recognize

Max power (kW) 3 3.5 2.2 Max welding velocity (mm/s) 16.0 10.0 5.0 Max wire-feed velocity (mm/s) 25 40 33 Max wire diameter (mm) 1.0 1.2 1.2 Max height deposition step (mm) 0.1 1 1 Max deposition rate (kg/h) 0.1 0.7 0.6 Max wall thickness (mm) 10 4-5 9.1

EBM LBD SMD

900–1000 MPa and a starting tension for the plastic behavior to 770 MPa.

alloy (SMA). In this chapter, the controls performed are several reported below:

Another material used with the AM technology is the AlSi10Mg, that is, a no-metallic material. This material, when produced using the AM, has very similar mechanical characteristics when compared to the traditional production methods, as it was demonstrated in a paper where this fact was demonstrated using a DMLS technology [18].

From the literature analysis about the mechanical characteristics of the materials worked with AM technology is quite evident that the materials worked with this kind of technology are comparable at all with the traditional ones, except for some defects about the porosity of the materials, that can be faced with several adjustments and practices in the cycle or using a postprocessing phase with traditional technologies.

Therefore, in conclusion, speaking in terms of quality of the final product, the AM processes are able to produce good parts, and so, it is possible to consider it in a production environment.

After the presentation of the process capability to realize good parts, let us continue with the operations management literature review.

Actually, different attention has to be paid to the operations management themes, since it is a research field for the AM not so much developed as the one about the materials and tolerances.

The operations management are the themes related to the: (i) production organization, (ii) production balancing, (iii) production quality, (iv) life cycle management, and (v) production sustainability.

The keywords investigated were strictly connected to the themes above mentioned with the adding of the AM words.

For the purpose of this chapter, they have analyzed 16 papers from several journals of the main scientific data base available nowadays.

A first contribution analyzed is from an Indian study about the costing in the operations for the prototypes production with metallic materials. In the paper [19], it is evidenced that the rapid prototyping (RP) finds great advantages from the AM application since it allows reducing dramatically the contribution of the production costs for a single piece.

Another contribution also in 2010 appeared in USA, the paper investigated the sustainability impact of AM production system [20]. In the paper, in particular, it is presented an assessment framework for the sustainability assessment for an AM production system, considering the following elements: (i) energy consumption, (ii) waste production, (iii) water usage, and (iv) environmental impact.

After these two first papers, for some years no particular research paper was published, maybe because the theme was too unripe. The theme was looked with interest once again in 2013, that is, after few years with the rising of the first possibility to bring the AM in the industrial context.

Actually, in 2013, they appeared some papers of interest for the AM operations research theme. In this year, they appeared a paper about a cost-benefit analysis between traditional and AM methods for RP operations performed [21]. The benefits are dominant to the costs when the AM is used instead of traditional production systems.

Therefore, the AM operations management facets are studied since the 2010 but they started to be mature in terms of study only from 2013, when some first publications about the possibility to analyze the AM from the costs and operations management perspective started to appear. After this year, (i.e. 2013) the number of papers in this field exponentially started to grow up

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Nevertheless, it is quite obvious from the literature review analyzed that the operations management using AM is not well covered yet from the research point of view. Actually, this lack is well understandable from Figure 2, in which the operations management theme is

The first points to be developed in the future are mainly focused on the definition of a proper

The aim of this chapter is to introduce the reader to some first definition attempts by the author

reported in terms of number of papers appeared in the years.

cost accounting model and also a scheduling model for the AM.

(see Figure 1).

about these two specific issues.

Figure 1. Number of operations management per year.

Figure 2. Number of publications per year and per theme.

In 2014, another study appeared to prove the convenience of an AM, from the supply chain management point of view [22]. It is defined a decision framework for the application of this technology to a specific product, and the AM resulted in the best choice, especially when the product to be produced is very complex in shape.

Always on 2014 another implementation framework for the AM technology was proposed by another research group [23]. In this paper the authors tried to understand if there is a model of decision capable to help the technology selection using also the AM in the production environment. Another contribution about a very similar theme appeared in 2015 by the US-Chinese research group demonstrated the convenience of the AM considering the supply chain and life-cycle cost models [24].

The technology selection decision was also analyzed by another paper published on 2014 [25]. The framework is based on the evaluation of parameters such as the part complexity, the quantity to be produced and the level of customization that the product requires.

With the 2015 the research started to be more focused on the operations management optimization. Indeed, on 2015 the creation of DSS able to put at the right level of a bill of materials a part produced using the AM technology appeared; the model does this minimizing the life cycle cost and maximizing the value chain of the product [26].

On 2015 another paper focused on the insertion of the AM technology in traditional production systems appeared [27]. The paper deals with the identification of the main facets to be solved to make easy the AM insertion, referring principally to the operations management issues.

Always in the same year, a paper about the economic consequences of the AM use in a production company appeared [28]. The paper depicted the main economic variables applicable to the AM, such as the marginal costs of the production, the quality impacts on production economics, and so on. Very similar works about the economic facets related to the AM implementation in traditional production systems are present also in other paper such as [29, 30].

Another great theme about the economics issues related to the AM was investigated in another paper appeared on 2015. This paper faces the problem to define a selling price for an AM built part; this was done using the gray theory [31].

Also, the environmental and the other variables of the sustainability were deeply investigated as demonstrated from several publications in the international literature [32, 33].

Another theme investigated in the international literature is about the possibility and the framework to make feasible the integration of an AM technology with the traditional subtractive machines [34].

Therefore, the AM operations management facets are studied since the 2010 but they started to be mature in terms of study only from 2013, when some first publications about the possibility to analyze the AM from the costs and operations management perspective started to appear. After this year, (i.e. 2013) the number of papers in this field exponentially started to grow up (see Figure 1).

Nevertheless, it is quite obvious from the literature review analyzed that the operations management using AM is not well covered yet from the research point of view. Actually, this lack is well understandable from Figure 2, in which the operations management theme is reported in terms of number of papers appeared in the years.

The first points to be developed in the future are mainly focused on the definition of a proper cost accounting model and also a scheduling model for the AM.

The aim of this chapter is to introduce the reader to some first definition attempts by the author about these two specific issues.

Figure 1. Number of operations management per year.

Actually, in 2013, they appeared some papers of interest for the AM operations research theme. In this year, they appeared a paper about a cost-benefit analysis between traditional and AM methods for RP operations performed [21]. The benefits are dominant to the costs when the

In 2014, another study appeared to prove the convenience of an AM, from the supply chain management point of view [22]. It is defined a decision framework for the application of this technology to a specific product, and the AM resulted in the best choice, especially when the

Always on 2014 another implementation framework for the AM technology was proposed by another research group [23]. In this paper the authors tried to understand if there is a model of decision capable to help the technology selection using also the AM in the production environment. Another contribution about a very similar theme appeared in 2015 by the US-Chinese research group demonstrated the convenience of the AM considering the supply chain

The technology selection decision was also analyzed by another paper published on 2014 [25]. The framework is based on the evaluation of parameters such as the part complexity, the

With the 2015 the research started to be more focused on the operations management optimization. Indeed, on 2015 the creation of DSS able to put at the right level of a bill of materials a part produced using the AM technology appeared; the model does this minimizing the life

On 2015 another paper focused on the insertion of the AM technology in traditional production systems appeared [27]. The paper deals with the identification of the main facets to be solved to make easy the AM insertion, referring principally to the operations management

Always in the same year, a paper about the economic consequences of the AM use in a production company appeared [28]. The paper depicted the main economic variables applicable to the AM, such as the marginal costs of the production, the quality impacts on production economics, and so on. Very similar works about the economic facets related to the AM implementation in traditional production systems are present also in other paper such as [29, 30].

Another great theme about the economics issues related to the AM was investigated in another paper appeared on 2015. This paper faces the problem to define a selling price for an AM built

Also, the environmental and the other variables of the sustainability were deeply investigated

Another theme investigated in the international literature is about the possibility and the framework to make feasible the integration of an AM technology with the traditional subtrac-

as demonstrated from several publications in the international literature [32, 33].

quantity to be produced and the level of customization that the product requires.

cycle cost and maximizing the value chain of the product [26].

part; this was done using the gray theory [31].

AM is used instead of traditional production systems.

product to be produced is very complex in shape.

and life-cycle cost models [24].

issues.

76 3D Printing

tive machines [34].

Figure 2. Number of publications per year and per theme.
