• MACHINE

    A nonstandard term when used for MECHANIZED.

  • MACHINE DESIGN

    The advances that have been made in welding and thermal cutting processes have provided the means of shaping and joining large sections of iron and steel. These processes have replaced castings in the production of machine frames, machine bases, and other structures. Correctly designed welded frames are stronger and lighter, but rigidity has not been sacrificed.

    Designing for Strength and Rigidity- Machine designs must have sufficient strength so that members will not fail by breaking or yielding when subjected to normal operating loads or reasonable overloads. Strength designs are common in road machinery, motor brackets, farm implements, and like structures.

    For weldments in machine tools and other machin- ery, rigidity as well as strength is important, since excessive deflection under load would result in lack of precision in the product. A design based on rigidity requires the use of design formulas for sizing members. Some parts of a weldment serve their design function without being subjected to loading much greater than their own weight (dead load). Some typical parts are dust shields, safety guards, and cover plates for access holes. Only casual attention is required in sizing such members.

    Design Formulas. The design formulas for strength and rigidity always contain terms representing the load, the stress, and the strain or deformation. If two of the three terms are known, the others can be calculated. All problems of design thus resolve into one of the following:

    (1) Finding the internal stress or deformation caused by an external load on a given member.

    (2) Finding the external load that may be placed on a given member for any allowable stress or deformation

    (3) Selecting a member to carry a given load without exceeding the specified stress or deformation.

    In designing within allowable limits, the designer should generally select the most efficient material section size and section shape. The properties of the material and those of the section determine the ability of a member to carry a given load.

    Sizing of Steel Welds- A weld is sized for its capability to withstand static or cyclic loading. Allowable stresses for welds for various types of loading are normally specified by the construction standards applicable to the job. They are usually based on a percentage of tensile or yield strength of the metal to ensure that a soundly welded joint can support the applied load for the expected service life. Allowable stresses or stress ranges are specified for various types of welds under static and cyclic loads. The allowable stress ranges for welded joints subjected to cyclic loading specified in current standards are based on testing of representative full-size welded joints in actual or mockup structures.

    The primary requirement of machine design for a machine and some of its members is rigidity. Such members are often thick sections so that the movement under load can be controlled within close tolerances. Whereas low-carbon steel has an allowable stress in tension of 138 MPa (20 ksi), a welded machine base or frame may have a working stress of only 14 to 28 MPa (2 to 4 ksi). In these cases the weld sizes should be designed for rigidity rather than load conditions.

    A practical method is to design the weld size to carry one-third to one-half of the load capacity of the thinner member being joined. This means if the base metal is stressed to one-third to one-half of the normal allowable stress, the weld would be strong enough to carry the load. Most rigid designs are stressed below these values.

    Welding Conditions- Designers specifying welding procedures for machinery fabrication should specify the following:

    (1) Joint type, groove angle, root opening and root face

    (2) Electrode type and size to be used

    (3) Current type, polarity and current in amperes

    (4) Arc length (arc voltage)

    (5) Travel speed

    (6) Welding position i.e., flat, horizontal, vertical, overhead

    (7) Test procedures for weld metal and joints 

  • MACROETCH

    Etching of a metal surface to accentuate the gross structural details and defects for observation by the unaided eye, or at a magnification not exceeding ten diameters. 

  • MACROETCH TEST

    A test in which a specimen is prepared with a fine finish, etched, and examined under low magnification. 

  • MACROGRAPH

    A graphic reproduction of the surface of a prepared specimen at a magnification not to exceed ten diameters. When photographed, the reproduction is called a photomacrograph. 

  • MACROSCOPIC

    Visible at magnifications from one to ten diameters. 

  • MACROSTRUCTURE

    The structure of metals as revealed by examination of the etched surface of a polished specimen at a magnification not greater than ten diameters. 

  • MAG

    Metal Active Gas; a little-used term for gas metal arc welding in which an active gas such as carbon dioxide is used. See GAS METAL ARC WELDING. 

  • MAGNESIUM

    (Chemical symbol: Mg). A light, white and fairly tough metal. It tarnishes slightly in air, and when fabricated into ribbon, wire or powder, ignites on heating and bums with a dazzling white flame. Magnesium is one of the most abundant elements; it is eighth in estimated amount in the earth's crust. It is removed commercially from sea water in the form of magnesium chloride (a mineral similar to table salt). Pure magnesium is obtained from molten magnesium chloride by the electrolysis process; the magnesium collects on the cathode. Atomic weight 24.32; atomic number, 12; melting point 651°C (1204°F); boiling point 1110°C (2030°F).

    In the pure state, magnesium does not have sufficient strength or other properties to make it suitable for structural purposes. However, it alloys readily with aluminum, zinc, silicon, manganese and tin, to form a variety of structural alloys. The strength of these alloys is comparable to aluminum alloys but they weigh only 65% as much as aluminum. See MAGNESIUM ALLOYS. 

  • MAGNESIUM ALLOYS

    Magnesium alloys are used in a wide variety of applications where light weight is important. Structural applications include industrial, materials-handling, commercial, and aerospace equipment. In industrial machinery, such as textile and printing machines, magnesium alloys are used for parts that operate at high speeds and must be lightweight to minimize inertial forces. Materials-handling equipment examples are dock boards, grain shovels, and gravity conveyors; commercial applications include such items as luggage and ladders. Good strength and rigidity at both room and elevated temperatures, combined with light weight, make magnesium alloys useful for some aerospace applications.

     

    Alloy Systems

    Most magnesium alloys are ternary types. They may be considered in four groups based on the major alloying element: aluminum, zinc, thorium, or rare earths. There are also binary systems employing manganese and zirconium. Magnesium alloys may also be grouped according to service temperature. The magnesium-aluminum and magnesium-zinc alloy groups are suitable only for room-temperature service. Their tensile and creep properties decrease rapidly when the service temperature is above about 150°C (300°F).  The magnesium-thorium and magnesium-rare earth alloys are designed for elevated-temperature service. They have good tensile and creep properties up to 370°C (700°F). Designation Method. Magnesium alloys are designated by a combination letter-number system composed of four parts. Part 1 indicates the two principal alloying elements by code letters arranged in order of decreasing percentage. The code letters are listed in Table M- 1.

    Part 2 indicates the percentages of the two principal alloying elements in the same order as the code letters. The percentages are rounded to the nearest whole number. Part 3 is an assigned letter to distinguish different alloys with the same percentages of the two principal alloying elements. Part 4 indicates the condition of temper of the product. It consists of a letter and number similar to those used for aluminum, as shown in Table M-2. They are separated from Part 3 by a hyphen.  An example is alloy AZ63A-T6. The prefix AZ indicates that aluminum and zinc are the two principal alloying elements. The numbers 6 and 3 indicate that the alloy contains nominally 6% aluminum and 3% zinc. The following A indicates that this is the first standardized alloy of this composition. The fourth part, T6, states that the product has been solution heat-treated and artificially aged.

    Commercial Alloys- Magnesium alloys are produced in the form of castings and wrought products including forgings, sheet, plate, and extrusions. A majority of the alloys produced in these forms can be welded. Commercial magnesium alloys are designed for either room-temperature or elevated-temperature service. Some of the more important magnesium alloys for room temperature service are listed in Table M-3. Those for elevated temperature service are listed in Table M-4.

     

    Wrought Alloys- Welded construction for room-temperature service is frequently designed with AZ3 1B alloy. It offers a good combination of strength, ductility, toughness, malleability, and weldability in all wrought product forms. The alloy is strengthened by work hardening. AZ80A and ZK60A alloys can be artificially aged to develop good strength properties for room temperature applications.  Weldments made with AZlOA, MIA, and ZK21A alloy are not sensitive to stress-corrosion cracking, so postweld stress relieving is not required for weldments made of these alloys. They are strengthened by work hardening for room-temperature service. HK31A, HM21A, and HM31A alloys are designed for elevated-temperature service. They are strengthened by a combination of work hardening followed by artificial aging.

    Cast Alloys- The most widely used casting alloys for room-temperature service are AZ9 1C and AZ92A. These alloys are more crack-sensitive than the wrought Mg-Al-Zn alloys with lower aluminum content. Consequently, they require preheating prior to fusion welding.  EZ33A alloy has good strength stability for elevated-temperature service and excellent pressure tightness. HK31A and HZ32A alloys are designed to operate at higher temperatures than is EZ33A. QH21A alloy has excellent strength properties up to 260°C (500°F). All of these alloys require heat treatment to develop optimum properties. They have good welding characteristics.

    Mechanical Properties- Typical strength properties at room temperature for magnesium alloys are given in Table M-5. For castings, the compressive yield strength is about the same as the tensile yield strength. However, the yield strength in compression for wrought products is often lower than in tension.  The tensile and creep properties of representative magnesium alloys at a service temperature of 315°C (600°F) are given in Table M-6. The alloys containing thorium (HK, HM, and HZ) have greater resistance to creep at 3 15°C (600°F) than do the Mg-Al-Zn alloys.

    Major Alloying Elements- With most magnesium alloy systems, the solidification range increases as the alloy addition increases. This contributes to a greater tendency for cracking during welding. At the same time, the melting temperature as well as the thermal conductivity and electrical conductivity decrease. Consequently, less heat input is required for fusion welding as the alloy content increases.  Aluminum and zinc show decreasing solubility in solid magnesium with decreasing temperature. These elements will form compounds with magnesium. Consequently, alloys containing sufficient amounts of aluminum and zinc can be strengthened by a precipitation-hardening heat treatment. Other alloying elements also behave similarly in ternary alloy systems. Beryllium, manganese, silver, thorium and zirconium are major alloying elements in magnesium alloys.

    Weldability- The relative weldability of magnesium alloys by gas shielded arc and resistance spot welding processes is shown in Table M-7. Castings are not normally resistance welded. The Mg-Al-Zn alloys and alloys that contain rare earths or thorium as the major alloying element have the best weldability. Alloys with zinc as the major alloying element are more difficult to weld. They have a rather wide melting range, which makes them sensitive to hot cracking. With proper joint design and welding conditions, joint efficiencies will range from 60 to 10096, depending on the alloy and temper.  Most wrought alloys can be readily resistance spot welded. Due to short weld cycles and heat transfer characteristics, fusion zones are fine-grained, and heat-affected zones experience only slight degradation.

    Arc Welding

    Applicable Processes- The gas tungsten arc and gas metal arc welding processes are commonly used for joining magnesium alloy components. Inert gas shielding is required with these processes to avoid oxidation and entrapment of oxide in the weld metal. Processes that use a flux covering do not provide adequate oxidation protection for the molten weld pool and the adjacent base metal. Procedures for arc welding magnesium are similar to those used for welding aluminum.

    Filler Metals- The weldability of most magnesium alloys is good when the correct filler metal is employed. A filler metal with a lower melting point and a wider freezing range than the base metal will provide good weldability and minimize weld cracking. The recommended filler metals for various magnesium alloys are given in Table M-8.  Casting repairs should be made with a filler metal of the same composition as the base metal when good color match, minimum galvanic effects, or good response to heat treatment is required. For these unusual service requirements, the material supplier should be consulted for additional information.

    Safe Practices- The welding fumes from all commercial magnesium alloys, except those containing thorium, are not harmful when the amount of fumes remains below the welding fume limit of 5 mg/m3. Welders should avoid inhalation of fumes from the thorium-containing alloys because of the presence of alpha radiation in the airborne particles. However, the concentration of thorium in the fumes is sufficiently low so that good ventilation or local exhaust systems will provide adequate protection. The radiation concern, however, is primarily responsible for the decline in use of the thorium-containing alloys. No external radiation hazard is involved in the handling of the thorium containing alloys.  The possibility of ignition when welding magnesium alloys in thicknesses greater than 0.25 mm (0.01 in.) is extremely remote. Magnesium alloy product forms will not ignite in air until they are at fusion temperature. Then, sustained burning will occur only if the ignition temperature is maintained. Inert gas shielding during welding prevents ignition of the molten weld pool. Magnesium fires may occur with accumulations of grinding dust or machining chips. Accumulation of grinding dust on clothing should be avoided. Graphite-based (G- 1) or proprietary salt-based powders recommended for extinguishing magnesium fires should be conveniently located in the work area. If large amount of fine particles, or fines,are produced, they should be collected in a waterwash-type dust collector designed for use with magnesium. Special precautions pertaining to the handling of wet magnesium fines must be followed.  The accumulation of magnesium dust in a water bath also can present a hazard. Dust of reactive metals like magnesium or aluminum can combine with the oxygen in the water molecule, leaving hydrogen gas trapped in a bubbly froth on top of the water. A heat source may cause this froth to explode.  Adequate ventilation, protective clothing, and eye protection must be used when working with these materials to avoid toxic effects, bums, or other injuries that they may cause. 

  • MAGNESIUM RESISTANCE WELDING

     

    Spot Welding- Magnesium alloy sheet and extrusions can be joined by resistance spot welding in thicknesses ranging from about 0.5 to 3.3 rmn(0.02 to 0.13 in.). Alloys recommended for spot welding are MIA, AZ31B, AZ61A, HK31A, HM21A, HM3lA, and ZK60A. Spot welding is used for low-stress applications where vibration is low or nonexistent. Magnesium alloys are spot welded using procedures similar to those for aluminum alloys.

    Electrodes- Spot welding electrodes for magnesium alloys should be made of RWMA Group A, Class 1 or Class 2 alloy. The faces of the electrodes must be kept clean and smooth to minimize the contact resistance between the electrode and the adjacent part. Cleaning should be done with an electrode dressing tool with the proper face contour covered with a very fine polishing cloth of 280-grit abrasive course.

    Copper pickup on the spot weld surfaces increases the corrosion susceptibility of magnesium. ‘Therefore, the copper should be completely removed from the surfaces by a suitable mechanical cleaning method. The presence of copper on spot welds can be determined by applying 10% acetic acid solution. A dark spot will form if copper is present on the surface.

    Joint Strength- Typical shear strengths for spot welds in several thicknesses of two magnesium alloys are shown in Table M-9. 

  • MAGNET

    A bar of steel, tungsten or cobalt steel in which the alignment of the atoms and the motion of the atomic electrons within the metal exert attractive forces on iron and steel. The ends of the bar are called poles. Every bar magnet has at least two poles, usually one near each end. Poles always exist in pairs. A magnet exerts the greatest attractive force at points near the ends. 

  • MAGNETIC ARC BLOW

    A nonstandard term for ARC BLOW. 

  • MAGNETIC CONTACTOR

    A device operated by an electromagnet which opens and closes an electrical circuit. 

  • MAGNETIC FIELD

    The region around a magnet in which magnetic force exists, and would act on a piece of iron or on another magnet brought into the region. In a compass, the direction in which the north-seeking pole of the compass needle points is called the direction of the magnetic field at that place. 

  • MAGNETIC FLUX

    The total amount of magnetism induced across a surface; the magnetic flux is equal to the number of magnetic lines of force in a magnetic circuit. See MAGNETIC LINES OF FORCE.

  • MAGNETIC FLUX DENSITY

    The number of lines of magnetic flux per square centimeter or per square inch. 

  • MAGNETIC FORCE

    The attractive (or repulsive) force exerted by one magnet on another or by a magnet on a ferromagnetic material. The force between two magnets at distances much larger than the lengths of the magnets varies inversely with the distance between the magnets. As the distance is increased, there is a rapid decrease in the force. 

  • MAGNETIC INDUCTION

    When iron is placed in a solenoid with current flowing through the solenoid circuit, the iron becomes magnetized, adding the lines of its own magnetic flux to the magnetic lines produced by the current. The total flux per square centimeter is no longer numerically equal to the magnetizing force, but to a larger quantity called the magnetic induction.

    This quantity is represented by the letter B, where B is the sum of the magnetic lines produced by the current and those produced by the iron. 

  • MAGNETIC INSPECTION OF WELDS

    A nonstandard term for MAGNETIC PARTICLE INSPECTION. See MAGNETIC PARTICLE INSPECTION. 

  • MAGNETIC LINES OF FORCE

    The concept of magnetic lines of force was invented by Michael Faraday and is useful in understanding magnetic and electrostatic phenomena. It is defined in the following way: on a sphere with a radius of one centimeter surrounding a unit pole, each square centimeter will contain a single line of force. The surface of a sphere is 4πr2, thus the total number of lines of force due to a unit pole is 4π. Again, it should be understood that these magnetic lines are purely imaginary. But the concept is a useful study tool, and many technicians are in the habit of referring to magnetic lines as if they actually exist in the space around every magnet. 

  • MAGNETIC MATERIALS

    All substances, whether in the form of liquid, solid, or gas, will respond in some manner to an applied magnetic field, although in varying degrees. The magnetic field can be produced by an electric current or it may be the flux from either a permanent magnet or an electromagnet.

    Ferromagnetism is the magnetic property of greatest interest in the context of welding metallurgy, because this particular magnetic behavior is frequently involved in welding operations. 

    Ferromagnetic Materials- Of all the elements in the periodic table only three, iron, cobalt, and nickel, are ferromagnetic at room temperature. However, ferromagnetic alloys can be formulated using various metallic elements which individually are not ferromagnetic. Alnico is an example of an Al-Ni-Co-Cu-Fe alloy used to make permanent magnets, although individually some of the elements of the magnet are not ferromagnetic. Ferromagnetic materials are divided into two classifications: magnetically soft materials, and hard or permanent magnet materials.

    Magnetically Soft Materials- Soft ferromagnetic materials are easy to magnetize, but retain little or none of the induced magnetism when the magnetizing force is removed. Magnetically soft materials made in large quantities include high-purity iron, silicon steels, iron-nickel alloys, iron-cobalt alloys, and ferrites.

    Permanent (Hard) Magnet Materials- Hard ferromagnetic materials are difficult to magnetize, but they retain a significant degree of magnetization when the applied magnetic force is removed. Permanent magnet materials include both plain high-carbon steels and high-carbon alloy steels, magnet alloys that have useful magnetic properties from the combination of specific elements but which are virtually free of carbon, and metallic oxides that possess unique magnetic properties that make them commercially important.

    Martensitic alloys are the best known and oldest of permanent magnet materials. The optimum magnetic properties result from untempered martensite in plain high-carbon steels (0.8 to 1.0percent carbon). Permanent magnet alloy materials include iron-chromium- carbon, and cobalt magnet steel.

    Alnico types are probably the most popular of permanent magnet steels. There are a number of Alnico alloys, with a typical alloy containing 12A1-28Ni-5Co. Some alloys have copper and titanium contents. All these alloys are hard, brittle, and unmachinable, so they must be cast or finish-ground to shape.

  • MAGNETIC PARTICLE INSPECTION (MT)

    Magnetic particle inspection (MT) is a nondestructive method used for locating surface or near surface discontinuities in ferromagnetic materials. Magnetic particle inspection is based on the principle that magnetic lines of force will be distorted by a change in material continuity; i.e., a discontinuity creating magnetic field leakage. See Figure M-1. Magnetic particles, scattered on the plate, will be retained at the location of magnetic flux leakage. The accumulation of particles will be visible under proper lighting conditions.

    A weld can be magnetized by passing an electric current through the weld (direct magnetization), or by placing it in a magnetic field (indirect magnetization).

     

    Direct Magnetization- The direct magnetization method is illustrated in Figure M-2. This method is  normally used with direct current (dc), half-wave  direct current (HWDC) or full-wave direct current  (FWDC).These types of current have penetrating abilities that generally enable slightly subsurface discontinuities to be detected. Direct magnetization can also be used with alternating current (ac), which is limited to the detection of surface discontinuities only. 

    Indirect Magnetization- Detection of subsurface discontinuities depends on several different variables--the magnetizing method, the type of current, the direction and density of the magnetic flux, and the material properties of the weld to be inspected.

    When evaluating surface discontinuities only, ac is preferred with the indirect magnetization method. See Figure M-3. Alternating current has a very low penetrating ability, which allows the magnetic field to be concentrated at the surface of the weld.

    The alternating nature of the current provides continuous reversal of the magnetic field. This action provides greater particle mobility, and, in turn, aids the detection of surface discontinuities. 

    When the magnetic field has been established within the weld, magnetic particles (medium) are applied to the inspection surface. After the excess particles are removed, the residual particles trapped in the leakage field of a discontinuity reveal the location, shape and size of a detectable discontinuity. These indications are usually distinguishable by their appearance as sharp, well defined lines of medium against the background of weld surface.

     

  • MAGNETIC PARTICLE INSPECTION (MT)

    Magnetic particle inspection (MT) is a nondestructive method used for locating surface or near surface discontinuities in ferromagnetic materials. Magnetic particle inspection is based on the principle that magnetic lines of force will be distorted by a change in material continuity; i.e., a discontinuity creating magnetic field leakage. See Figure M-1. Magnetic particles, scattered on the plate, will be retained at the location of magnetic flux leakage. The accumulation of particles will be visible under proper lighting conditions.

    A weld can be magnetized by passing an electric current through the weld (direct magnetization), or by placing it in a magnetic field (indirect magnetization).

     

    Direct Magnetization- The direct magnetization method is illustrated in Figure M-2. This method is  normally used with direct current (dc), half-wave  direct current (HWDC) or full-wave direct current  (FWDC).These types of current have penetrating abilities that generally enable slightly subsurface discontinuities to be detected. Direct magnetization can also be used with alternating current (ac), which is limited to the detection of surface discontinuities only. 

    Indirect Magnetization- Detection of subsurface discontinuities depends on several different variables--the magnetizing method, the type of current, the direction and density of the magnetic flux, and the material properties of the weld to be inspected.

    When evaluating surface discontinuities only, ac is preferred with the indirect magnetization method. See Figure M-3. Alternating current has a very low penetrating ability, which allows the magnetic field to be concentrated at the surface of the weld.

    The alternating nature of the current provides continuous reversal of the magnetic field. This action provides greater particle mobility, and, in turn, aids the detection of surface discontinuities. 

    When the magnetic field has been established within the weld, magnetic particles (medium) are applied to the inspection surface. After the excess particles are removed, the residual particles trapped in the leakage field of a discontinuity reveal the location, shape and size of a detectable discontinuity. These indications are usually distinguishable by their appearance as sharp, well defined lines of medium against the background of weld surface.

    Advantages of MT Inspection- Magnetic particle inspection is considerably less expensive than radiography (RT) or ultrasonic inspection (UT). Magnetic particle inspection equipment is relatively low in price compared to equipment required by the RT and UT methods of nondestructive inspection. Less training time is generally required for personnel to become competent in performing magnetic particle inspection and evaluating discontinuities.

    Using the MT method, the inspector obtains an instant visual indication that assists in locating a defect. Compared to penetrant inspection (PT),the MT method has the advantage of revealing discontinuities that are not open to the surface (i.e., cracks filled with carbon, slag or other contaminants) and therefore not detectable by penetrant inspection. Magnetic particle inspection is generally faster, requires less surface preparation, and is usually more economical than penetrant inspection.

    Disadvantages of MT Inspection- The MT method is limited to ferromagnetic material. This method cannot be used to inspect non-ferromagnetic materials such as aluminum, magnesium or austenitic stainless steel. Difficulties may arise when inspecting welds where the magnetic characteristics of the weld differ appreciably from those of the base metal, e.g., austenitic steel surfacing on a low-carbon steel weld. Welded joints between metals of dissimilar magnetic characteristics may create magnetic particle indications even though the welds themselves are sound. Most weld surfaces are acceptable for magnetic particle inspection after the removal of slag, spatter, and other extraneous material that may mechanically hold the medium. 

     

  • MAGNETICALLY IMPELLED ARC WELDING

    An arc welding process in which an arc is created between the butted ends of tubes and propelled around the weld joint by a magnetic field, followed by an upsetting operation.