No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher ISBN No. 978-1-943694-05-1 ? 2016 Metal Powder Industries Federation 105 College Road East Princeton, New Jersey 08540-6692 USA All rights reserved Produced in the U.S.A. 2?
MPIF Standard 35—2016 Materials Standards for Metal Injection Molded Parts Explanatory Notes and Definitions
Minimum Value Concept
The Metal Powder Industries Federation has adopted the concept of minimum property values for metal injection molded (MIM) materials. These values may be used to determine the material best suited to the particular application as it is manufactured by the metal injection molding (MIM) process.
As an aid to the user in selecting materials, in addition to minimum property values, typical values for other properties are listed. This makes it possible for the user to select and specify the exact MIM material and properties most suitable for a specific application. The data provided define minimum values for listed materials and display typical properties achieved under commercial manufacturing procedures. Enhanced mechanical properties and other improvements in performance characteristics may be attained through more complex processing. To select a material optimum in both properties and cost effectiveness, it is essential that the part application be discussed with the MIM parts manufacturer.
Minimum Mechanical Property Values
The minimum mechanical property values for MIM materials are expressed in terms of yield strength (0.2% offset method), ultimate tensile strength and percent elongation for all materials in the as-sintered and/or heat treated conditions. MIM materials exhibit properties similar to wrought materials because they are processed to near full density.
The tensile properties utilized for establishing this Stan-dard were obtained from tensile specimens prepared spe-cifically for evaluating MIM materials. Tensile properties of test specimens machined from commercial parts or from non-standard MIM test specimens, may vary from those obtained from specimens prepared according to MPIF Standard 50. (See MPIF Standard 50 for additional details)
Minimum Magnetic Property Values
The minimum magnetic property values for MIM materials are expressed in terms of part density, maximum permeability, maximum coercive force and magnetic saturation. The specified minimum magnetic saturation is measured with an applied field of 25 oersteds. All magnetic test data reported are for DC testing only.
The magnetic properties utilized for establishing this Standard were obtained from specimens prepared and tested in accordance with ASTM A773.
Minimum Controlled-Expansion Property Values A minimum density level is expressed for the MIM controlled-expansion alloys due to their use in electronics applications to provide hermetic seals with materials such as glasses and ceramics.
Practical Methods of Demonstrating Part Performance
For structural parts, the practical method of demonstrating minimum values is through the use of a static or dynamic proof test by the manufacturer and the purchaser using the first production lot of parts and a mutually agreed upon method of stressing the part. For example, from the design of a given part, it is agreed that the breaking load should be greater than a given force. If that force is exceeded in proof tests, the minimum strength is demonstrated. The first lot of parts can also be tested in service and demonstrated to be acceptable. The static or dynamic load to fracture is determined separately and these data are statistically analyzed to determine a minimum breaking force for future production lots. Exceeding that minimum force on future lots is proof that the specified strength has been met.
For parts that require minimum magnetic characteristics, the practical method of demonstrating acceptable mag-netic properties is through the use of a magnetic proof test. For example, from the design of a given part, it is agreed that the magnetic force generated by the part when a specified magnetic field is applied should be greater than a mutually agreed upon value between the parties concerned. If that force is exceeded in proof tests, the minimum magnetic performance is demonstrated. Exceeding this minimum value on future lots is proof that the specific magnetic properties have been met.
Utilization of MPIF Standard 35 to specify a MIM material means that unless the purchaser and manufacturer have agreed otherwise, the material will have the minimum value specified in the Standard. (See Material Properties section.)
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MPIF Standard 35, Metal Injection Molded Parts—2016 Edition Typical Values Most MIM materials respond well to normal wrought For each MIM material listed, a set of typical values is heat treating practices and procedures. It is recommended shown for properties, e.g., density, hardness, elongation, that the heat-treatment procedures for any MIM etc., some or all of which may be important for a specific material be established in cooperation with the MIM application. Typical values are shown for properties, part manufacturer to achieve the desired balance of e.g., elongation, hardness, coercive field, etc., some or final properties in the finished part. all of which may be important for a specific application. The property data were compiled from test specimens Surface Finish processed by individual MIM producers. The overall finish and surface reflectivity of MIM materi-The typical values are listed for general guidance als depends on density, tool condition, particle size and only. They should not be considered minimum values. secondary operations. Effective surface smoothness of While achievable through normal manufacturing as-sintered MIM components is usually better than an processing, they may vary somewhat depending upon investment cast surface. Surface finish can be further the area of the component chosen for evaluation, or improved by secondary operations such as coining, the specific manufacturing process utilized. Those honing, burnishing or grinding. The surface finish properties listed under the “typical value section” for requirements and methods of determination must be each material which are required by the purchaser established by mutual agreement between purchaser should be thoroughly discussed with the MIM parts and producer. (See MPIF Standard 58 for additional manufacturer before establishing the specification. details.) Required property values, other than those expressed as minimum should be separately specified for each Microstructure MIM part, based on its intended use. MIM materials generally contain less than 5% porosity, approaching the density of wrought materials. Chemical Composition The examination of the microstructure of a MIM part The chemical composition of each material lists its can serve as a diagnostic tool and reveal the degree of principal elements and allowable ranges. sintering and other metallurgical information critical to the metal injection molding process. There are several Mechanical Properties observations common to most sintered MIM materials, Mechanical property data indicate the minimum and as briefly described below. Comments on specific typical properties that may be expected from test materials will be found in the subsections devoted to specimens conforming to the density and chemical those particular materials. composition criteria listed. It should be understood that Sintered parts are normally examined first in the mechanical properties used in this standard were unetched condition. With a proper sinter, there will be derived from individual test specimens prepared no original particle boundaries seen at 200X. Small, specifically for material evaluation and sintered under uniformly distributed, well rounded discrete pores lead commercial production conditions. to higher strength, ductility and impact resistance. Hardness values of heat treated specimens are given first as apparent hardness and second, when available, MIM Material Designation as equivalent particle or matrix hardness values. The Metal Injection Molding Association has chosen Residual porosity found in MIM components will slightly to use the designation system similar to AISI-SAE affect the apparent hardness readings. Microin-where applicable. These designations were chosen
dentation hardness values shown as Rockwell C were because MIM parts are likely to be used as converted from 100 g load (0.981 N) Knoop microin-replacements for wrought products already in service.
dentation hardness measurements. When specifying a material made by the MIM process, it should be so designated with a “MIM” prefix to the Heat Treatment material grade. For example, a part fabricated from MIM materials may be heat treated to increase strength, Type 316L stainless steel by MIM would be designated hardness and wear resistance. The percentages of car-as \
bon, alloying elements and residual porosity determine the degree of hardening possible. Tempering or stress Material Selection relief is required after quenching for optimum strength Before a particular material can be selected, a careful and durability. Ferrous MIM parts processed with little analysis is required of the design of the part and its or no final carbon may be surface carburized for end use. In addition, the final property requirements of increased surface hardness while retaining core the finished part should be agreed upon by the toughness. Martensitic and precipitation hardening manufacturer and the purchaser of the MIM part.
Issues such as static and dynamic loading, wear stainless steels may also be heat treated for increased
resistance, machinability and corrosion resistance may hardness and strength.
also be specified.
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Grade Selection
For certain magnetic materials, the material designation will specify the material as either “Grade 1” or “Grade 2”. The Grade 1 material, as compared with Grade 2, will exhibit improved magnetic characteristics. The difference between a Grade 1 and Grade 2 material can usually be found in the material’s microstructure, with a high density, large grain size and low amounts of interstitials (carbon, oxygen, nitrogen, etc.) all contributing to improved magnetic properties. A careful analysis of the design and function of the part should determine what grade material is required for a given application. It is recommended that a discussion of the required magnetic performance take place between the manufacturer and the purchaser before the final grade selection.
Density
Density is expressed in grams per cubic centimeter
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(g/cm) and may be determined by various standardized methods. Some common methods of MIM density deter-mination include:
MPIF Standard 54: This method is generally used for products that contain less than 2% porosity (impermeable PM). It is based on the principle of water displacement.
MPIF Standard 63: This method comprises use of a gas pycnometer. Any open porosity will not be included as part of measured volume. The density obtained by the gas pycnometer method will typically be higher than the density obtained by water displacement.
MPIF Standard 42: This method is generally used for PM products having surface-connected porosity and is based on the use of Archimedes’ principle. MIM materials generally contain less than 5% porosity, so impregnation is not applicable. Ultimate Tensile Strength Ultimate tensile strength, expressed in 103 psi (MPa) is the ability of a test specimen to resist fracture when a pulling force is applied in a direction parallel to its longitudinal axis. It is equal to the maximum load divided by the original cross-sectional area. (See MPIF Standard 50 for additional details.) Yield Strength Yield Strength, expressed in 103 psi, is the load at which a material exhibits a 0.2% offset from proportionality on a stress-strain tension curve divided by the original cross-sectional area. (See MPIF Standard 50 for additional details.)
MPIF Standard 35, Metal Injection Molded Parts—2016 Edition
Elongation Elongation (plastic), expressed as a percentage of the original gage length (typically 1.0 in. [25.4mm]), is based on measuring the increase in gage length after fracture, providing the fracture takes place within the gage length.
Elongation can also be measured with a break-away extensometer on the tensile specimen. The recorded stress strain-curve displays total elongation (elastic and plastic). The elastic strain at the 0.2% yield strength must be subtracted from the total elongation to give the plastic elongation. (See MPIF Standard 59 for additional details.)
Elastic Constants Data for the elastic constants in this standard were generated from resonant frequency testing. An equation relating the three elastic constants is:
Young’s Modulus (E)
Young’s modulus, expressed in 106 psi (GPa), is the ratio of normal stress to corresponding strain for tensile or compressive stresses below the proportional limit of the material. Shear Modulus (G) Shear modulus, expressed in 106 psi (GPa), is the ratio of shear stress to corresponding shear strain below the proportional limit of the material. Poisson’s Ratio (?) Poisson’s ratio is the absolute value of the ratio of transverse strain to the corresponding axial strain resulting from uniformly distributed axial stress below the proportional limit of the material. Impact Energy Impact energy, measured in foot-pounds-force (Joules), is a measure of the energy absorbed in fracturing a specimen in a single blow. An unnotched 5 mm X 10 mm cross- section Charpy specimen was used to establish the MIM impact energy values. (See MPIF Standard 59 for additional details.) Macroindentation Hardness (Apparent) The hardness value of a MIM part when using a conventional indentation hardness tester is referred to as \hardness\because it represents a combination of matrix hardness plus effect of residual porosity. The effect of residual porosity on hardness values is minor for MIM parts. Apparent hardness measures the resistance to indentation.
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MPIF Standard 35, Metal Injection Molded Parts—2016 Edition The manufacturer and the purchaser should agree on
the hardness, the measuring procedure, and the
hardness scale for each part tested. (See MPIF
Standard 43 for additional details.)
Microindentation Hardness Microindentation hardness is determined by utilizing Knoop (HK) or Vickers (HV) indentors with a microinden-tation hardness tester. It measures the true hardness of the structure by eliminating the effect of porosity, and thus is a measure of resistance to abrasive and adhesive
wear. Microindentation hardness measurements are
convertible to equivalent Rockwell hardness values
for comparison with other materials. A description of the microstructure must be reported.
The specimen shall be polished to reveal the porosity
and lightly etched to view the phases in the micro-structure and to determine where to place the hardness indentation. If the indentor strikes an undisclosed pore,
the diamond mark will exhibit curved edges and the
reading must be discarded. Since the data tend to be
scattered compared with pore-free material, it is recom-mended that a minimum of 5 indentations be made, anomalous readings discarded, and an average taken of
the remainder. (See MPIF Standard 51 for additional
details.) Corrosion Resistance Three media and test methods were used to rate the resistance of the MIM stainless steel alloys to corrosion. Sulfuric Acid Testing - Standard 5 mm X 10 mm X 55 mm test specimens were immersed in a 2% sulfuric acid solution at room temperature (72 °F ± 4 °F [22 °C ± 2 °C])
for 1,000 hours. Two replicates were tested. The loss
in mass for each was determined and then
converted into a mass loss per surface area (in dm2)
per day factor, in units of
g
(dm2) (day)
(See MPIF Standard 62 for additional details.)
Copper Sulfate Testing - The copper sulfate test consists of mixing 22.5 ml of distilled water with 1 g cupric sulfate crystals and 2.5 g sulfuric acid. Specimens are immersed in this solution for 6 minutes at a temperature between 63 Specimens that show no visual ° and 67 signs °F (17 of copper ° and 19 plating °C). are classified as passing this test. (See ASTM F1089 for additional details.)
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Boiling Water Testing - The boiling water test consists of immersing the specimen in boiling, distilled water for 30 minutes. After 30 minutes, the heat source is shut off and the specimen remains in the water for 3 hours. The specimen is then removed and left to dry for 2 hours. Specimens that show no visual corrosion are classified as passing this test. (See ASTM F1089 for additional details.)
Soft-Magnetic Properties The magnetic data presented in this standard were developed in accordance with ASTM Standard A773. Magnetizing Field (H) The magnetic field applied to a test specimen, measured in oersteds (Oe) or amperes/metre (A/m). Induction (B) The measured magnetic field generated in a test specimen due to an applied magnetic field, measured in kilo- gauss (kG) or tesla (T). Maximum Induction (Bm) The maximum value of induction in a DC hysteresis loop. This value depends on the magnetizing field applied. Data are reported at magnetizing fields of 25 Oe
and 500 Oe, (1,990 A/m and 39,800 A/m), in units of kilogauss (kG) or tesla (T). Maximum Permeability (μmax) The slope of the line from the origin to the knee of the initial B-H magnetization curve. This parameter is dimensionless.
Coercive Field (Hc) The DC magnetizing field required to restore the magnetic induction to zero after the material has been symmetrically, cyclically magnetized, measured in Oe (A/m).
Residual Induction (Br) The retained magnetism in the specimen after
the applied field has been reduced to zero Oe (A/m).
This is reported in kG or T.