Secondaries for MIM Parts

Table of Contents

• 8.1 Introduction

• 8.2 Operations to improve dimensional control

• 8.2.1 Mechanical deformation (sizing)

• 8.2.2 Machining

• 8.3 Operations to enhance mechanical properties

• 8.4 Operations to improve appearance and surface properties

• 8.5 Operations to reduce tooling cost and enhance applications

• 8.6 Resume-Outlook

8.1 Introduction

The classical flow sheet for metal injection molding usually ends with the sintering process. After final inspection and packing, parts are ready for shipment to the customers. The metal injection molding process is considered to be a netshape technique, thus any additional process steps would lead to increased production cost and therefore should be avoided. For a lot of metal injection molding (MIM) parts, this is true. Especially if the final customer is willing to adapt the design of its parts to the specifics of the MIM process, shipping these as-sintered parts to the customer is no big challenge. A good example of such a part is the hydraulic connector shown in Fig. 8.1. This 316L stainless steel part does not require any additional operations, thanks to a minor design change. After adding a small support aid, the part does not require any special support nor does it bend during sintering.

Fig. 8.1 316L hydraulic connector-ready to use without any secondary operations thanks to the MIM friendly design

On the other hand, MIM parts will always be in competition with alternative production routes such as investment casting, conventional press-and-sinter, stamping and recently additive manufacturing. In the last decades, MIM has demonstrated its cost advantage for complex-shaped parts in medium to large numbers. Still cost is only one decision factor. Good product designers will make their choice with regards to a wide range of properties and requirements and these will strongly depend on the production method selected, as can be seen in Table 8.1.

Obviously, there is no technology providing perfect solutions for all properties. Many MIM companies experience this in their discussions with new customers. Attracted by a prospect of high-cost reduction, initial optimism may turn into skepticism, when other properties are discussed. For example, if dimensional tolerances are critical, the PIM process can neither match modern machining operations, nor compete with conventional press and sinter. While the situation described in Table 8.1 may appear discouraging to newcomers in the MIM industry, experienced MIM manufacturers know this would be shortsighted. The MIM process does not have to end after the sintering.

Table 8.1 Competing technologies (MIM-Expertenkreis, n.d.)

Due to the high density and closed porosity, MIM parts can be treated as conventional fully dense materials and consequently all conventional operations such as machining, forming, heat treatment, surface finishing, coating, and joining can be applied to it (see also Table 8.2).

Table 8.2 Overview of secondary operations for MIM parts

Although secondary operations will increase production cost, many of today's MIM manufactures are already using these operations to meet the growing demands of their customers, and to add value to their product. Distinctions can be made concerning operations to

1. improve dimensional tolerances

2. enhance mechanical properties

3. improve appearance and surface properties

4. reduce tooling cost and widen applications of the MIM process.

In the following paragraph, each of the operations is described in more detail.

8.2 Operations to improve dimensional control

As described in previous chapters, MIM parts shrink substantially during the sintering process, depending on the material between 12% and 22% linearly. Although lots of efforts have been made to predict shrinkage rates, numerous factors that will cause dimensional variations remain difficult to control the following:

1. Homogeneity in the feedstock, including batch-to-batch variation

2. Size of the part

3. Large variations in wall thickness

4. Sticking to sintering supports

5. Gravity forces during sintering

6. Variations of sintering between each cycle

7. Variations of sintering parameters inside the equipment.

Actually, the effect of variability of molding parameters is negligible compared to the factors named above. Still, if all of the above variations are kept to a minimum, typical tolerances of an industrial scale production will be around ±0.5% for larger dimensions (30mm or 11/4 00 and more), but can be as high as ±1.5% for smaller dimensions (below 3mm or 1 /8 ''), as agreed and published by the German MIM Expertenkreis (MIM-Expertenkreis, n.d.). On top of that, there are restrictions in straightness, flatness, and parallelism of 0.5% of the longest dimension as well as an uncertainty in angle of ±0.5 degree. For precision, partslikethe machine part depictedin Fig. 8.2 thisistoo much.

Fig. 8.2 High precision machine parts with tolerances around
0.1% on critical dimensions. Courtesy ITB Precisietechniek, the Netherlands.

8.2.1 Mechanical deformation (sizing)

As stated previously, during the sintering, the brown parts will shrink between 12% and 22% linearly. After debinding, the so-called brown parts still contain a substantial amount of binder, depending on the binder system between 10% and 70% of its starting volume. In the first stage of sintering, these binder residues are removed by thermal degradation usually in a temperature range between 200℃ and 600℃. Already in this phase deformation may occur, as no metallic bonding between the particles is available yet. Initial sintering (neck formation) will start from approx. 600℃ depending on the type of powder used. It would be incorrect to assume that from that point on, no deformation will take place.

During the sintering, the component is shrinking and consequently there will be a lot of movement along the surface of the support. A big part of 100mm may show movements up to 11mm at the edges. The ceramic support should not hinder these movements as this may lead to unpredictable deformation.

An example of a complex ceramic support is shown in Fig. 8.3. As the parts "move" over the surface, incidental sticking may occur, which even regular cleaning of supports cannot always prevent.

These deformations are unwanted and have to be eliminated if exceeding the allowable tolerances. Typical operations are straightening, bending, and sizing. Although seemingly straightforward, the part designer has to be aware that the maximum degree of deformation strongly depends on the material properties like yield strength (YS) and elongation. For example, a flat medical device made of 316L (YS 180MPa/elongation 50%) will be easy to straighten, but the same shape in a 17-4-PH material will require much more force and the maximum deformation is limited (YS 660MPa, elongation to fracture 3%). An additional challenge is that 17-4-PH mechanical properties are influenced by the cooling rates in the furnace (slower cooling rates result in a higher degree of precipitation making it more difficult to straighten a part). Less controlled heat treatment results in more scattered mechanical properties, and consequently the effect of the deformation will be more difficult to predict.

Fig. 8.3 Complexly shaped ceramic support, required to reduce deformation during sintering

For some materials such as titanium, the situation is even more complex as parts have to be heated to allow for an acceptable degree of deformation. Depending on the complexity of the part, partial heating using hot air guns or induction coils may be enough, but for complexly shaped parts an additional furnace may be required. In all cases, there will be a huge impact on logistics and layout of the operations. Parts as hot as 500℃ or more must be moved to calibration presses, requiring additional safety measurements for the operators.

In summary, mechanical deformation is a complicated operation depending on the design of the part and its alloy, requiring special tools and continuous controls to guarantee a positive outcome. Best results are obtained if the complete operation is automated. In Fig. 8.4, an advanced solution of a sizing operation for machine parts is shown. A robot system positions the parts in the automatic sizing operation. An optical measurement system is used to check the final sizes and to adjust forces if necessary. Final tolerances of 0.01% are obtained with this operation.

Fig. 8.4 Fully automated calibration system including automated measurement and quality control

In summary, mechanical deformation is a complicated operation depending on the design of the part and its alloy, requiring special tools and continuous controls to guarantee a positive outcome. Best results are obtained if the complete operation is automated. In Fig. 8.4, an advanced solution of a sizing operation for machine parts is shown. A robot system positions the parts in the automatic sizing operation. An optical measurement system is used to check the final sizes and to adjust forces if necessary. Final tolerances of 0.01% are obtained with this operation.

8.2.2 Machining

MIM parts are almost fully dense and their mechanical properties are comparable with wrought products. Consequently, they can be machined accordingly. Typical machining operations are as follows:

-Drilling, especially in combination with thread tapping of ISO threads

-Milling

-Grinding of high accuracy dimensions and to improve flatness

-Spark erosion

Fig. 8.5 Grinding in large scale operations require special tools for fixation of dozens of parts at once

The challenging part is to minimize machining as much as possible or, if unavoidable, allow for the use of automated systems. The parts depicted in Fig. 8.2 are a good example of MIM parts that require secondary operations including machining. The critical tolerances are as small as 0.1%. This can only be achieved by selective grinding operations as shown in Fig. 8.5. Using specially designed fixtures, the manufacturer can machine 50-100 parts at once. Intense quality control and frequent calibration of tooling ensures a high reproducibility during several years of production.

Although secondary machining will always increase the fabrication cost per part, total costs for the customer may be less as compared to competing forming technologies. This is particularly the case for small series (typically several hundreds of parts). A small series for a highly complex part would require an expensive tool (multiple cores and sliders). Thus, in some cases, a simple tool combined with secondary operations such as drilling may offer the most economical solution.

8.3 Operations to enhance mechanical properties

MIM parts can be made of different alloys. After sintering, most alloys behave like conventional metallic materials and thus can be heat-treated alike. In Table 8.3, a few examples of possible heat treatments are listed.

Table 8.3 Common heat treatments of MIM parts

Heat treatment of MIM parts has become very common. Depending on the base materials, many options for heat treatment are available.

For example, carbon steel MIM parts can be heat-treated just like any other heat treatable carbon steel, by heating the parts above the austenisation temperature followed by a suitable quenching operation, usually in oil. To regain some toughness, parts are usually annealed afterwards, reducing some of the hardness in trade of ductility.

A typical problem for MIM producers arises when parts must be both heat-treated and straightened. Straightening may build up strains, released during heat treatment. On the other hand, parts may deform during heat treatment because of different cooling gradients inside the part. Consequently, some high-precision MIM parts have to be machined and/or straightened after heat treatment causing high wear on the machines.

Case hardening operations, where hardening is limited to the surface only, can also be applied using standard nitriding and carburizing techniques. As all hardening operations require extensive know-how and quality control, smaller MIM operations outsource these activities to specialized heat treatment companies. In Table 8.4, examples of the impact of heat treatment on the mechanical properties of a MIM part are shown.

Table 8.4 Examples of effect of heat treatment on mechanical properties of MIM alloys

An increasingly popular heat treatment is hot isostatic pressing (HIP). In this operation, the sintered MIM parts are exposed to temperatures between 900℃ and 1250℃ under a high pressure protective gas atmosphere. With pressures between 100 and 200MPa, any remaining porosity of the parts will be eliminated. As gas pressure acts uniformly in all directions, the resulting shrinkage is isotropic, as will be the improvement in mechanical properties. A schematic drawing of a HIP furnace is depicted in Fig. 8.6.

Fig. 8.6 Schematic layout of a HIP furnace

Most MIM parts already feature a porosity of <4%. Consequently, the improve- ment of YS and ultimate tensile strength (UTS) is not noticeable as described by Mash (2014). Removing the remaining porosity will influence the ductility. For example, HIPing a Co-28Cr-6Mo alloy improves the ductility from 15 to 205 as reported by Sago, Bradley, and Eckert (2012). Even more classical materials, for example, 17-4PH stainless steel may benefit from the HIP operations as demonstrated by LaGoy and Bulger (2009): impact energy is increased from 5.4 J of the sintered only part to up to 24.4 J after HIP treatment.

Therefore, for critical parts in medical or aerospace applications, HIP operations are usually compulsory. In the last decade, costs for HIP operations have decreased up to 50%. But even pilot scale equipment as depicted in Fig. 8.7 requires substantial investments and cannot be installed without extensive safety precautions. Therefore, most HIP operations are outsourced to specialized companies.

Fig. 8.7 Pilot scale HIP plant with a useable volume of 80L and a maximum operating temperature of 2000℃

8.4 Operations to improve appearance and surface properties

The sintering step is the key to success in the MIM process. If performed appropriately, many parts can be used in the as-sintered condition. When fine powder and high sintering temperatures are used in combination with reducing sintering atmospheres (either at atmospheric or reduced pressures) the metallic surface of the sintered parts is usually free of any discoloration. Densely sintered parts display an attractive bright surface structure with a typical roughness of Ra 1.2μm (0.047mil). Fig. 8.8 depicts parts just after leaving a continuous sintering furnace after being sintered under 100% hydrogen. These 17-4-PH stainless steel parts will be used in their as-sintered condition.

Fig. 8.8 Stainless steel 17-4-PH parts directly after sintering in a continuous furnace. Courtesy ITB Precisietechniek, the Netherlands.

For the automotive industry and mechanical engineering, parts are commonly supplied in their as-sintered condition. However, depending on the application and competing technologies, a product designer may request a modification of the surface structure or appearance of a MIM part, for example.

-to reduce the surface roughness

-to improve optics

-to add color

-to enhance corrosion resistance

As MIM parts are almost fully dense, almost every surface treatment can be applied.

Classical operations aim at mechanical improvement of surfaces by removing flaws mechanically. Examples are given in Table 8.5. By combining these operations, surface roughness can be reduced up to a mirror finish with a Ra<0.01μm.

Table 8.5 Examples of mechanical operations employed to improve the surface quality of MIM parts

High-end polishing always starts with blasting or tumbling of the sintered parts, followed by the actual polishing operation. In Fig. 8.9, an example of such parts in the different stages is shown. It is important to realize however, that good polishing requires flawless parts to begin with. Surface defects must be avoided at all times. Internal defects can also cause unwanted dents in a polished part, as shown in Fig. 8.10.

Fig. 8.9 Stages of surface improvement showing (A) the green part; (B) a sintered part; (C) a tumbled part; and (D) a high-gloss manually polished part

Fig. 8.10 Examples of surface flaws caused by internal failures

Potential internal flaws are one reason why high-end users are critical with regards to remaining porosity of parts. Using HIP, remaining porosity can be reduced to zero. However, voids underneath the surface may lead to indenting of the surface structure during HIP and consequently visible dents.

During manual polishing as shown in Fig. 8.11, these defects can still be detected and manually sorted. However, manual polishing is extremely expensive and can only be recommended for very small production numbers (a few hundred at most).

Fig. 8.11 Manual polishing operations (A) for leisure and medical applications in 316L and titanium (B)

Today even high-end ceramic parts are polished in automatic lines as shown in Fig. 8.12. Depending on the degree of initial surface roughness, several consecutive polishing steps are necessary, using different mediums for each step. The use of modern polishing equipment guarantees constant quality, independent of the skills of human operators, resulting in parts as shown in Fig. 8.13.

Fig. 8.12 Automatic high-end polishing for watch industry

Fig. 8.13 Technical ceramic PIM parts with "perfect" surface finish

Other types of surface coatings use chemical or physical processes, such as

1. Black oxidation

2. Manganese phosphating

3. Anodizing

4. Salt bath nitriding (quench polish quench, QPQ)

5. Plating

6. Passivation (stainless 316L)

7. Coating (17-4-PH Aerospace)

8. Physical or chemical vapor deposition.

As the MIM parts behave like ordinary raw materials, there is no limit to the coatings.

Nevertheless, these kinds of coating are very individual and depend strongly on the final application and the standards in the particular field. Some typical examples are described later.

Black oxidation and manganese phosphating are classical examples of surface treatments used for low alloy steels in mechanical engineering or fire arm parts.Besides providing the black color, the coatings improve corrosion resistance.

In the black oxidation process, an amorphous Fe3O4 oxide layer is generated on the surface of the parts by putting them in a hot salt bath of approx. 135-145℃. Critical steps in the process are the cleaning and degreasing of the parts prior to the salt baths. This so-called 2-steps oxidizing process actually consists of 10 consecutive process steps as described in Table 8.6.

Table 8.6 Differences between black oxidation and manganese phosphating

The manganese phosphating process is a similar procedure, but the protective layer is clearly thicker and crystalline. Consequently, the layer will provide a much better adherence for oil, helping to reduce friction and enhance corrosion resistance even more. From an environmental point of view, the process is favorable as the concentration and amount of chemicals used is four times less.

Both processes are usually carried out in automated lines consisting of several baths and purification systems. A critical issue in the MIM process is contamination. Although the automated oxidation and phosphating processes are designed to clean and degrease machined parts, the MIM process poses two more challenges. One is the risk of contamination from the setters. During the many sintering cycles, impurities tend to build up on the setters and may cause contamination on the surface of the parts. Some companies use fine ceramic powders on the setters to minimize sticking of larger parts; however these powders tend to stick to the MIM parts as well and integrate into the surface structure. These contaminations cannot be removed by conventional methods and will cause markings on the parts.

In Fig. 8.14, examples of titanium MIM parts are shown. Sintered titanium parts are usually treated, for example blasted, to obtain a smooth homogenous surface. Additionally, titanium is very suitable for anodizing, which achieves brightly colored parts and can be applied to both blasted or polished parts.

In Fig. 8.14B, a combination oftwo differentMIM partsis shown.Thetitanium housing carriestwo 17-4-PH stainless steelthread halves. Thesethreads are passivated followed by a surface treatment with a low-friction coating according to aerospace standards.

Fig. 8.14 Anodized titanium dental inserts (A) and fasteners (B). Both as-sintered as well as polished parts have been anodized.

Passivation is a process typically used for stainless steel parts to guarantee their corrosion resistance. Parts are submersed in a nitric or citric acid solution to remove free iron from the surface and to expedite the formation of a passive oxide layer of chromium on the surface. Similar to the automated blackening lines, passivation of stainless steel requires several steps:

1. Alkine cleaning

2. Water rinse

3. Immersion in nitric or citric acid

4. Water rinse (2×)

5. Drying

Again, it is important to avoid excessive contamination of the sintering setters, as any impurities tend to adhere to the MIM parts and cannot be removed prior to the passivation. Consequently, corrosion resistance is jeopardized.

For the parts depicted in Fig. 8.15, the customer required a tough black coating capable to endure stress and continuous movement. Instead of ordinary black paint, it was decided to use salt bath nitriding (QPQ Process). This offered an increased surface hardness which minimized wear, and at the same time provided a black color suitable for the final application.

Fig. 8.15 Salt bath nitrided stainless steel parts. The tough nitride coating protects against wear and has a much longer lifetime than any polymer-based coating

8.5 Operations to reduce tooling cost and enhance applications

The MIM process is particularly suitable for large scale applications. One of the major reasons for this is the cost of the molds. Depending on its complexity, a single cavity mold can cost 20.000 $ and more. These costs areindependent ofthe number of piecesto be produced, and usually assumed by the customer. Consequently, it can be hard to convince new customers of the benefits of MIM for their application. A second problem is how to spread the cost of the mold over the total production number. In some classical MIM applications, the number of parts required can be as low as 1000 per year.

Therefore, for new applications and (expected) low total volumes, it is essential to limit costs for a mold. The mold cost can be contained by removing features that can be machined in after molding. Depending on the feedstock, the MIM parts can already be machined in the green condition. Fig. 8.16 explains how a small series of titanium parts (for example for medical applications as shown in Fig. 8.17) are produced by a combination of different secondary operations.

Fig. 8.16 Schematic work flow of a small volume of titanium MIM parts from the molded part to the final component.

Fig. 8.17 Example of green machining of Ti parts for an endoscope, including some of the intermediate stages-molded parts (A and B), machining (C), deburring (D), after sintering and blasting (E), and polished (F).

In the end, the parts are hardly recognizable as MIM parts, and cost savings can be as high as 50%.

8.6 Resume-Outlook

MIM has been around for several decades. However, only recently MIM producers started to use secondary operations to their benefit, instead of trying to avoid them at all cost. In doing so, MIM producers can improve their position in the market and obtain added value. Secondary operations offer the possibility to the following:

-Improve tolerances

-Improve properties

-Improve looks

-Reduce total costs

In fact, depending on their background, not all MIM companies are familiar with these possibilities. Particularly the knowledge about surface properties and coatings sometimes is a soft spot that offers possibilities for further improvement. Green machining is an option which is often misunderstood, but can actually enable MIM producers to extend their field toward additive manufacturing. Using simple base geometries in combination with green machining provides MIM producers with a possibility to produce pilot parts without the need to manufacture complex tools. In this way, they may convince customers in a much earlier stage of the product design. Specialized companies operating in the field of technical ceramics demonstrate that even with smaller production volumes, a thriving business is possible.

In large scale applications such as automotive, MIM industry is not as advanced as the conventional press and sinter industry. In the latter, heat treatment of parts has become so much of a standard that usually the heat treatment cycle is integrated into the sintering process. These so-called high-temperature sinter-hardening lines are depicted in Fig. 8.18. These advanced furnace units enable the P/M producers to harden and anneal their parts in one system, exceeding cooling rates of 6 K/s as described by Heuer (2016).

Fig. 8.18 Rest debinding and sintering furnace with integrated carbon control and quenching capabilities suitable for integrated heat treatment of MIM parts.

A final topic which needs further development is joining. Both laser welding and brazing are already applied on a large scale in automotive P/M steel components. Depending on the final application, a fully automated laser welding unit may be a worthwhile investment. Obviously, it has to be checked how easily these systems can be converted to laser welding of stainless steel components, such as 17-4-PH, 316L or even titanium.

When it comes to brazing, there is already abundant experience in the conventional P/M industry. For over a decade, P/M companies have already integrated the brazing of several components in the sintering process. However, MIM companies hardly ever consider to join parts by brazing. Surprisingly, if you consider that batch type MIM sintering furnaces are very suitable for brazing and consequently no additional investment in hardware is required.